\(\newcommand{L}[1]{\| #1 \|}\newcommand{VL}[1]{\L{ \vec{#1} }}\newcommand{R}[1]{\operatorname{Re}\,(#1)}\newcommand{I}[1]{\operatorname{Im}\, (#1)}\)

Functions are objects

In Python, functions are objects, like any other object.

>>> # - compatibility with Python 2
>>> from __future__ import print_function  # print('me') instead of print 'me'
>>> from __future__ import division  # 1/2 == 0.5, not 0

If I make a string in Python:

>>> name = 'Matthew'

then I have a Python object of type str:

>>> type(name)
<class 'str'>

Let’s say I defined a function add:

>>> def add(a, b):
...     return a + b

Now I have another Python object, of type function:

>>> type(add)
<class 'function'>

With my string, I can refer to the same string object, with a different variable name:

>>> prisoner = name
>>> prisoner
'Matthew'

It’s the same for functions, because functions are objects too:

>>> my_add = add
>>> type(my_add)
<class 'function'>

Functions are objects you can “call” by appending parentheses enclosing arguments you want to pass:

>>> add(1, 2)
3

>>> my_add(1, 2)
3

As for any other object in Python, you can pass function objects to other functions:

>>> def run_a_func(func, arg1, arg2):
...     result = func(arg1, arg2)
...     print('Result was', result)

>>> run_a_func(add, 1, 2)
Result was 3

>>> run_a_func(my_add, 1, 2)
Result was 3

>>> def sub(a, b):
...     return a - b

>>> run_a_func(sub, 1, 2)
Result was -1