An introduction to Python for absolute beginners

1
An introduction to Python
for absolute beginners

2
Sources
● An introduction to Python for absolute
beginners
Bob Dowling
University Information Services
scientific-computing@ucs.cam.ac.uk
http://www.ucs.cam.ac.uk/docs/course-notes/unix-courses/PythonAB
● Introduction to Python
Chen Lin clin@brandeis.edu
● Learn Python the hard way
http://learnpythonthehardway.org/book/
● http://python.org/

3
Advices
● Go through each exercise.
● Type in each sample exactly.
● Make it run.
● Reading and Writing
● Attention to Detail
● Spotting Differences
● Do Not Copy-Paste
● A Note on Practice and Persistence

4
Course outline ― 1
Who uses Python & what for
What sort of language it is
How to launch Python
Python scripts
Reading in user data
Numbers
Conversions
Comparisons
Names for values
Text and Comments
Truth & Falsehood

5
Assignment
Names
Our first “real” program
Loops
if… else…
Indentation

6
Lists
Indices
Lengths
Changing items
Extending lists
Methods
Creating lists
Testing lists
Removing from lists
for… loop
Iterables
Slices

7
Files
Reading & writing
Writing our own functions
Tuples
Modules
System modules
External modules
Dictionaries
Formatted text

8
What is Python
● Cross platform
● Multi-purpose (Web, GUI, Scripting, …)
● Object Oriented
● Interpreted
● Stringly typed
● Dynamically typed
● Focus on readability and productivity
● Powerful

9
Features
● Everything is an Object
● Interactive shell
● IDE
● Huge ecosystem

10
History
● Created in 1989 by Guido Van Rossum
● 1994 – Python 1.0
● 2000 – Python 2.0
● 2008 – Python 3.0

11
Who uses Python?
On-line games
Web services
Applications
Science
Instrument control
Embedded systems
en.wikipedia.org/wiki/List_of_Python_software

12
What sort of language is Python?
Explicitly
compiled
to machine
code
Purely
interpreted
C, C++,
Fortran
Shell,
Perl
Explicitly
compiled
to byte
code
Java, C#
Implicitly
compiled
to byte
code
Python
Compiled Interpreted

Major versions of Python
●
“Python” or “CPython” is written in C/C++
– Version 2.7 came out in mid-2010
– Version 3.1.2 came out in early 2010
– Latest stable version is 3.5.1 of 3.x series
●
“Jython” is written in Java for the JVM
●
“IronPython” is written in C# for the .Net
environment
●
PyPy Go To Website

Downloading Python
● https://www.python.org/downloads
– 3.5.1
● Download Windows x86-64 executable installer
– 2.7.11
● Download Windows x86-64 MSI installer

17
Running Python ― 2
$ python3
Unix prompt
Unix command
Introductory blurb
Python prompt
[GCC 4.6.3] on linux2
(default, May 3 2012, 15:54:42)
Python version
Python 3.2.3
>>>

18
Quitting Python
>>> exit()
>>> quit()
>>> Ctrl D+
Any one
of these

19
PEP 8
● Python Enhancement Proposals (PEP)
– https://www.python.org/dev/peps/
● PEP 8
– Style Guide for Python Code
– https://www.python.org/dev/peps/pep-0008/

20
A first Python command
>>> print('Hello, world!')
Hello, world!
>>>
Python prompt
Python command
Output
Python prompt

21
Python commands
print
Python “function”
(
Function’s “argument”
Round brackets
― “parentheses”
( )'Hello, world!'
“Case sensitive”print PRINT≠

22
Python text
The body
of the text
Quotation marks
' 'Hello, world!
!
The quotes are not
part of the text itself.

23
Python comments
● Notes, examples in a single or multiple lines
● Single line:
# print('Hello, world!')
● Multiple lines:
'''
You can writhe anything here:
print('Hello, world!')
Same in English
'''

24
Quotes?
print Command
'print' Text

25
Alternative Pythons
iPython
idle

26
Python scripts
hello1.py
File in home directory
$ python3
Hello, world!
$
Unix prompt
Unix command
to run Python
Python script
Python script’s output
Unix prompt
hello1.py
print('Hello, world!')
Run from Unix prompt

27
Editing Python scripts — 1

28
Editing Python scripts — 2

Development Environments
what IDE to use? http://stackoverflow.com/questions/81584
1. PyDev with Eclipse
2. Komodo
3. Emacs
4. Vim
5. TextMate
6. Gedit
7. Idle
8. PIDA (Linux)(VIM Based)
9. NotePad++ (Windows)
10.BlueFish (Linux)

31
PyCharm
● Download
– https://www.jetbrains.com/pycharm/download/
– Community version

32
Progress
Interactive Python
Python scripts
print() command
Simple Python text

33
Exercise 1
1. Print “Rock and roll” from interactive Python.
2. Edit exercise1.py to print the same text.
3. Run the modified exercise1.py script.
2 minutes
❢ Please ask if you have questions.

34
A little more text
hello2.py
print(' эłℏ Ꮣዐ, ω☺ ∂‼')ⲗրFull “Unicode” support
www.unicode.org/charts/

35
Getting characters
ğ
AltGr Shift+ #+ g
“LATIN SMALL
LETTER G
WITH BREVE”
u011f
Character Selector
Linux
˘

36
Text: a “string” of characters
>>> type('Hello, world!')
<class 'str'> A string of characters
H e l l o , ␣ w o r l d !13
Class: string
Length: 13
Letters
str

37
Text: “behind the scenes”
str 13 72 101 108 108 111 44 32 33100…
011f16
ğ
>>> chr(287)
'ğ'
>>> ord('ğ')
287
>>> 'u011f'
'ğ'
28710

38
Adding strings together: +
print('Hello, ' + 'world!')
hello3.py
“Concatenation”
>>> 'Hello, ' + 'world!'
'Hello, world!'
>>>

39
Pure concatenation
>>> 'Hello,␣' + 'world!'
'Hello, world!'
>>> 'Hello,' + '␣world!'
'Hello, world!'
>>> 'Hello,' + 'world!'
'Hello,world!'
Only simple
concatenation
No spaces added
automatically.

40
Single & double quotes
>>> 'Hello, world!'
'Hello, world!'
>>>
'Hello, world!'
Single quotes
"Hello, world!" Double quotes
Single quotes
Single quotes

41
Python strings: input & output
'Hello, world!'
'Hello, world!'
"Hello, world!"
Single or double
quotes on input.
Single quotes on output.
Create same
string object.
H e l l o , ␣ w o r l d !13str

42
Uses of single & double quotes
>>>
He said "hello" to her.
print('He said "hello" to her.')
>>>
He said 'hello' to her.
print("He said 'hello' to her.")

43
Why we need different quotes
>>>
File "<stdin>", line 1
print('He said 'hello' to her.')
^
SyntaxError: invalid syntax
print('He said 'hello' to her.')
✘

44
Adding arbitrary quotes
>>> print('He said 'hello' to her.')
He said 'hello' to her.
'
"
'
" “Escaping”
Just an ordinary
character.
H e s a i␣ ' h e l l23str o ' t o h e r .d ␣␣ ␣

45
Putting line breaks in text
>>> print('Hello,
Hello,
world! What we want
world')
↵ Try this
>>> print('Hello, ↵
File "<stdin>", line 1
print('Hello,
^
>>>
SyntaxError: EOL while
scanning string literal “EOL”: End Of Line
✘

46
Inserting “special” characters
>>> print('Hello,
Hello,
world!
world!')n
Treated as
a new line.
str 13 e l l o , ↵ w o r l d !H
n Converted into a
single character.
>>> len
13 len() function: gives
the length of the object
('Hello,nworld!')

47
The backslash
'
"
'
"
Special Ordinary
n
t
↵
⇥
Ordinary Special

48
n: unwieldy for long text
'SQUIRE TRELAWNEY, Dr. Livesey, and then
rest of these gentlemen having asked men
to write down the whole particularsnabou
t Treasure Island, from thenbeginning to
the end, keeping nothingnback but the b
earings of the island,nand that only bec
ause there is stillntreasure not yet lif
ted, I take up mynpen in the year of gra
ce 17__ and gonback to the time when my
father keptnthe Admiral Benbow inn and t
he brownnold seaman with the sabre cut f
irstntook up his lodging under our roof.'
Single
line

49
Special input method for long text
Triple
quotes
'''
rest of these gentlemen having asked me
to write down the whole particulars
about Treasure Island, from the
beginning to the end, keeping nothing
back but the bearings of the island,
and that only because there is still
treasure not yet lifted, I take up my
pen in the year of grace 17__ and go
back to the time when my father kept
the Admiral Benbow inn and the brown
old seaman with the sabre cut first
took up his lodging under our roof.
SQUIRE TRELAWNEY, Dr. Livesey, and the
'''
Multiple
lines

50
Python’s “secondary” prompt
>>> '''Hello,
world'''...
Python asking for more
of the same command.

51
It’s still just text!
>>> 'Hello,nworld!'
'Hello
>>>
...
'''Hello,
world!'''
'Hellonworld'
world'n
Python uses n to represent
line breaks in strings.
Exactly the same!

52
Your choice of input quotes:
'Hello,nworld!' "Hello,nworld!"
"""Hello,
world!"""
'''Hello,
world!'''
str 13 e l l o , ↵ w o r l d !H
Same result:
Four inputs:

53
Progress
International text
print()
Concatenation of strings
Long strings
Special characters

54
Exercise 2
3 minutes
1. Replace XXXX in exercise2.py so it prints the following
text (with the line breaks) and then run the script.
coffee
Kaffee
café
caffè
é
è u00e8
u00e9
AltGr
AltGr
+
+
#
;
e
e

55
Attaching names to values
message = 'Hello, world!'
print(message)
hello3.py
str 13 e l l o , ␣ w o r l d !Hmessage
“variables”
>>> message='Hello, world!'
>>> message
'Hello, world!'
>>> type(message)
<class 'str'>

56
Attaching names to values
message = 'Hello, world!'
print(message)
hello4.py
print function
str 13 e l l o , ␣ w o r l d !Hmessage
>>> type(print)
<class 'builtin_function_or_method'>

57
Reading some text into a script
$ python3
Yes?
input1.py
Boo!
Boo!
message = input('Yes?␣')
print(message)
input1.py
input('Yes?␣')
message = …
print(message)

58
Can't read numbers directly!
number = input('N?␣')
print(number + 1)
input2.py
$ python3 input2.py
N? 10
Traceback (most recent call last):
File "input2.py", line 2, in <module>
print( + )
string integer
number
TypeError:
Can't convert 'int' object
to str implicitly
✘
1

59
input(): strings only
number = input('N?␣')
print(number + 1)
input2.py
✘$ python3 input2.py
N? 10
input('N? ')␣ str 2 01
int 10≠

60
Some more types
>>> type('Hello, world!')
<class 'str'>
>>> type(42)
<class 'int'>
>>> type(3.14159)
<class 'float'>
string of characters
integer
floating point number

61
Converting text to integers
>>> int('10')
10
>>> int('␣-100␣')
-100
>>> int('100-10')
ValueError:
invalid literal for int() with base 10: '100-10'
str 2 01 int 10
str 6 -␣
int -100
01 ␣0
✘

62
Converting text to floats
>>> float('10.0')
10.0
>>> float('␣10.␣')
10.0
'10.0' is a string
10.0 is a floating
point number

63
Converting between ints and floats
>>> float(10)
10.0
>>> int(10.9)
10 Truncates
fractional part
>>> int(-10.9)
-10

64
Converting into text
>>> str(10)
'10'
>>> str(10.000)
'10.0'
integer
float
string
string

65
Converting between types
int()
float()
anything
anything
integer
float
str() anything string
Functions named after the type they convert into.

66
Reading numbers into a script
text = input('N? ')␣
number = int(text)
print(number + 1)$
N?
11
python3 input3.py
10

67
Stepping through our script — 1
text = input('N?␣')
number = int(text)
print(number + 1)
str 3 ?N ␣

68
text = input('N?␣')
number = int(text)
print(number + 1)
str 3 ?N ␣
input function
str 2 01 NB: text, not number

69
number = int(text)
print(number + 1)
input function
str 2 01text

70
number = int(text)
print(number + 1)
input function
str 2 01text
int function
int 10

71
number = int(text)
print(number + 1)
input function
str 2 01text
int function
int 10number

72
number = int(text)
print(number + 1)
int 10number

73
number = int(text)
print(number + 1)
int 10number
function
int 11
+
int 1

74
number = int(text)
print(number + 1)
int 10number
int 11
functionprint

75
Progress
Names
Types
Type conversions
Values
Reading in text
str() int() float()
input(prompt)
name = value
strings
integers
floating point numbers

76
Exercise 3
Replace the two XXXX in exercise3.py to do the following:
1. Prompt the user with the text “How much?␣”.
2. Convert the user’s answer to a floating point number.
3. Print 2.5 plus that number.
3 minutes

77
Integers
ℤ{… -2, -1, 0,
1, 2, 3, 4 …}

78
Integer addition & subtraction
>>> 20+5
25
>>> 20␣-␣5
15
Spaces around the
operator don’t matter.
“No surprises”

79
Integer multiplication
There is no “×” on the keyboard. Linux:
,+ShiftAltGr +
Use “*” instead
>>> 20␣*␣5
100
Still no surprises

80
Integer division
There is no “÷” on the keyboard. Linux:
.+ShiftAltGr +
Use “/” instead
>>> 20␣/␣5
4.0 This is a floating point number!
Surprise!

81
Integer division gives floats !
Fractions
>>> 20␣/␣40
0.5
>>> 20␣/␣30
0.6666666666666666
Floats sometimes
Consistency Floats always
!

82
Integer division gives floats !
>>> 20␣//␣4
5
!
// Special operation to stick with integers
>>> 21␣//␣4
5
>>> -21␣//␣4
-6
>>> 20␣/␣4
5.0
>>> 21␣/␣4
5.25
>>> -21␣/␣4
-5.25

83
Integer powers
There is no “42
” on the keyboard.
Use “**” instead
>>> 4␣**␣2
16
>>> 4*␣*2
SyntaxError: invalid syntax
Spaces around the
operator don’t matter.
Spaces in the
operator do!

84
Integer remainders
e.g. Is a number even or odd?
>>> 4␣%␣2
0
>>> 5␣%␣2
1
Use “%”
>>> -5␣%␣2
1 Remainder is always non-negative

85
How big can a Python integer be?
>>> 2**2
4
>>> 4**2
16
>>> 16**2
256
>>> 256**2
65536
>>> 65536**2
4294967296
>>> 256**2
65536
>>> 256**2
65536
>>> 2**2
4

86
>>> 4294967296**2
18446744073709551616
>>> 18446744073709551616**2
340282366920938463463374607431768211456
>>> 340282366920938463463374607431768211456**2
1157920892373161954235709850086879078532699846
65640564039457584007913129639936
>>> 115792089237316195423570985008687907853269
1340780792994259709957402499820584612747936582
0592393377723561443721764030073546976801874298
1669034276900318581864860508537538828119465699
46433649006084096
984665640564039457584007913129639936**2

87
10443888814131525066917527107166243825799642490473837803842334832839
53907971557456848826811934997558340890106714439262837987573438185793
60726323608785136527794595697654370999834036159013438371831442807001
18559462263763188393977127456723346843445866174968079087058037040712
84048740118609114467977783598029006686938976881787785946905630190260
94059957945343282346930302669644305902501597239986771421554169383555
98852914863182379144344967340878118726394964751001890413490084170616
75093668333850551032972088269550769983616369411933015213796825837188
09183365675122131849284636812555022599830041234478486259567449219461
70238065059132456108257318353800876086221028342701976982023131690176
78006675195485079921636419370285375124784014907159135459982790513399
61155179427110683113409058427288427979155484978295432353451706522326
90613949059876930021229633956877828789484406160074129456749198230505
71642377154816321380631045902916136926708342856440730447899971901781
46576347322385026725305989979599609079946920177462481771844986745565
92501783290704731194331655508075682218465717463732968849128195203174
57002440926616910874148385078411929804522981857338977648103126085903
00130241346718972667321649151113160292078173803343609024380470834040
3154190336
There is no limit!
Except for
machine
memory

88
Big integers
2
4
16
256
65536
4294967296
18446744073709551616
3402823669209384634…
63374607431768211456
C / C++
Fortran
Out of the reach
of C or Fortran!
long long
INTEGER*16
long
INTEGER*8
int
INTEGER*4

89
Floating point numbers
1.0
0.33333333
3.14159265
2.71828182
ℝ✗

90
Basic operations
>>> 20.0 + 5.0
25.0
>>> 20.0 - 5.0
15.0
>>> 20.0 * 5.0
100.0
>>> 20.0 / 5.0
4.0
>>> 20.0 ** 5.0
3200000.0
Equivalent to integer arithmetic

91
Floating point imprecision
>>> 1.0 / 3.0
0.3333333333333333
>>> 10.0 / 3.0
3.3333333333333335
≈ 17 significant figures
If you are relying on
this last decimal place,
you are doing it wrong!

92
Hidden imprecision
>>> 0.1
0.1
>>> 0.1 + 0.1
0.2
>>> 0.1 + 0.1 + 0.1
0.30000000000000004
Really: if you are relying on
this last decimal place,
you are doing it wrong!
!

93
How big can a Python float be? ― 1
>>> 65536.0**2
4294967296.0
>>> 4294967296.0**2
1.8446744073709552e+19
So far, so good.
Switch to
“scientific notation”
1.8446744073709552
1.8446744073709552
×1019
e+19

94
Floats are not exact
>>> 4294967296**2
18446744073709551616
>>> 4294967296.0**2
1.8446744073709552e+19
Integer
Floating point
1.8446744073709552×1019
18446744073709552000
18446744073709551616-
384

95
How big can a Python float be? ― 2
>>> 1.8446744073709552e+19**2
3.402823669209385e+38
>>> 3.402823669209385e+38**2
1.157920892373162e+77
>>> 1.157920892373162e+77**2
1.3407807929942597e+154
>>> 1.3407807929942597e+154**2
OverflowError: (34,
'Numerical result out of range')
So far, so good.
Too big!

96
Floating point limits
1.2345678901234567×10N
17 significant figures
-325 < N < 308
4.94065645841×10-324
< N < 8.98846567431×10307
Positive values:

97
z
z2
Complex numbers
>>> (1.25+0.5j)**2
(1.3125+1.25j)
ℂ ℜ
ℑ

98
Progress
Arithmetic
Integers
Complex numbers
+ - * / ** %
No limits!
Limited size
Limited precision

99
Exercise 4
3 minutes
Replace the XXXX in exercise4.py to evaluate
and print out the following calculations:
1. 223 ÷ 71
2. (1 + 1/100)100
3. (1 + 1/10000)10000
4. (1 + 1/1000000)1000000
Note: Brackets work in
Python arithmetic just like they
do in normal mathematics.

100
Comparisons
5 < 10
5 > 10 ✘✘✘
✔
✘✘✘✘✘✘

101
Comparisons
>>> 5 < 10
True
>>> 5 > 10
False
Asking the question
Asking the question
✘✘✘
✔
✘✘✘✘✘✘

102
True & False
>>> type(True)
<class 'bool'> “Booleans”
5 10+
5 10<
int int
15
True
int
bool

103
True & False
bool ✓
bool ✗
True
False
Only two values

104
Six comparisons
Maths
=
≠
<
>
≤
≥
Python
==
!=
<
>
<=
>=
Double equals sign

105
Equality comparison & assignment
name = value
=
value1
== value2
==
Attach a name to a value.
Compare two values

106
Textual comparisons
>>> 'cat' < 'dog'
True
Alphabetic ordering
>>> 'Cat' < 'cat'
True
>>> 'Dog' < 'cat'
True
Uppercase before lowercase
All uppercase before lowercase

107
Ordering text is complicated
German:
Swedish:
z < ö
ö < z
Python inequalities use Unicode character numbers.
This is over-simplistic for “real” use.
Alphabetical order?
“Collation” is a whole field of computing in itself

108
“Syntactic sugar”
0 < number < 10
0 < number
and
number < 10
>>> number = 5
>>> 0 < number < 10
True

109
Converting to booleans
float() Converts to floating point numbers
int() Converts to integers
str() Converts to strings
bool() Converts to booleans
<class 'float'>
<class 'int'>
<class 'str'>
<class 'bool'>

110
Useful conversions
'Fred' True
'' False
Non-empty string
Empty string
1 True
0 False
Non-zero
Zero
12 True
-1 True

111
Boolean operations
Numbers have +, –, * …
What do booleans have?
bool ✓
bool ✗
bool
bool
bool?

112
Boolean operations ― “and”
bool
bool
booland
True Trueand
True Falseand
False Trueand
False Falseand
True
False
False
False
Both have
to be True

113
Boolean operations ― “and”
>>> 4 < 5 and 6 < 7
True
4 < 5
6 < 7
True
True
and True
>>> 4 < 5 and 6 > 7
False
4 < 5
6 > 7
True
False
and False

114
Boolean operations ― “or”
bool
bool
boolor
True Trueor
True False
False True
False False
True
False
True
True
At least
one has
to be Trueor
or
or

115
Boolean operations ― “or”
>>> 4 < 5 or 6 < 7
True
4 < 5
6 < 7
True
True
or True
>>> 4 < 5 or 6 > 7
True
4 < 5
6 > 7
True
False
or True

116
Boolean operations ― “not”
bool boolnot
Truenot
False True
False
not

117
Boolean operations ― “not”
>>> not 6 < 7
False
6 < 7 True not False
>>> not 6 > 7
True
6 > 7 False not True

118
Ambiguity in expressions
3 + 6 / 3
(3 + 6) / 3 3 + (6 / 3)
3 5
✘✘✘ ✔✘✘✘✘✘✘

119
Division before addition
5
3 6 / 3+
3 2+
Division first
Addition second

120
“Order of precedence”
x**y x*yx/yx%y x+yx-y+x-x
x==y x>yx>=yx!=y x<yx<=y
not x x or yx and y
First
Last

121
Progress
Comparisons == != < > <= >=
Booleans True False
Numerical comparison
Alphabetical ordering
5 < 7
'dig' < 'dug'
Boolean operators and or not
Conversions ''
0
0.0
False
False
False
other True

122
Exercise 5
3 minutes
Predict whether these expressions will evaluate to True or False.
Then try them.
'dog' > 'Cat' or 45 % 3 == 0
'sparrow' > 'eagle'
60 - 45 / 5 + 10 == 1
1.
2.
3.

123
Names and values: “assignment”
>>> alpha = 100
1. alpha = 100
2. alpha = 100
int 100
Python creates
an “integer 100”
in memory.
int 100alpha
Python attaches
the name “alpha”
to the value.

124
Assignment: right to left
alpha 100=
“RHS”: right hand side
Evaluated first
“LHS”: left hand side
Processed second

125
Simple evaluations
>>> beta = 100 + 20
RHS1. 100 20+
int 100 function + int 20
int 1202.
int 1203. beta
LHS

126
Changing value — 1
>>> gamma = 1
>>> gamma = 2

127
>>> gamma = 1
int 1
RHS

128
>>> gamma = 1
int 1gamma
LHS

129
>>> gamma = 1
>>> gamma = 2
int 1gamma
int 2
RHS

130
>>> gamma = 1
>>> gamma = 2
int 1gamma
int 2
LHS

131
>>> gamma = 1
>>> gamma = 2
int 1gamma
int 2
✘
garbage collection

132
>>> gamma = 1
>>> gamma = 2
gamma
int 2

133
Changing value — a subtlety
>>> gamma = 1
>>> gamma = 2
gamma
int 2
int 1
✘
Two separate
integer objects.
Python doesn’t change an integer’s value;
Python replaces the integer.
Python integers are “immutable”

134
Names on the RHS — 1
>>> delta = alpha + 40
RHS
1.
alpha 40+
function + int 40int 100
alpha
int 140
2.
3.

135
Names on the RHS — 2
>>> delta = alpha + 40
LHS
4. delta int 140

136
Same name on both sides — 0
delta int 140
Starting
position
>>> print(delta)
140

137
>>> delta = delta + 10
RHS
1. delta 10+
function + int 10int 140
delta
int 150
2.
3.
4.

138
5.
6.
LHS
int 150
int 140delta
RHS
int 150
int 140delta
RHS

139
7.
8.
LHS
int 150delta
int 150
int 140delta
✗ No longer
used.

140
“Syntactic sugar”
thing += 10 thing = thing + 10
thing -= 10 thing = thing - 10
thing *= 10 thing = thing * 10
thing /= 10 thing = thing / 10
thing **= 10 thing = thing ** 10
thing %= 10 thing = thing % 10
is equivalent to

141
Deleting a name ― 1
>>> print(thing)
File "<stdin>", line 1, in <module>
>>> thing = 1
>>> print(thing)
1
NameError: name 'thing' is not defined
Unknown
variable

142
Deleting a name ― 2
>>> print(thing)
>>> del thing
>>> print(thing)
1
NameError: name 'thing' is not defined
Unknown
variable
Known
variable

143
Progress
Assignment
Strictly right to left
+= etc. “syntactic sugar”
thing = thing + 10
thing = thing + 10
2nd 1st
thing += 10
Deletion del thing

144
Our first “real” program
1.414213562373095
$
Number?
python3 sqrt.py
2.0
We have to write sqrt.py
First, the maths.
Then, the Python.

145
Square root of 2.0 by “bisection”
“Interval of uncertainty”
0.0 too small for √2
2.0 too large for √2
0.0 < √2 < 2.0

146
Square root of 2.0 by bisection — 1
Mid-point: 1.0
(0.0 + 2.0) / 2.0
1.0
1.0**2
1.0

147
1.0**2 < 2.0
1.0 < √2 < 2.0
so change lower bound
midpoint √2
22
<

148
Mid-point: 1.5
(1.0 + 2.0) / 2.0
1.5
1.5**2
2.25

149
1.5**2 > 2.0
so change upper bound
1.0 < √2 < 1.5

150
Mid-point: 1.25
(1.0 + 1.5) / 2.0
1.25
1.25**2
1.5625

151
1.25**2 < 2.0
1.25 < √2 < 1.5

152
Mid-point: 1.375
(1.25 + 1.5) / 2.0
1.375
1.375**2
1.890625

153
1.375**2 < 2.0
1.375 < √2 < 1.5

154
Exercise 6
1.375 < √2 < 1.5
One more iteration.
Find the mid-point.
Square it.
Compare the square to 2.0.
Do you change the
lower or upper bound?
2 minutes

155
Understanding before Programming

156
And now using Python!
lower = 0.0
upper = 2.0
middle = (lower+upper)/2

157
And now using Python — 2
middle**2 < 2.0
True
lower = middle
print(lower, upper)
2.01.0

158
print( middle**2)middle,
2.251.5

159
middle**2 < 2.0
False
upper = middle
print(lower, upper)
1.51.0

160
1.56251.25

161
middle**2 < 2.0
True
lower = middle
print(lower, upper)
1.51.25

162
1.8906251.375

163
middle**2 < 2.0
True
lower = middle
print(lower, upper)
1.51.375

164
Looking at the Python code
lower = 0.0
upper = 2.0
print(middle, middle**2)
middle**2 < 2.0
lower = middle upper = middle
?✔ ✘
print(lower, upper)

165
Looking at the Python structures
lower = 0.0
upper = 2.0
print(middle, middle**2)
middle**2 < 2.0
?✔ ✘
print(lower, upper)
Set up
Loop
Choice

166
Looping
Before
Loop test
Loop body
After
✘ ✔
Should the
loop run
(again)?
What is
run each
loop?

167
Loop example: Count from 1 to 10
number = 1
number <= 10
print(number)
number += 1
print('Done!')
✘ ✔
Before
Loop test
Loop body
After
✘ ✔

168
number = 1
number <= 10
print(number)
number += 1
print('Done!')
✘ ✔
number = 1
while
print(number)
number += 1
print('Done!')
:number <= 10
␣␣␣␣
␣␣␣␣

169
Loop test: Count from 1 to 10
number = 1
while
print('Done!')
:number <= 10
“while” keyword
loop test
colon
print(number)
number += 1
␣␣␣␣
␣␣␣␣

170
Loop body: Count from 1 to 10
number = 1
while
print('Done!')
:number <= 10
loop body
indentation
print(number)
number += 1
␣␣␣␣
␣␣␣␣

171
number = 1
while number <= 10 :
print(number)
number += 1
print('Done!')
while1.py
$
1
2
3
4
5
6
7
8
9
10
Done!
python3 while1.py
$

172
Python’s use of indentation
number = 1
while number <= 10 :
p
␣␣␣␣
␣␣␣␣
print(number)
number += 1
Four spaces’ indentation
indicate a “block” of code.
The block forms
the repeated lines.
The first unindented line
marks the end of the block.
rint('Done!')

173
c.f. “legalese”
1
1(a)
1(b)
1(b)(i)
1(b)(ii)
1(b)(iii)
1(c)
2
3

174
Other languages
Shell
C
while ...
do
␣␣␣␣...
␣␣␣␣...
done
while ...
{
␣␣␣␣...
␣␣␣␣...
}
do ... done
{ ... }
Syntax
Syntax
␣␣␣␣...
␣␣␣␣...
Clarity
Clarity

175
Progress
while ... : before
while test :
␣␣␣␣action1
␣␣␣␣action2
␣␣␣␣action3
afterwards
test to keep looping
code blocks
␣␣␣␣indentation

176
Exercise 7
5 minutes
For each script:
Predict what it will do.
Run the script.
Were you right?
while2.py
while3.py
while4.py
while5.py␣␣␣␣
␣␣␣␣
␣␣␣␣
To kill a running script: Ctrl C+
while6.py

177
1.51.375
Back to our square root example
1.0 2.0
1.0
0.0 2.0
2.0
1.0 1.5
0.5
1.25 1.5
0.25
0.125
×½
×½
×½
×½
uncertainty
1.0×10–15
tolerance
What we get
What we want

178
Keep looping while … ?
tolerance>uncertainty
while uncertainty > tolerance :
␣␣␣␣
␣␣␣␣
␣␣␣␣
␣␣␣␣
Do stuff.

179
Square root: the loop
lower = 0.0
upper = 2.0
tolerance = 1.0e-15
uncertainty = upper - lower
middle = (lower + upper)/2
print(lower, upper)
?
Set up
Loop
Choice

180
Choosing
middle**2 < 2.0
?✔ ✘
Choice
middle**2 < 2.0 True Falseor
True
False
lower = middle
upper = middle

181
Simple example
text = input('Number? ')
number = int(text)
if number % 2 == 0:
print('Even number')
else:
print('Odd number')
print('That was fun!')
ifthenelse1.py
$
Number?
Even number
That was fun
python3 ifthenelse1.py
$
Number?
Odd number
That was fun
python3 ifthenelse1.py
8
7

182
if…then… else… ― 1
if
else :
␣␣␣␣print('Even number')
␣␣␣␣upper = middle
:number % 2 == 0
if keyword
Test
Colon

183
if
else :
␣␣␣␣ print('Even number')
:number % 2 == 0
Run if test is True
Indentation

184
if
else :
␣␣␣␣ upper = middle
:number % 2 == 0
else: keyword
Run if test is False
Indentation

185
if
else :
␣␣␣␣ upper = middle
:number % 2 == 0
Run afterwards
regardless of test

186
Our square root example
if
else :
print(lower, upper)
␣␣␣␣lower = middle
Before
:middle**2 < 2.0
After
if… block

187
Progress
if ... : before
if test :
␣␣␣␣action1
␣␣␣␣action2
else:
␣␣␣␣action3
afterwards
else:
choice of two
code blocks
␣␣␣␣indentation

188
Exercise 8
5 minutes
For each script:
Predict what it will do.
Run the script.
Were you right?
ifthenelse2.py
ifthenelse3.py
ifthenelse4.py
␣␣␣␣
␣␣␣␣
␣␣␣␣

189
Back to our example
lower = 0.0
upper = 2.0
tolerance = 1.0e-15
print(lower, upper)
if middle**2 < 2.0 :
else :
upper = middle
lower = middle
Doubly
indented
if starts
indented

190
Levels of indentation
lower = 0.0
upper = 2.0
tolerance = 1.0e-15
print(lower, upper)
else :
upper = middle
lower = middle
␣␣␣␣
␣␣␣␣
␣␣␣␣␣␣␣␣
␣␣␣␣
␣␣␣␣␣␣␣␣
␣␣␣␣
␣␣␣␣
4 spaces
8 spaces

191
Trying it out
tolerance = 1.0e-15
lower = 0.0
upper = 2.0
if middle**2 < 2.0:
lower = middle
else:
upper = middle
print(lower, upper)
sqrt1.py
$
1.0 2.0
1.0 1.5
1.25 1.5
1.375 1.5
1.375 1.4375
1.40625 1.4375
1.40625 1.421875
...
1.414213... 1.414213...
python3 sqrt1.py
☺

192
Script for the square root of 2.0
lower = 0.0
tolerance = 1.0e-15
print(lower, upper)
upper = 2.0
else :
upper = middle
lower = middle
␣␣␣␣
␣␣␣␣
␣␣␣␣␣␣␣␣
␣␣␣␣
␣␣␣␣␣␣␣␣
␣␣␣␣
␣␣␣␣
√2.0
√2.0

193
Input target
number = float(text)
if middle**2 < number :
…

194
Initial bounds?
x > 1.0 1.0 < √x < x
1.0 > x 1.0 > √x > x
lower = ?
upper = ?
if...then...else...

195
Initial bounds
if number < 1.0 :
␣␣␣␣lower = number
␣␣␣␣upper = 1.0
else :
␣␣␣␣lower = 1.0
␣␣␣␣upper = number

196
Generic square root script?
text = input('Number?␣')
if number < 1.0:
␣␣␣␣upper = 1.0
else:
␣␣␣␣lower = 1.0
tolerance = 1.0e-15
while uncertainty > tolerance:
␣␣␣␣middle = (lower+upper)/2.0
␣␣␣␣if middle**2 < number:
␣␣␣␣␣␣␣␣lower = middle
␣␣␣␣else:
␣␣␣␣␣␣␣␣upper = middle
␣␣␣␣uncertainty = upper - lower
print(lower, upper)
User input
Initialization
Processing
Output sqrt2.py

197
exit()␣␣␣␣
Negative numbers?
Need to catch negative numbers
if number < 0.0:
␣␣␣␣print('Number must be positive!')
Quit immediately
"else" is optional

198
“Chained” tests
if number < 0.0:
␣␣␣␣exit()
if number < 1.0:
␣␣␣␣upper = 1.0
else:
␣␣␣␣lower = 1.0
...
User input
Input validation
Initialization

199
elif
“Chained” tests ― syntactic sugar
if number < 0.0:
␣␣␣␣exit()
number < 1.0:
␣␣␣␣upper = 1.0
else:
␣␣␣␣lower = 1.0
...
elif: “else if”
sqrt3.py

200
Without elif…
if number < 0.0:
␣␣␣␣print('Number is negative.')
else:
␣␣␣␣if number < 1.0:
␣␣␣␣␣␣␣␣print('Number is between zero and one.')
␣␣␣␣else:
␣␣␣␣␣␣␣␣if number < 2.0:
␣␣␣␣␣␣␣␣␣␣␣␣print('Number is between one and two.')
␣␣␣␣␣␣␣␣else:
␣␣␣␣␣␣␣␣␣␣␣␣if number < 3.0:
␣␣␣␣␣␣␣␣␣␣␣␣␣␣␣␣print('Number is between two and three.')
␣␣␣␣␣␣␣␣␣␣␣␣else:
␣␣␣␣␣␣␣␣␣␣␣␣␣␣␣␣print('Number is three or more.')
Stacked clauses get unwieldy

201
With elif…
if number < 0.0:
␣␣␣␣print('Number is negative.')
elif number < 1.0:
␣␣␣␣print('Number is between zero and one.')
elif number < 2.0:
␣␣␣␣print('Number is between one and two.')
elif number < 3.0:
␣␣␣␣print('Number is between two and three.')
else:
␣␣␣␣print('Number is three or more.')

202
Progress
Nested structures
if … :
…
elif … :
…
elif … :
…
else:
…
while … :
if … :
Chained tests
Testing inputs to scripts
exit()

203
Exercise 9
10 minutes
exercise9.py
1. Edit the square root script to catch negative numbers.
Only attempt each part after
you have the previous part working!
2. Edit the square root script to ask for the tolerance.
3. Edit the square root script to catch negative tolerances.

204
Comments
We have written our first real Python script
What did it do?
Why did it do it?
Need to annotate the script

205
Python comment character
The “hash” character
Lines starting with “#” are ignored
Partial lines starting “#” are ignored
Used for annotating scripts
#
a.k.a. “pound”, “number”, “sharp”

206
Python commenting example
# Script to calculate square roots by bisection
# (c) Bob Dowling 2012. Licensed under GPL v3.0
number = float(text) # Need a real number
# Test number for validity,
# set initial bounds if OK.
if number < 0.0:
␣␣␣␣print('Number must be non-negative!')
␣␣␣␣exit()
elif number < 1.0:
␣␣␣␣upper = 1.0
else:
␣␣␣␣lower = 1.0

207
On a real Unix system…
#!/usr/bin/python3
# Script to calculate square roots by bisection
# (c) Bob Dowling 2012. Licensed under GPL v3.0
number = float(text) # Need a real number
Magic line for executable files
$
instead of
fubar.py
$ python3 fubar.py

208
Progress
Comments
#“#” character

209
Exercise 10
2 minutes
Comment your square root script from exercise 9.

210
Recap: Python types so far
Whole numbers
Text
Booleans
-127
3.141592653589793
'The cat sat on the mat.'
True False
Complex numbers (1.0 + 2.0j)

211
Lists
[ 'hydrogen', 'helium', 'lithium', 'beryllium',
'boron', …, 'thorium', 'protactinium', 'uranium' ]
[ -3.141592653589793, -1.5707963267948966,
0.0, 1.5707963267948966, 3.141592653589793 ]
[ 2, 3, 5, 7, 11, 13, 17, 19 ]

212
What is a list?
A sequence of values
Individual value identified
by position in the sequence
The names of the elements
“helium” is the name of the
second element
Values stored in order Atomic number order
hydrogen, helium, lithium, beryllium, …, protactinium, uranium

213
What is a list?
A sequence of values
Individual value identified
by position in the sequence
The prime numbers
less than sixty
7 is the fourth prime
Values stored in order Numerical order
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59

214
Creating a list in Python
>>> primes = [ 2, 3, 5, 7, 11, 13, 17, 19]
>>> primes
[2, 3, 5, 7, 11, 13, 17, 19]
>>> type(primes)
<class 'list'>
The whole list
A Python type
A literal list

215
How Python presents lists
[ ]1917,13,11,7,5,3 ,,2
Square brackets
at the ends
Commas between items

216
Square brackets
primes = [2, 3, 5, 7, 11] Literal list

217
Python counts from zero
[ ]1917,13,11,7,5,3 ,,2
76543210“index”
“value”

218
Looking things up in a list
>>> primes = [ 2, 3, 5, 7, 11, 13, 17, 19]
[ ]1917,13,11,7,5,3 ,,2
76543210
>>> primes
2
>>>
17
]0[
index
primes ]6[
square brackets

219
Square brackets
primes = [2, 3, 5, 7, 11] Literal list
primes[3] Index into list

220
Counting from the end
>>> primes = [ 2, 3, 5, 7, 11, 13, 17, 19]
[ ]1917,13,11,7,5,3 ,,2
76543210
>>>
19
primes ]-1[
getting at the last item
-1-2-3-4-5-6-7-8

221
Inside view of a list
list
8
int 19
…
primes
int 17
int 3
int 2 primes[0]
primes[1]
primes[6]
primes[7]

222
Length of a list
primes
list
8
>>> len
8
(primes)
0
7
Maximum
index is 7
len() function:
length of list

223
Changing a value in a list
>>> data = ['alpha', 'beta', 'gamma']
>>> data[2]
'gamma'
>>> data[2] = 'G'
>>> data[2]
'G'
>>> data
['alpha', 'beta', 'G']
The list
Initial value
Change value
Check change
Changed list

224
Changing a value in a list ― 1
list
3 str 5 a l p h a
str 4 b e t a
str 5 g a m m a
Right to left

225
data
list
3 str 5 a l p h a
str 4 b e t a
str 5 g a m m a
Right to left

226
>>> data[2] = 'G'
Right to left
data
list
3 str 5 a l p h a
str 4 b e t a
str 5 g a m m a
str 1 G New value

227
>>> data[2] = 'G'
Right to left
data
list
3 str 5 a l p h a
str 4 b e t a
str 5 g a m m a
str 1 G
No longer
referenced

228
>>> data[2] = 'G'
Right to left
data
list
3 str 5 a l p h a
str 4 b e t a
str 1 G

229
Removing an entry from a list ― 1
>>> del data[1]
data
list
3 str 5 a l p h a
str 4 b e t a
str 5 g a m m a
data[0]
data[1]
data[2]
✗

230
>>> del data[1]
data
list
2 str 5 a l p h a
str 4 b e t a
str 5 g a m m a
data[0]
data[1]
No longer
referenced

231
>>> del data[1]
data
list
2 str 5 a l p h a
str 5 g a m m a
data[0]
data[1]

232
Running off the end
list
8
int 19
…
primes
int 17
int 3
int 2
primes[8]

233
Running off the end
>>> len(primes)
8
>>> primes[7]
19
>>> primes[8]
IndexError: list index out of range
Type of error Description of error

234
Running off the end
>>> primes[8] = 23
IndexError: list assignment index out of range
Same type
of error
Similar description of error
but with “assignment”

235
Progress
Lists
index
Count from zero
Length
Over-running
[2, 3, 5, 7, 11, 13, 17, 19]
primes[4]
primes[0]
len(primes)
primes[8]
Deletion del primes[6]

236
Exercise 11
5 minutes
Track what the value of numbers is at each stage of this
sequence of instructions.
numbers = [5, 7, 11, 13, 17, 19, 29, 31]>>>
numbers[1] = 3>>>
del numbers[3]>>>
numbers[3] = 37>>>
numbers[4] = numbers[5]>>>
1
2
3
4
5

237
How can we add to a list?
list
8
int 19
…
int 17
int 2
list
9
int 19
…
int 17
int 2
int 23
?
Same list
Extra item
New length

238
Appending to a list
>>> primes
[2, 3, 5, 7, 11, 13, 17, 19]
>>> primes
>>> primes
[2, 3, 5, 7, 11, 13, 17, 19,
(23)append.
The list is now updated
]23
A function built into a list

239
primes.append() ?
>>> primes (23)append.
The list
A connecting dot
append()
The value to append
All lists have this
function built in.

240
“Methods”
(arguments)object function.
a function that has
special access to
the object’s data.
Behaves just
like a function

241
Using the append() method
>>> print(primes)
[2, 3, 5, 7, 11, 13, 17, 19]
>>> print(primes)
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37]
The function doesn’t
return any value.
It modifies
the list itself.

242
Other methods on lists: reverse()
>>> numbers = [4, 7, 5, 1]
>>> numbers.reverse()
>>> print(numbers)
[1, 5, 7, 4]
The function doesn’t
return any value.
It modifies
the list itself.

243
Other methods on lists: sort()
>>> numbers = [4, 7, 5, 1]
>>> numbers.sort()
>>> print(numbers)
[1, 4, 5, 7]
Numerical order.
The function does not
return the sorted list.
It sorts the
list itself.

244
Other methods on lists: sort()
>>> greek = ['alpha', 'beta', 'gamma', 'delta']
>>> greek.sort()
>>> print(greek)
['alpha', 'beta', 'delta', 'gamma']
Alphabetical order
of the words.

245
Other methods on lists: insert()
>>> greek = 'gamma',['alpha',
0 1 2
>>> greek.insert(
'delta']
Where to insert What to insert
'beta'1, )
>>> greek
['alpha', 'gamma', 'delta']'beta',
1
Displaced
0

246
Other methods on lists: remove()
>>> numbers = [7, 4,
>>> numbers.remove
>>> print(numbers)
[7, 4,
7, 2, 5, 4]8,
(8)
7, 2, 5, 4]
c.f. del numbers[2]
Value to remove
Index to remove

247
Other methods on lists: remove()
>>>
>>> numbers.remove
>>> print(numbers)
[7,
(4)
print(numbers)
[7, 7, 2, 5,4, 4]
7, 2, 5, 4]
Only the first instance is removed
There are two instances of 4.

248
What methods are there?
>>> help(numbers)
Help on list object:
class list(object)
...
| append(...)
| L.append(object) -- append object to end
...
Pagination: ␣
B
next page
back one page
Q quit

249
Help on a single method
>>> help(numbers.append)
Help on built-in function append:
append(...)
L.append(object) -- append object to end

250
Sorting a list redux
>>> greek.sort()
>>> print(greek)
Recall: greek.sort()
sorts the list “in place”.

251
Sorting a list redux: “sorted()”
>>> print(sorted(greek))
>>> print(greek)
['alpha', 'beta', 'gamma', 'delta']
sorted() function
returns a sorted list…
…and leaves the
list alone

252
Adding to a list redux: “+”
>>> primes
[2, 3, 5, 7, 11, 13, 17, 19]
>>> primes
[2, 3, 5, 7, 11, 13, 17, 19,
[23, 29, 31]+
23, 29, 31]
Concatenation
operator
List to add

253
Concatenation
>>> newlist
>>>
>>> primes
= primes + [23, 29, 31]
primes = primes + [23, 29, 31]
Create a new list
Update the list
[23, 29, 31]+=
Augmented assignment

254
Creating lists from text ― 1
>>> list('Hello')
['H', 'e', 'l', 'l', 'o']
str 5 H e l l o
list
5
str 1 H
str 1 e
str 1 l
str 1 l
str 1 o

255
Creating lists from text ― 2
>>> 'Hello, world!'
['Hello,', 'world!']
list
2
H e l l o , ␣ w o r l d !13str
str 6 H e l l o ,
str 6 w o r l d !
.split()
Built in method
Splits on spaces

256
Progress
“Methods”
append(item)
reverse()
sort()
insert(index,item)
remove(item)
Concatenation
+
+=
object.method(arguments)
[1,2,3] + [4,5,6]
primes += [29, 31]
Sorting list.sort()
sorted(list)
Help help(object)
help(object.method)

258
Is an item in a list? ― 1
>>> odds = [3, 5, 7, 9] Does not include 2
>>> odds.remove(2)
ValueError:
Hard error
list.remove(x): x not in list
x must be in the
list before it can
be removed
Try to remove 2
✗

259
Is an item in a list? ― 2
>>> odds = [3, 5, 7, 9]
>>> 2 in odds
False
>>> 3 in odds
True
>>> 2 not in odds
True

260
Precedence
x**y x*yx/yx%y x+yx-y+x-x
x==y x>yx>=yx!=y x<yx<=y
not x x or yx and y
First
Last
x not in y x in y
The list now contains
every operator we
meet in this course.

261
Safe removal
if number in numbers :
numbers.remove(number)
while number in numbers :
numbers.remove(number)
What’s the
difference?

262
Working through a list ― 1
e.g. Printing each element on a line
['The', 'cat', 'sat', 'on', 'the', 'mat.']
The
cat
sat
on
the
mat.

263
e.g. Adding the elements of a list
[45, 76, -23, 90, 15]
203
What is the sum of an empty list? [] ?

264
e.g. Squaring every number in a list
[4, 7, -2, 9, 1]
[16, 49, 4, 81, 1]

265
The “for loop” ― 1
words ['The', 'cat', 'sat', 'on', 'the', 'mat.']=
for
␣␣␣␣
:wordsinword
print(word)
name of list list
A new Python
looping construct
print: What we want
to do with the list items.

266
for
␣␣␣␣
:wordsinword
print(word)
keywords
colon followed by
an indented block

267
word)
for :wordsinword
print(
Defining the loop variable
Using the loop variable
␣␣␣␣

268
Running the “for loop”
list
6
str 3 T h e
str 3 c a t
str 3 s a t
str 2 o n
str 3 t h e
str 4 m a t .
words
word
for :wordsinword
word)print(␣␣␣␣
First loop

269
list
6
str 3 T h e
str 3 c a t
str 3 s a t
str 2 o n
str 3 t h e
str 4 m a t .
words
word
for :wordsinword
Second loop

270
list
6
str 3 T h e
str 3 c a t
str 3 s a t
str 2 o n
str 3 t h e
str 4 m a t .
words
word
for :wordsinword
Third loop

271
list
6
str 3 T h e
str 3 c a t
str 3 s a t
str 2 o n
str 3 t h e
str 4 m a t .
words
word
for :wordsinword
Fourth loop

272
list
6
str 3 T h e
str 3 c a t
str 3 s a t
str 2 o n
str 3 t h e
str 4 m a t .
words
word
for :wordsinword
Fifth loop

273
list
6
str 3 T h e
str 3 c a t
str 3 s a t
str 2 o n
str 3 t h e
str 4 m a t .
words
word
for :wordsinword
Final loop

274
The “for loop” for printing
word)
for :wordsinword
print(␣␣␣␣
for1.py

275
The “for loop” for adding
numbers = [45, 76, -23, 90, 15]
for :in
total +=
total = 0
number numbers
print(total)
number
Set up before the loop
Processing in the loop
Results after the loop
␣␣␣␣
for2.py

276
The “for loop” for creating a new list
numbers = [4, 7, -2, 9, 1]
for :in
squares.append(
squares = [ ]
number numbers
print(squares)
Set up before the loop
Processing in the loop
Results after the loop
number**2)␣␣␣␣
for3.py

277
The loop variable persists!
numbers = [4, 7, -2, 9, 1]
for :in
squares.append(
squares = [ ]
number numbers
print(
)number
Loop variable only
meant for use in loop!
But it persists!

278
“for loop hygeine”
numbers = [4, 7, -2, 9, 1]
for :in
squares.append(
squares = [ ]
number numbers
del
number Delete it after use

279
Progress
Testing items in lists
for loops
3 in [1,2,3,4]
total = 0
for number in [1,2,3,4]:
total += number
del number
True
loop variables for number in [1,2,3,4]:
total += number
del number

280
Exercise 13
5 minutes
What does this print?
numbers = [0, 1, 2, 3, 4, 5]
total = 0
total_so_far = []
for number in numbers:
total += number
total_so_far.append(total)
print(total_so_far)

281
“Sort-of-lists”
Python “magic”:
Treat it like a list and it will
behave like a useful list
What can “it” be?

282
Strings as lists
Recall:
list('Hello') ['H', 'e', 'l', 'l', 'o']
for letter in H
e
l
l
o
print(letter)
:'Hello'
Gets turned
into a list. for4.py

283
Creating lists of numbers
Built in to Python:
range(start,limit)
for number in
print(number)
:range(3,8) 3
4
5
6
7
8 not included

284
ranges of numbers
Not actually lists: >>>
range(0,5)
range(0,5)
But close enough: >>>
[0, 1, 2, 3, 4]
list(range(0,5))
Treat it like a list and
it will behave like a list

285
Why not just a list?
Most common use: for number in
…
range(0, 10000):
Inefficient to make a
huge list just for this
“iterator” : anything that can be treated like a list
list(iterator) Explicit list

286
Ranges of numbers again
range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
range(3, 10) [3, 4, 5, 6, 7, 8, 9]
range(3, 10, 2) [3, 5, 7, 9]
via list()
Start at 0
Every nth
number
range(10, 3, -2) [10, 8, 6, 4] Negative steps

287
Indices of lists
>>> primes = [ 2, 3, 5, 7, 11, 13, 17, 19]
[ ]1917,13,11,7,5,3 ,,2
76543210
>>> len(primes)
>>> list(range(
8
[0, 1, 2, 3, 4, 5, 6, 7]
8))
valid indices

288
Direct value or via the index?
primes = [2, 3, 5, 7, 11, 13, 17, 19]
for prime in primes:
print(prime)
for index in range(len(primes)):
print(primes[index])
Equivalent
Simpler

289
Working with two lists: “dot product”
list1 = [ 0.4]0.0,0.3,
list2 = [ 0.6]0.5,0.2,
×××
0.240.00.06 + + 0.3
∙

290
Working with two lists: indices
list1 = [0.3, 0.0, 0.4]
list2 = [0.2, 0.5, 0.6]
for index in
sum = 0.0
print(sum)
0 1 2
indices
:range(len(list1))
sum += list1[index]*list2[index]
Dealing with
values from
both lists at
the same time.

291
iter
4
2
A little more about iterators ― 1
str 5 a l p h a
str 4 b e t a
str 5 g a m m a
str 5 d e l t a
>>> greek_i = iter(greek)
>>> next(greek_i)
'alpha'
>>> next(greek_i)
'beta'
Offset

292
iter
4
✗
A little more about iterators ― 2
str 5 a l p h a
str 4 b e t a
str 5 g a m m a
str 5 d e l t a
>>> next(greek_i)
'gamma'
>>> next(greek_i)
'delta'
>>> next(greek_i)
✗StopIteration

293
Progress
Non-lists as lists
“Iterators”
range()
Indices of lists
Parallel lists
range(limit)
range(start,limit)
range(start,limit,step)
range(3,7) [3,4,5,6]
for letter in 'Hello':
...
for index in range(len(things)):
greek_i = iter(greek)
next(greek_i)

294
Exercise 14
5 minutes
list1 = [ 0.4]0.0,0.3,
list2 = [ 0.6]0.5,0.2,
-0.2-0.50.1
Complete exercise14.py
0.040.250.01 0.3++
Difference
Square
Add

295
List “slices”
primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]>>>
primes>>>
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
primes[3]>>>
7
primes[3:9]>>>
[7, 11, 13, 17, 19, 23]
The list
An item
Part of the list

296
Slices ― 1
primes = [2, 3, 5, 7, 11, 13, 17, 19, 29, 31]
3 9
[7,primes[3:9]
Item 9 not
included
23,
11, 13, 17, 19, 23]
primes[ ]9:3
to
from
Up to but not
including…

297
Slices ― 2
primes 7, 11, 13, 17, 19, 29, 31]23,
primes[ ]9:3 [7, 11, 13, 17, 19, 23]
[2, 3, 5,
primes[ ]9: [2, 3, 5, 7, 11, 13, 17, 19, 23]
primes[ ]:3 [7, 11, 13, 17, 19, 23, 29, 31]
primes[ ]: [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31]

298
Slices ― 3
primes 7, 11, 13, 17, 19, 29, 31]23,
primes[ ]9:3 [7, 11, 13, 17, 19, 23]
[2, 3, 5,
primes[ :9:3 [7, 13, 19 ]]2
primes[ :9:3 ]3 [7, 17 ]

299
list
3
alphabet
letters
Copies and slices ― 1
letters = ['a','b','c']>>>
str 1 a
str 1 b
str 1 c
alphabet = letters>>>

300
list
3
alphabet
letters
letters[0] = 'A'>>>
str 1 A
str 1 b
str 1 c
print(alphabet)>>>
['A', 'b', 'c']

301
letters alphabet
list
3
letters = ['a','b','c']>>>
str 1 a
str 1 b
str 1 c
alphabet = letters[:]>>>
list
3
Slices are
copies.

302
letters[0] = 'A'>>>
print(alphabet)>>>
Slices are
copies.
['a', 'b', 'c']
letters
alphabet
list
3
str 1 a
str 1 b
str 1 c
list
3
str 1 A

303
Progress
Slices
items[from:to]
items[:to]
items[from:]
items[:]
items[from:to:stride]
items[:to:stride]
items[from::stride]
items[::stride]
End-limit excluded
Slices are copies

304
Exercise 15
3 minutes
Predict what this Python will do.
Then run it.
Were you right?
exercise15.py
foo = [4, 6, 2, 7, 3, 1, 9, 4, 2, 7, 4, 6, 0, 2]
bar = foo[3:12:3]
bar[2] += foo[4]
foo[0] = bar[1]
print(bar)

305
list
5
Diversion: Lists and strings
Text: “a string of characters”
list()
str 1 H
str 1 e
str 1 l
str 1 l
str 1 o
str 5 H e l l o

306
Indexing into strings
'Hello, world!'[0]>>>
'H'
'Hello, world!'[:4]>>>
'Hell'
Simple indexing
Slicing

307
Strings are immutable
'Hello, world!'[0] = 'C'>>>
TypeError: 'str' object does not support
item assignment

308
Files
.txt
.dat
.csv
Reading Writing
Input Output

309
Reading a text file
File name
File object
File contents
'treasure.txt'
book
'TREASURE ISLAND'
string
file
string
“opening” the file
reading from the file
Finished with the file
“closing” the file

310
Opening a text file
>>> book )'r','treasure.txt'(open=
Read-only [text]
File name
Python function
Python file object
“mode”
book )'treasure.txt'(open=
Read-only is
the default

311
Reading from a file object
>>> line1 book=
File object
First line of file
>>> line1
' n'TREASURE ISLAND
Includes the
“end of line”
next( )
“next line, please”

312
File object are iterable
>>> line2 next(book)=
Second line of file
>>> line2
'n'
>>> line3 next(book)=
>>> line3
'PART ONEn'
A blank line
Third line of file

313
Closing the file
>>> .close()book
Method built in to file object
Frees the file for other
programs to write to it.

314
The file object ― 1
>>> book
<_io.TextIOWrapper
encoding='UTF-8'>
name='treasure.txt'
Text Input/Output
File name
Character encoding:
how to represent
letters as numbers.

315
The file object ― 2
file 29 treasure.txt UTF-8
TREASURE ISLAND↵
↵
PART ONE↵
↵
The Old Buccaneer↵
↵
☞
Pointer to the file
on the file system
“offset”: how far into
the file we have read
File name
Text encoding

316
Reading through a file
Treat it like a list and
it will behave like a list
list(file_object) List of lines in the file
>>> book = open('treasure.txt', 'r')
>>> lines = list(book)
>>> print(lines)

317
Reading a file moves the offset
!>>> book = open('treasure.txt', 'r')
>>> lines_a = list(book)
>>> print(lines_a)
>>> lines_b = list(book)
>>> print(lines_b)
[] Empty list
Huge output…

318
Reading a file moves the offset
>>> book
>>> lines_a =
>>> print(lines_a)
>>>
>>> print(lines_b)
[]
…
File object starts with offset at start.
= open('treasure.txt', 'r')
Operation reads entire file from offset.
list(book)
lines_b = list(book)
Offset changed to end of file.
Operation reads entire file from offset.
So there's nothing left to read.

319
Resetting the offset
>>> book
>>> lines_a =
>>> print(lines_a)
>>>
>>>
…
= open('treasure.txt', 'r')
list(book)
lines_b = list(book)
>>> print(lines_b)
book.seek(0) Set the offset explicitly

320
Typical way to process a file
book = open('treasure.txt', 'r')
for line in book :
…
Treat it like a list…

321
Example: lines in a file
n_lines = 0
for line in book :
n_lines += 1
print(n_lines)
Line count
Read each line of the file
Increment the count
Print out the count
book.close()

322
Example: characters in a file
n_chars = 0
for line in book :
n_chars
print(n_chars)
len(line)
Number of characters on the line
Increase the count by that many
+=
book.close()

323
Progress
Opening file to read
Reading file
Closing file
File offset
book = open(filename, 'r')
book.close()
book.seek(0)
for line in book:
...

324
Exercise 16
5 minutes
Complete a script to count
lines, words and characters
of a file.
exercise16.py

325
Writing files
File name
Data to write
File object
'treasure.txt'
book
'TREASURE ISLAND'
string
file
string
“opening” the file
writing to the file
Finished with the file
“closing” the file

326
Opening a text file for writing
>>> output )'w','output.txt'(open=
Open for
writing [text]
!
This will truncate
an existing file

327
Opening a text file for appending
>>> output )'a','output.txt'(open=
Open for
appending [text]

328
Writing to a file object ― 1
>>> .writeoutput
Method built in
to file object
File object
(line1)
Data being written6
Number of
characters
actually written
>>> len(line1)
6

329
Writing to a file object ― 2
>>> .writeoutput ('alpha Typical use: whole line.
Includes “end of line”
n')
>>> .writeoutput ('be')
>>> .writeoutput ('tan')
Doesn’t have
to be whole lines
>>> .writeoutput ('gammandeltan')
Can be multiple lines
6
2
3
12

330
Closing the file object
>>> .close()output Vital for written files

331
Importance of closing
Data “flushed” to disc on closure.
Python script File object File system
write “flush”
!

332
Importance of closing promptly !
Files locked for other access
open ['w'] open ['r']
✗
(More a problem for Windows than Unix)

333
Writing non-text values
>>> output.write('Boo!n') Writing text (str)
5
>>> output.write(42) Writing non-text (int)
TypeError: must be str, not int
write() only accepts text
✗

334
Writing non-text values
>>> output.write( Convert to text: str()
2
>>> output.write('n') Explicit end-of-line
1
str(42))
Text formatting (later in the course)
provides a more elegant solution.

335
Progress
Opening files for writing
Explicit ends of lines
book = open(filename, 'w')
book.write('n')
Writing data book.write(str(data))
Writing text book.write(text)
Closing the file is important book.close()

336
Exercise 17
5 minutes
The script exercise17.py prints
a series of numbers ending in 1.
Change the script to write to a file called
output.txt instead of printing to the console.

338
Functions we have met
print(line)
open(filename, mode)
range(from, to, stride)
float(thing)
int(thing)
iter(list)
str(thing)
len(thing)
type(thing)
input(prompt)
Not that many! “The Python Way”:
If it is appropriate to an object,
make it a method of that object.
bool(thing)
ord(char)
chr(number)
list(thing)

339
Why write our own functions?
… read
… test
… fix
… improve
… add to
… write
Easier to …
“Structured
programming”
… develop

340
Defining a function
(y1
, y2
, y3
) (x1
, x2
, x3
, x4
, x5
)= f
Identify the inputs
Identify the processing
Identify the outputs

341
A function to define: total()
Sum a list
[1, 2, 3]
[7, -4, 1, 6, 0]
[ ]
6
10
0 “Edge case”

342
Defining a Python function ― 1
def :( … )total colon
inputs
name of function
define a function called …

343
def total( ):numbers
name for the input
This name is
internal to
the function.

344
def total(numbers): Colon followed by indentation
⇨

345
def total(numbers):
sum_so_far = 0
sum_so_far += number “Body” of function

346
def total( ):numbers
sum_so_far
for
sum_so_far += number
= 0
in numbers:number
These variables exist only
within the function’s body.

347
def total(numbers):
sum_so_far = 0
return sum_so_far This value
is returned
return this value

348
def total(numbers):
sum_so_far = 0
return sum_so_far
And that’s it!
Unindented
after this

349
And that’s it!
All internal names cleaned up No need for del
All internal names internal No need to avoid reusing names

350
Using a Python function ― 1
def total(numbers):
sum_so_far = 0
return sum_so_far
print(total([1, 2, 3]))
The list we
want to add up

351
def total(numbers):
sum_so_far = 0
return sum_so_far
The function we
have just written

352
def total(numbers):
sum_so_far = 0
return sum_so_far
Printing out
the answer

353
def total(numbers):
sum_so_far = 0
return sum_so_far
total1.py
$ python3 total1.py
6
nb: Unix prompt

354
def total(numbers):
sum_so_far = 0
return sum_so_far
print(total([7,-4,1,6,0]))
print(total([]))
total2.py
$ python3 total2.py
6
10
0
Use the function
multiple times

355
Functions’ private names ― 1
def total(numbers):
sum_so_far = 0
return sum_so_far
data = [1, 2, 3]
data_sum = total(data)
print(data_sum)
Function definition
Main script

356
def total(numbers):
...
total
main script function
“namespace”

357
data = [1, 2, 3]
list 3
int 1
int 2
int 3
total
data
main script function

358
... = total(data)
total
data
main script
list 3
int 1
int 2
int 3
def total(numbers):
total
numbers
function

359
... = total(data)
total
function
data
main script
list 3
int 1
int 2
int 3
sum_so_far = 0
total
numbers
sum_so_far
int 0

360
... = total(data)
total
function
data
main script
list 3
int 1
int 2
int 3
return sum_so_far
total
numbers
sum_so_far
int 6

361
total
function
data
main script
list 3
int 1
int 2
int 3
return sum_so_far
total
numbers
sum_so_far
int 6
data_sum

362
total
function
data
main script
list 3
int 1
int 2
int 3
int 6
data_sum

363
Progress
Functions
“Structured programming”
Defining a function
Returning a value
def function(input):
...
return output
Private name spaces

364
Exercise 18
5 minutes
Edit the script exercise18.py.
It currently defines and uses a function total() which adds
the elements of a list.
Change it to create a function product() which multiplies them.
Three examples are included in the script.
Running it should produce the following output:
$ python3 exercise18.py
6
0
1

365
Reminder about indices
def total(numbers):
sum_so_far = 0
return sum_so_far
def total(numbers):
sum_so_far = 0
for index in range(len(numbers)):
sum_so_far += numbers[index]
return sum_so_far
Equivalent
total3.py
total2.py

366
Want a function to add two lists of the same length term-by-term:
Example of multiple inputs
[1, 2, 3] [5, 7, 4] [6, 9, 7]&
[10, -5] [15, 14] [25, 9]&
[11, 11, -2, 2, 7][3, 7, 4, 1, 7] [8, 4, -6, 1, 0]&
Two inputs

367
Functions with multiple inputs
def add_lists(
sum_list = []
for
sum
sum_list.append(sum)
return sum_list
):b_list,a_list
:range(len(a_list))inindex
b_list[index]+a_list[index]=
Multiple inputs are
separated by commas

368
Functions with multiple inputs
def add_lists(
sum_list = []
for
sum
sum_list.append(sum)
return sum_list
):b_list,a_list
:range(len(a_list))inindex
b_list[index]+a_list[index]=
We have
two lists…
…so we have
to use indexing

369
Multiple outputs
Write a function to find minimum and maximum value in a list
[1, 2, 3]
[10, -5]
[3, 7, 4, 1, 7]
Two outputs
1 &
-5 &
1 &
3
10
7

370
Finding just the minimum
def min_list(a_list):
min_so_far = a_list[0]
for
if a < min_so_far:
return
:a_listina
min_so_far = a
min_so_far Returning a single value
List cannot be empty!
minlist.py

371
Finding just the maximum
def max_list(a_list):
max_so_far = a_list[0]
for
if a > max_so_far:
return
:a_listina
max_so_far = a
max_so_far Returning a single value
Only real change
maxlist.py

372
Finding both
def minmax_list(a_list):
for
if a > max_so_far:
return what?
:a_listina
max_so_far = a
if a < min_so_far:
min_so_far = a
This is the real question

373
Returning both
def minmax_list(a_list):
for
if a > max_so_far:
return
:a_listina
max_so_far = a
if a < min_so_far:
min_so_far = a
min_so_far, max_so_far A pair of values
minmaxlist.py

374
“Tuples”
e.g. min_value max_value,
min_value avg_value, max_value,
Pair
Triple
Often written with parentheses:
min_value max_value,
min_value avg_value, max_value,
(
( )
)
Commas

375
Using tuples to return values
min_value max_value,return
minimum
In the function definitiondef …
Using the function
= minmax_list(values)maximum,

376
Using tuples to attach names
alpha =beta, 56,12
alpha =
beta 56
12
=
( ) ( )

377
Swapping values
>>> alpha = 12
>>> beta = 56
>>> (alpha, beta) = (beta, alpha)
>>> print(alpha)
56
>>> print(beta)
12
Swapping values

378
Assignment works right to left
alpha = 12
beta = 56
(alpha, beta) = (beta, alpha)
Stage 1: (beta, alpha) (56, 12)
Stage 2: (alpha, 12)(56,=beta)

379
Progress
Multiple inputs
Multiple outputs
“Tuples”
Simultaneous assignment
def thing(in1
, in2
, in3
):
return (out1
, out2
, out3
)
(a, b, c)
(a, b) = (a+b, a-b)

380
Exercise 19
10 minutes
The script exercise19.py is an answer to exercise 16.
Edit it to:
1. define a function file_stats() that takes a file
name and returns a triple (n_lines, n_words, n_chars)
2. use input() to read in a file name
3. use file_stats() with that file name
4. end with print(filename, file_stats(filename))

381
Tuples and lists: similarities
>>> >>>x = ['alpha','beta'] y = ('alpha','beta')
>>> >>>x[1] y[1]
'beta'
'beta'
>>> >>>len(x) len(y)
2
2
>>> (a, b) = (1, 2)
>>> a
1
>>> [c, d] = [1, 2]
>>> c
1
Indexing
Length
Simultaneous
asignment

382
Tuples and lists: differences
>>> >>>x = ['alpha','beta'] y = ('alpha','beta')
>>> >>>x[1] = 'B' y[1] = 'B'
>>> x
['alpha','B']
TypeError:
'tuple' object does not
support item assignment
Lists are
mutable
Tuples are
immutable

383
Tuples and lists: philosophy
Sequential:
Concept of “next item”
Lists Tuples
Simultaneous:
All items “at once”
Best with all items
of the same type
Safe to have
multiple types
[2, 3, 5, 7, 11, 13,
17, 19, 23, 29, 31]
('Dowling', 'Bob',
50, 105.0, 'rjd4')
Sequence of prime numbers Surname, forename,
age, weight, user id
Serial Parallel

384
Functions we have written so far
total(list)
add_lists(list1
,list2
)
minmax_list(list)

385
Reusing functions within a script
def square(limit):
...
...
squares_a = square(34)
...
five_squares = squares(5)
...
squares_b = squares(56)
...
One definition
Multiple uses in
the same file
Easy!

386
...
squares_a = squares(34)
...
...
five_squares = squares(5)
...
Reusing functions between scripts?
def squares(limit):
... One definition
How?
...
squares_b = squares(56)
...
Multiple uses in
multiple files

387
“Modules”
def
...
square(limit)
five = squares(5)
: ...
...
Definition Use
Module: a container of functions
def
...
cubes(limit):

388
def squares(limit):
answer = []
for n in range(0,limit):
answer.append(n**2)
return answer
def total(numbers):
sum_so_far = 0
return sum_so_far
number = int(text)
squares_n = squares(number)
total_n = total(squares_n)
print(total_n)
Modules: a worked example ― 1a
sum_squares.py
Starts empty
utils.py

389
Modules: a worked example ― 1b
$ python3 sum_squares.py
Number? 5
30
Number? 7
91
= 0 + 1 + 4 + 9 + 16
= 0 + 1 + 4 + 9 + 16 + 25 + 36

390
number = int(text)
print(total_n)
def squares(limit):
answer = []
answer.append(n**2)
return answer
def total(numbers):
sum_so_far = 0
return sum_so_far
utils.py
sum_squares.py
Move the definitions
into the other file.

391
Number? 5
File "sum_squares.py", line 4, in <module>
NameError: name 'squares' is not defined
Because we have (re)moved its definition.

392
import utils
number = int(text)
print(total_n)
def squares(limit):
answer = []
answer.append(n**2)
return answer
def total(numbers):
sum_so_far = 0
return sum_so_far
utils.py
sum_squares.py
import: Make
a reference to
the other file.
import utils
and not
import utils.py

393
Number? 5
File "sum_squares.py", line 4, in <module>
NameError: name 'squares' is not defined
Still can’t find the function(s).

394
import utils
number = int(text)
squares_n = utils.squares(number)
total_n = utils.total(squares_n)
print(total_n)
sum_squares.py
utils.…: Identify
the functions
as coming from
the module.
squares()
utils.squares()
total()
utils.total()

395
Number? 5
30
Number? 7
91
Working again!
☺

396
Progress
Sharing functions between scripts
“Modules”
Importing modules
Using functions from modules
import module
module.function()

397
Exercise 20
5 minutes
The script exercise20.py is an answer to exercise19.
Move the function file_stats() from exercise19.py
into utils.py and edit exercise19.py so that it still works.

398
The Python philosophy
math
sys
string
A small core
language …
… plus lots
of modules
“Batteries
included”

399
Example: the “math” module
>>> import
>>> math.
1.4142135623730951
math Load in the “math” module.
Run the sqrt() function…
… from the math module.
2.0)sqrt(

400
import module as alias
>>> import math
>>> math.
1.4142135623730951
>>> import math as
>>> m
1.4142135623730951
sqrt(2.0)
Too long to keep typing?
m
.sqrt(2.0)
“Alias”

401
Don’t do these
>>> from math import sqrt
>>> sqrt(2.0)
1.4142135623730951
>>> from math import *
>>> sqrt(2.0)
1.4142135623730951
!
!!
Much better
to track the
module.

402
What system modules are there?
sys
os
string
re
csvargparse
webbrowser
math
cmath
colorsys
pickle
datetime
email
getpass
glob
html
http
io
json
logging
random
signal
sqlite3
subprocess
tempfile
unicodedata
unittest xml
Python 3.2.3 comes with over 250 modules.

403
“Batteries included”
>>> help('modules')
Please wait a moment while I gather a list
of all available modules...
CDROM binascii inspect shlex
bdb importlib shelve
Enter any module name to get more help. Or,
type "modules spam" to search for modules whose
descriptions contain the word "spam".
263 modules
Not quite this simple

404
Additional downloadable modules
numpy
scipy psycopg2
MySQLdb
cx_oracle
ibm_db
pyodbc
Numerical
PostgreSQL
MySQL
Oracle
DB2
Databases
pymssql SQL Servermatplotlib
Graphics

405
An example system module: sys
import sys
print(sys.argv)
argv.py
$ python3 argv.py one two three
['argv.py',
$ python3 argv.py 1 2 3
['argv.py', '3']'2','1',
Always strings
'three']'two','one',
0 1 2 3index

406
sys.exit()
exit() What we have been using
sys.exit What we should use(rc)
“Return Code”: an integer
0: Everything was OK
≠0:Something went wrong

407
An example system module: sys
But also…
sys.path
sys.version
sys.modules
sys.stdin
sys.stdout
sys.stderr
Directories Python searches for modules
Version of Python
All modules currently imported
Where input inputs from
Where print prints to
Where errors print to
…and there’s more!
sys.float_info All the floating point limits

408
Modules in Python
“How do I do
X in Python?”
“What’s the Python
module to do X?”
“Where do I find
Python modules?”

409
Finding modules
Python:
PyPI:
Built-in modules
Python Package Index
SciPy: Scientific Python modules
Search: “Python3 module for X”

410
Help with modules
>>> import math
>>> help(math)
NAME
math
DESCRIPTION
This module is always available. It provides
access to the mathematical functions defined
by the C standard.
…

411
Help with module functions
…
FUNCTIONS
acos(x)
Return the arc cosine (measured in
radians) of x.
…
>>> math.acos(1.0)
0.0

412
Help with module constants
DATA
e = 2.718281828459045
pi = 3.141592653589793
…
…
>>> math.pi
3.141592653589793

413
Help for our own modules?
def squares(limit):
answer = []
answer.append(n**2)
return answer
def total(numbers):
sum_so_far = 0
return sum_so_far
utils.py
>>> import utils
>>> help(utils)
NAME
utils
FUNCTIONS
squares(limit)
total(numbers)
FILE
/home/y550/utils.py
Basic help already
provided by Python

414
Adding extra help text
def squares(limit):
answer = []
answer.append(n**2)
return answer
def total(numbers):
sum_so_far = 0
return sum_so_far
utils.py
>>> help(utils)
NAME
utils
>>> import utils Fresh start"""Some utility functions
from the Python for
Absolute Beginners course
"""
FUNCTIONS
squares(limit)
total(numbers)
DESCRIPTION
Some utility functions
from the Python for

415
Adding extra help text to functions
"""Some utility functions
from the Python for
"""
def squares(limit):
answer = []
answer.append(n**2)
return answer
utils.py
"""Returns a list of
squares from zero to
limit**2.
"""
>>> help(utils)
NAME
utils
DESCRIPTION
...
>>> import utils Fresh start
FUNCTIONS
squares(limit)
Returns a list of
limit**2.

416
Adding extra help text to functions
"""Some utility functions
from the Python for
"""
def squares(limit):
answer = []
answer.append(n**2)
return answer
utils.py
"""Returns a list of
limit**2.
"""
>>> help(utils.squares)
>>> import utils Fresh start
squares(limit)
Returns a list of
limit**2.

417
Progress
Python a small language…
…with many, many modules
System modules
Modules provide help
Foreign modules
Doc strings
Functionality Module
help(module)
help(module.function)

418
Exercise 21
5 minutes
Add help text to your utils.py file.

419
Recap: Lists & Indices
list
3
Position 2
Position 1
Position 0
greek = ['alpha', 'beta', 'gamma']
str 5 a l p h a
str 4 b e t a
str 5 g a m m a
greek[0]
greek[1]
greek[2]

420
“Index in ― Value out”
listindex value
1 greek 'beta'[1]
Must be
a number
start at 0
no gaps in
sequence

421
Other “indices”: Strings?
dictionaryEnglish Spanish
'cat'
'dog'
'mouse'
'gato'
'perro'
'ratón'
dictionary

422
Other “indices”: Tuples?
dictionary(x, y) Objects
(2,5)
(4,5)
'Treasure'
'Fortress'
dictionary

423
Python “dictionaries”
>>> en_to_es = { 'cat':'gato' , 'dog':'perro' }
>>> en_to_es['cat']
'gato'

424
Creating a dictionary — 1
'dog':'perro'{ },'cat':'gato'
Curly brackets
Comma
'cat' → 'gato' 'dog' → 'perro'

425
Creating a dictionary — 2
'dog'{ },'cat' 'gato': 'perro':
'cat' 'gato'
“key” “value”
:
:

426
Using a dictionary — 1
>>> en_to_es['cat']
'gato'
Creating the dictionary
Using the dictionary

427
Using a dictionary — 2
en_to_es ]'cat'[ 'gato'
dictionary key value
Square brackets

428
Missing keys
>>> en_to_es['dog']
'perro'
>>> en_to_es['mouse']
KeyError: 'mouse' Error message
✓
✗

429
Dictionaries are one-way
!
>>> en_to_es =
>>> en_to_es['dog']
'perro'
>>> en_to_es['perro']
KeyError:
✓
✗
'perro'
Looking for a key
{ }'dog':,'cat': 'perro''gato'

430
Adding to a dictionary
>>> en_to_es
>>> en_to_es['mouse'] = 'ratón'
>>> en_to_es['mouse']
'ratón'
= { 'cat':'gato' , 'dog':'perro' }
Initial dictionary has no 'mouse'
Adding 'mouse' to the dictionary

431
Removing from a dictionary
>>> print(en_to_es)
{'mouse': 'ratón',
>>> del en_to_es[
>>> print(en_to_es)
{'mouse': 'ratón', 'cat': 'gato'}
'cat': 'gato'}'dog': 'perro',
]'dog'

432
Progress
Dictionaries Key Value
{ key1
:value1
, key2
:value2
, key3
:value3
}
dictionary[key] valueLooking up values
dictionary[key] = valueSetting values
del dictionary[key]Removing keys

433
Exercise 22
5 minutes
Complete exercise22.py to create
an English to French dictionary.
cat chat
dog chien
mouse souris
snake serpent

434
What’s in a dictionary? ― 1
>>> en_to_es
{'mouse': 'ratón', 'dog': 'perro', 'cat': 'gato'}
>>> en_to_es.keys()
dict_keys
>>> en_to_es.values()
dict_values
Just treat them like lists
Orders
match
(['mouse', 'dog', 'cat'])
(['ratón', 'perro', 'gato'])
(or convert them to lists)

435
>>> en_to_es
dict_items([('mouse', 'ratón'), ('dog', 'perro'),
('cat', 'gato')])
>>> for (english, spanish) in en_to_es.items():
... print(spanish, english)
...
ratón mouse
perro dog
gato cat
Most useful method.items()
(Key,Value) pairs/tuples

436
>>> list
[('mouse','ratón'), ('dog','perro'),('cat','gato')]
Common simplification
(en_to_es.items())

437
Getting the list of keys
dictionary
list of
keys
list()
{'the': 2, 'cat': 1, 'sat': 1, 'on': 1, 'mat': 1}
['on', 'the', 'sat', 'mat', 'cat']

438
Is a key in a dictionary?
>>> en_to_es['snake']
KeyError: 'snake' Want to avoid this
>>> 'snake' in en_to_es
False We can test for it

439
Example: Counting words ― 1
words = ['the','cat','sat','on','the','mat']
counts = {'the':2,'cat':1,'sat':1,'on':1,'mat':1}

440
counts = {}
for word in words:
Do something
Start with an empty dictionary
Work through all the words

441
counts = {}
for word in words:
counts[word] += 1
✗ This will not work
counter1.py

442
Why doesn’t it work?
counts = {'the':1, 'cat':1}
counts['the'] += 1
counts['sat'] += 1
✗
✓
counts['the'] = + 1counts['the']
The key must already
be in the dictionary.
counts['sat'] = + 1counts['sat']
Key is not in
the dictionary!

443
counts = {}
for word in words:
if word in counts:
counts[word] += 1
else:
Do something Need to add the key

444
counts = {}
for word in words:
if word in counts:
counts[word] += 1
else:
counts[word] = 1 counter2.py
print(counts)

445
$ python3 counter2.py
{'on': 1, 'the': 2, 'sat': 1, 'mat': 1, 'cat': 1}
You cannot predict the order of the
keys when a dictionary prints out.
!

446
print(counts)
items = list(dictionary.items())
items.sort()
for (key, value) in items:
print(key, value)
✗ Too ugly
Better
counter3.py

447
cat 1
mat 1
on 1
sat 1
the 2

448
Progress
Testing keys in dictionaries
Creating a list of keys
if key in dictionary:
...
keys = list(dictionary)
Inspection methods dictionary.keys()
dictionary.values()
dictionary.items()

449
Exercise 23
5 minutes
Complete exercise23.py to write
a script that reverses a dictionary.
{
{
'perro'}'dog':'gato','cat':'ratón','mouse':
:'dog'}'perro''cat','gato':'mouse','ratón':

450
Formatted output
cat 1
mat 1
sat 1
the 2
on 1 Ugly!
cat
mat
on
sat
the
1
1
1
1
2
We want data
nicely aligned

451
The format() method
>>>
'xxxAyyy100zzz'
'xxx{}yyy{}zzz'.format('A', 100)
'
'
zzz{}yyy{}xxx '
'zzz100yyyAxxx

452
String formatting ― 1
>>>
'xxxA␣␣␣␣yyy'
'xxx{:5s}yyy'.format('A')
'
'
yyy{:5s}xxx '
'yyyA␣␣␣␣xxx
{:5s}
s ― substitute a string
5 ― pad to 5 spaces
(left aligns by default)

453
>>>
'xxxA␣␣␣␣yyy'
'xxx{:<5s}yyy'.format('A')
'
'
yyy{:<5s}xxx '
'yyyA␣␣␣␣xxx
{:<5s}
< ― align to the left (←)

454
>>>
'xxx␣␣␣␣Ayyy'
'xxx{:>5s}yyy'.format('A')
'
'
yyy{:>5s}xxx '
'yyy␣␣␣␣Axxx
{:>5s}
> ― align to the right (→)

455
Integer formatting ― 1
>>>
'xxx␣␣123yyy'
'xxx{:5d}yyy'.format(123)
'
'
yyy{:5d}xxx '
'yyy␣␣123xxx
{:5d}
d ― substitute an integer
5 ― pad to 5 spaces
(right aligns by default)
(digits)

456
>>>
'xxx␣␣123yyy'
'xxx{:>5d}yyy'.format(123)
'
'
yyy{:>5d}xxx '
'yyy␣␣123xxx
{:>5d}
> ― align to the right (→)

457
>>>
'xxx␣␣123yyy'
'xxx{:>5d}yyy'.format(123)
'
'
yyy{:<5d}xxx '
'yyy123␣␣xxx
{:<5d}
< ― align to the left (←)

458
>>>
'xxx00123yyy'
'xxx{:05d}yyy'.format(123)
'
'
yyy{:05d}xxx '
'yyy00123xxx
{:05d}
0 ― pad with zeroes

459
>>>
'xxx␣+123yyy'
'xxx{:+5d}yyy'.format(123)
'
'
yyy{:+5d}xxx '
'yyy␣+123xxx
{:05d}
+ ― always show sign

460
>>>
'xxx␣+0123yyy'
'xxx{:+05d}yyy'.format(123)
'
'
yyy{:+05d}xxx '
'yyy+0123xxx
{:05d}
+ ― always show sign
0 ― pad with zeroes

461
>>>
'xxx1,234yyy'
'xxx{:5,d}yyy'.format(1234)
'
'
yyy{:5,d}xxx '
'yyy1,234xxx
{:5,d}
, ― 1,000s

462
Floating point formatting ― 1
>>>
'xxx 1.20yyy'
'xxx{:5.2f}yyy'.format(1.2)
'
'
yyy{:5.2f}xxx '
'yyy␣1.20xxx
{:5.2f}
f ― substitute a float
5 ― 5 places in total
.2 ― 2 places after the point

463
Floating point formatting ― 2
>>>
'xxx1.200000yyy'
'xxx{:f}yyy'.format(1.2)
'
'
yyy{:f}xxx '
'yyy1.200000xxx
{:f}
{:.6f}

464
Ordering and repeats ― 1
>>>
'X
'X )1.23123,'abc',X'.format({:f}X{:d}X{:s}
X'1.230000X123Xabc
0 1 2
0 1 2
0 1 2
>>> 'X )1.23123,'abc',X'.format({2:f}X{1:d}X{0:s}
'X X'1.230000X123Xabc
0 1 2
0 1 2
Equivalent

465
Ordering and repeats ― 2
>>> 'X )1.23123,'abc',X'.format({2:f}X{1:d}X{0:s}
'X X1.230000 X123Xabc
0 12
0 1 2
'
>>> 'X )1.23123,'abc',X'.format(X{1:d}X{0:s}
'X X123Xabc
0 1
0 1 2
X'
{1:d}
123
1

466
Formatting in practice
cat
mat
on
sat
the
1
1
1
1
2
...
formatting = '{:3} {:1}'
for (word, number) in items:
print(formatting.format(word,number))
$

467
Progress
string.format(args)Formatting
Numbered parameters
Substitutions
{0} {1} {2}
{:>6s}
{:+06d}
{:+012.7f}

468
Exercise 24
5 minutes
Complete exercise24.py
to format the output as shown:
Joe 9
Samantha 45
Methuselah 969
[(Joe,9), (Samantha,45), (Methuselah,969)]

469
And that's it! (And “it” is a lot!)
Text
Prompting
Numbers
Arithmetic
Comparisons
Booleans
Variables
Deleting names
“while” loop
“if” test
Indented blocks
Lists
Indices
Object methods
Built-in help
“for” loops
“Treat it like a list…”
Values direct/via index
Reading files
Writing files
Functions
Tuples
“for” loops
Modules
Dictionaries
Formatting

470
But wait! There’s more…
Advanced topics: Self-paced introductions to modules
Object-oriented programming in Python

471
Text
Prompting
Numbers
Arithmetic
Comparisons
Booleans
Variables
Deleting names
“while” loop
“if” test
Indented blocks
Lists
Indices
Object methods
Built-in help
“for” loops
“Treat it like a list…”
Values direct/via index
Reading files
Writing files
Functions
Tuples
“for” loops
Modules
Dictionaries
Formatting
Congratulations!
Please don’t forget the feedback.

An introduction to Python for absolute beginners

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