# Document Functions

Define a function document

>>> def example():

... """This is an example function."""

... print("Example function")

...

>>> example.__doc__

'This is an example function.'

>>> help(example)

# Default Arguments

>>> def add(a, b=0):

... return a + b

...

>>> add(1)

1

>>> add(1, 2)

3

>>> add(1, b=2)

3

# Option Arguments

>>> def example(a, b=None, *args, **kwargs):

... print(a, b)

... print(args)

... print(kwargs)

...

>>> example(1, "var", 2, 3, word="hello")

1 var

(2, 3)

{'word': 'hello'}

# Unpack Arguments

>>> def foo(a, b, c='BAZ'):

... print(a, b, c)

...

>>> foo(*("FOO", "BAR"), **{"c": "baz"})

FOO BAR baz

# Keyword-Only Arguments

New in Python 3.0

>>> def f(a, b, *, kw):

... print(a, b, kw)

...

>>> f(1, 2, kw=3)

1 2 3

>>> f(1, 2, 3)

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: f() takes 2 positional arguments but 3 were given

# Annotations

New in Python 3.0

>>> def fib(n: int) -> int:

... a, b = 0, 1

... for _ in range(n):

... b, a = a + b, b

... return a

...

>>> fib(10)

55

>>> fib.__annotations__

{'n': <class 'int'>, 'return': <class 'int'>}

# Callable

>>> a = 10

>>> def fun():

... print("I am callable")

...

>>> callable(a)

False

>>> callable(fun)

True

# Get Function Name

>>> def example_function():

... pass

...

>>> example_function.__name__

'example_function'

# Lambda

>>> fn = lambda x: x**2

>>> fn(3)

9

>>> (lambda x: x**2)(3)

9

>>> (lambda x: [x*_ for _ in range(5)])(2)

[0, 2, 4, 6, 8]

>>> (lambda x: x if x>3 else 3)(5)

5

# Generator

>>> def fib(n):

... a, b = 0, 1

... for _ in range(n):

... yield a

... b, a = a + b, b

...

>>> [f for f in fib(10)]

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

# Decorator

New in Python 2.4

PEP 318 - Decorators for Functions and Methods

>>> from functools import wraps

>>> def decorator(func):

... @wraps(func)

... def wrapper(*args, **kwargs):

... print("Before calling {}.".format(func.__name__))

... ret = func(*args, **kwargs)

... print("After calling {}.".format(func.__name__))

... return ret

... return wrapper

...

>>> @decorator

... def example():

... print("Inside example function.")

...

>>> example()

Before calling example.

Inside example function.

After calling example.

# Equals to

... def example():

... print("Inside example function.")

...

>>> example = decorator(example)

>>> example()

Before calling example.

Inside example function.

After calling example.

# Decorator with Arguments

>>> from functools import wraps

>>> def decorator_with_argument(val):

... def decorator(func):

... @wraps(func)

... def wrapper(*args, **kwargs):

... print("Val is {0}".format(val))

... return func(*args, **kwargs)

... return wrapper

... return decorator

...

>>> @decorator_with_argument(10)

... def example():

... print("This is example function.")

...

>>> example()

Val is 10

This is example function.

# Equals to

>>> def example():

... print("This is example function.")

...

>>> example = decorator_with_argument(10)(example)

>>> example()

Val is 10

This is example function.

# Cache

New in Python 3.2

# Without Cache

>>> import time

>>> def fib(n):

... if n < 2:

... return n

... return fib(n - 1) + fib(n - 2)

...

>>> s = time.time(); _ = fib(32); e = time.time(); e - s

1.1562161445617676

# With Cache (dynamic programming)

>>> from functools import lru_cache

>>> @lru_cache(maxsize=None)

... def fib(n):

... if n < 2:

... return n

... return fib(n - 1) + fib(n - 2)

...

>>> s = time.time(); _ = fib(32); e = time.time(); e - s

2.9087066650390625e-05

>>> fib.cache_info()

CacheInfo(hits=30, misses=33, maxsize=None, currsize=33)