A Gentle Introduction to Coding ... with Python

A Gentle
Introduction to
Programming
(with Python)
Tariq Rashid, July-August 2015

Ambition
Very Gentle Introduction
to coding and Python, zero expertise assumed
Confidence
to explore further,
understand suppliers and techies

The Journey
The “1s and 0s”
languages
tech overview
“Hello World!” working with
data
repetitive work reusable
blocks
graphics
using libraries
data science?
digital
services?

Deep Inside Tech is .... Numbers
Computers do stuff with numbers:
add, multiply, subtract, divide, shift, chop, invert, ….
That’s it!
There is no other magic.

It’s all numbers ...
Networks

Desktops
Applications

Smartphone Apps
Android / iOS

Digital Movies
Digital Music

Digital Web Services

The 1s and 0s
This is a computer …. (kinda)
on / off
switch
lightbulb
battery

The 1s and 0s
on / off
switch
lightbulb
battery
input output

The 1s and 0s
This one is a bit more useful.
on / off
switches
lightbulb
battery
0
0
0

The 1s and 0s
on / off
switches
lightbulb
battery
1
0
0

The 1s and 0s
on / off
switches
lightbulb
battery
1
1
1

The 1s and 0s
on / off
switches
lightbulb
battery
Computation:
Only when both inputs are on
Turn on the output light
1
1
1

The 1s and 0s
on / off
switches
lightbulb
battery
Computation:
Only when both inputs are on
Turn on the output light
1
1
1
Recipe:
IF
temperature is low
AND it’s after 8pm
THEN
Turn on the heating

Programming
Programming
Is Telling Computers What to Do
... and How …
“Recipes”

Ye Olde Programming!
… those wires again!

Modern Programming!
… live demo!

Loads of Programming Languages!
different tools for
different jobs

Popular Languages 2014
…. Go Python!

Enough Theory!
Let’s code!
( fire up your Python )

Traditional First Program
Hello World!

Let’s Decode Some Python ...
print ( “Hello World!” )

IPython … Python in a Browser

IPython
Python instructions
… and the result!
Play button
“Execute my command with extreme
prejudice”
awaiting your next instruction

Can you print out other words?
Have a Play!

What About Simple Arithmetic?
2 times 3
… is 6
the star * symbol means “multiply” in
many programming languages

Have a Play Again!
Can you do other kinds of arithmetic?
What does this code do? Why?
print ( “3 * 4” )

Well Done!
You're now coding!
Dennis Ritchie, CKen Thompson, UNIX
Richard Stallman, Tech Freedom

World’s First Programmer!
Ada Lovelace, World’s First Programmer

Next Time ...
Working with more data - with variables, arrays, ...
Automating lots of work - with loops, iterators, ...

Welcome back to Part 2!
Working with more data - with variables, arrays, ...
Automating lots of work - with loops, iterators, ...

Data Nitty Gritty
3
memory
data

Variable - the name of a storage box
3
a
variable name

Variable - the name of a storage box
3
a
variable names
5
b

Just like school maths
3
a
In maths we write:
a = 3

Data Nitty Gritty
3
a
In Python we also
write:
a = 3
… try it!

Python Variables
code starting with the hash
symbol # are ignored
we can use them as comments
shortcut!

Changing Data is Useful
animation is … changing data

Changing Data
3
a
7
replace the 3 with a 7

Changing Data
7
a
the box name stays
the same
… where did the 3 go?

Changing Data - Try it!
before ... a is 3
after ... a is 7

Arithmetic on Variables
What does this do?
a = 7
b = a * 3
print (b)

Arithmetic on Variables - Try It!

Different Kinds of Data
Numbers:
a = 7 Decimals:
b = 3.14159
Text:
c = “cat”

Different Kinds of Data
Numbers:
a = 7 Decimals:
b = 3.14159
Text:
c = “cat”
“integer”
“floating point”
“string”

A Collection of Data is Useful
List (of numbers, for example)
9 3 4 8 1 5

Python Lists
List
9 3 4 8 1 5
0 1 2 3 4 5
the zeroth element is 9
the data in box 3 is 8
index

Lists are variables too
9 3 4 8 1 5
0 1 2 3 4 5
a[0] = 9
a[3] = 8
a =
index
variable name
Selecting from a List

Making a Python List
a = [ 9, 3, 4, 8, 1, 5 ]
9 3 4 8 1 5
0 1 2 3 4 5
a =

Python Lists - Try it!
make the list using
square brackets
selecting individual list items
using the index
yup, it checks out fine

Changing List Data
a = [ 9, 3, 4, 8, 1, 5 ]
9 3 4 8 1 5
0 1 2 3 4 5
a =
a[2] = 0
9 3 0 8 1 5
0 1 2 3 4 5
a =

Changing List Data
a = [ 9, 3, 4, 8, 1, 5 ]
9 3 4 8 1 5
0 1 2 3 4 5
a =
a[2] = 0
9 3 0 8 1 5
0 1 2 3 4 5
a =
Try it!

Oops!
what happened ?!?!
error message!

Reaching Too Far Out!
9 3 4 8 1 5
0 1 2 3 4 5
a[0] = 9
a[3] = 8
a =
index
a [6] doesn’t exist!
a[5] = 5

Classic Coder Errors
Forgetting lists start at … 0 .. not 1
Software Bug!
Going beyond the end of a list is a major
method of security hacking!
Buffer Overflow!

Computers Love Repetitive Work!
Computers don’t mind doing lots of
repetitive number calculations.
Working through a list is useful start ...

Working Through a List - Try it!
what does this new code do?

it’s called
iterating
over a list

woah!
what’s all this?
can you work it out?

Next Time ...
Reusable code - functions, libraries, …
Working with more data - arrays, ...
Visualising data - bitmaps, plots, ...

Reusable code - functions, libraries, …
Working with more data - arrays, ...
Visualising data - bitmaps, plots, ...

Functions
Python functions are kinda like school maths functions
do some working out
functioninputs output

Useful Recipe - Volume of a Block
w
l
h
volume = width * length * height
v = w * l * h
In Python …
v = a * b * c
volume

Our First Function
def volume(w, l, h):
v = w * l * h
return v

Our First Function
def volume(w, l, h):
v = w * l * h
return v
create a new
function
… give it a name ... … takes these inputs
...
… does this work ...
… finally returns an
answer ...

Our First Function - Block Volume
v = w * l * h
w
h
l
v
functioninputs output

Functions - Try it!
reusable code
reused here!
.. and again here!

Functions - Try it!
reusable code
reused here!
.. and again here!
Hurrah!
You’ve just created some
reusable code!

Functions - Puzzle 1
radius
circumference = 2 * pi * radius

Functions - Puzzle 2
palindrome !

Google is a Coder’s Best Friend!
Google is really really good at helping coders ...
explains that string
reversal we saw

Libraries of Reusable Code
It’s how coding is done at scale, with lots of coders.
And how Open Source projects often share cool useful stuff.
Linux

Let’s Use Somebody Else’s Work
Libraries - extend your Python’s powers with
somebody else’s code
my Python
my functions
somebody else’s module
importing
libraries

Python Documentation is Good
Have a look at the Python docs for the math
library
https://docs.python.org/3/library/math.html
(just an example, you could look at other Python libraries yourself)

Imported Powers - Try it!
import that library
so we can use it
library name dot function
not a function, just a variable …
… remember them?

Another Kind of Collection
List 9 3 4 8 1 5
Array
9 3 4 8 1 5
4 2 5 0 0 1
2 7 3 1 4 0

Python Arrays
Array
9 3 4 8 1 5
4 2 5 0 0 1
2 7 3 1 4 0
0 1 2 3 4 5
a[2,0] = 2
a[1,3] = 0
a =
indexvariable name
2
1
0
index

Python Arrays
Array
9 3 4 8 1 5
4 2 5 0 0 1
2 7 3 1 4 0
0 1 2 3 4 5
a[2,0] = 2
a[1,3] = 0
a =
indexvariable name
2
1
0
index
array [ row, column ]
in fact you can have many dimensions ...array [dim1, dim2, dim3 …]

We need HELP!!
Numpy to the Rescue!
Numpy is a very popular and useful library.
It’s used loads in data science.
Numpy does arrays really rather well...

Numpy Arrays
it’s a function
just like we saw
before
a = np.array ( )

Numpy Arrays
a = np.array (
[ ]
)
arrays are enclosed by
square brackets

Numpy Arrays
a = np.array (
[ [1, 2, 3],
[4, 5, 6],
[7, 8, 9] ]
)
3 by 3
array of numbers
row by row

Numpy Arrays - Try It!
import numpy library
but we’ll call it np from
now on
create the 3 by 3 array
and print it out to be sure
accessing elements with
indices
taking row and column
slices

Array Functions - All At Once!
you can do stuff to an
array as a whole!
vs doing it to each element at a time
powerful
so can write concise clean code
efficient
for working on large data

Array Functions - Try it!
multiply every element by 10
find the mean (average)
sine function …
(trigonometry from school)
sum over all elements

Graphics! … What Does This Do?

… and this?
making a List … remember
them !
applying sin() to the array

.. and there’s loads of libraries
Lots of example online:
http://scipy-lectures.github.io/intro/matplotlib/matplotlib.html
http://matplotlib.org/basemap/users/examples.html
Have an explore yourself!

Images
Image processing
with Python is easy
decode it for homework if you fancy a
challenge!

Next Time ...
Web Application - we’ll make our own little app ... (not just a static page)
Mini Challenge! - two teams …. ;)

Web Application - we’ll make our own little app ... (not just a static page)
Mini Challenge! - two teams …. ;)

Let’s Make a Web Application
Not just a web page …
.. a proper interactive web application.

How do these work?
What’s under the hood?
What are the main moving parts?

Anatomy of (many) Web Applications
user
browser
internet
web application

web application server
front end builder
logic
data storage

web application server
front end builder
logic
data storage
visual design
business
process
glue and
coordination
customer
records

Talking to a Web Application
browser
web application
GET /contact
OK … stuff ...
talking in a
language called
HTTP

Demo - Let’s Talk to a Web Server
connect to web server
request a page
response code; OK
content of response
this is what the browser
uses to compose the page

Flask - A Small Python Web Framework
GET /contact
OK … stuff ...
handlers
route URLs to handlers

Flask - Let’s Try It!
import flask library
so we can use it
run the web server
.. until you kill it
registering URL handlers -
the URL and code to run
create a flask object -
this contains all the machinery like
routing requests

Add a Pinch of HTML
a string which contains HTML
code
if we return it, the browser will
decode and display it
head and body
“hello” and horizontal rule

Add a Pinch of HTML
<h2>Hello</h2>
<hr>
<title>My Hello Page</title>

Twitter Clone - The Logic
URL is /
form to submit new
tweet
list of
previous tweets
URL is /submit
add tweet to list
redirect back to /
submit

Twitter Clone - Python Code
import Flask
list of tweets
html code template
the XXXX will be replaced later
main home page shows tweets
submit page with form
redirect back to home page

Twitter Clone - copy and paste this code if you need to
# import flask
import flask
# python list of tweets
tweets = ["First tweet - Hello, World!"]
# html for page
html_template = """
<!DOCTYPE html>
<html>
<head>
<title>My Tweet App</title>
<style>
body {background-color: linen; }
body {font-family: Sans-Serif;}
</style>
</head>
<body>
<h2>Submit a Tweet</h2>
<form action="submit" method="post">
<textarea name="content" cols="50" rows="4"></textarea>
<input type="submit" value="submit my tweet">
</form>
<h2>Previous Tweets</h2>
XXXX
</body>
</html>
"""
#create the application object
app = flask.Flask(__name__)
@app.route('/')
def home_page():
# replace XXXX with tweets
html = html_template.replace('XXXX', '<hr><p>'.join(tweets))
return html
@app.route('/submit', methods = ['POST'])
def submit():
tweet = flask.request.form['content']
tweets.append(tweet)
return flask.redirect('/')
app.run()

Twitter Clone - Let’s Try It!

Twitter Clone - Let’s Try It!
SUCCESS!

Spy Challenge!
1. Write a function to encrypt a
message.
2. Write a function to decrypt a
message.
* use Google to help

Encrypt and Decrypt
Encrypt:
● lowercase the text message
● rotate up the letters by the number given by the key
(if you go past “z”, wrap around to “a”)
Decrypt: - just the opposite
● rotate down the letters by the number given by the key

Encrypt - Illustrated
Encrypt
a p p l e z
c r r n g b
key is 2
shift up by 2 wrap around
A p p l E z

Decrypt
Decrypt just do the opposite

Functions - Reminder
def function_name (parameters):
do something
do more
return the_answer

Competition!
TEAM 1 TEAM 2
message message
encrypted
message
encrypt decrypt

In just 4 sessions we’ve …
The “1s and 0s”
languages
tech overview
“Hello World!” working with
data
repetitive work reusable
blocks
graphics
using libraries
data science?
digital
services?

Link to These Slides
https://goo.gl/6eQaMi

A Gentle Introduction to Coding ... with Python

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