Functions in Python

  • Convention is to leave a double line-break after a function definition (but not with nested functions - here, a single linebreak is sufficient)
  • Scope within functions is demarcated by indents, as everything in Python
  • We use a docstring within the function body, to document our function. This text will then show up in Intellisense etc.

Basic examples


# No params, no return
def print_msg():
    """ A function that prints hello world """
    print('Hello World!')

print_msg()
# Hello World!

print(type(print_msg))
# <class 'function'>

# Params, no return
def print_my_msg(msg):
    """ A simple function to print a message """
    print(msg)

print_my_msg('Good day')
# Good day

# Params and return
def square(n):
    return n * n

print(square(2))
# 4

result = square(4)
print(result)

# 16

Default parameters

def greeter(name, message='Live Long and Prosper'):
    print('Welcome', name, '-', message)

greeter('Eloise')

# Welcome Eloise - Live Long and Prosper

Optional parameters

def func_with_optional(non_optional, optional_param=None):
  if optional_param is not None:
    # Do something with specific value
  else:
    # Run standard process


## Function with arbitrary parameter list

```python
def greeter(*args):
    for name in args:
        print('Welcome', name)

greeter('John', 'Denise', 'Phoebe', 'Adam', 'Gryff', 'Natalia')

"""
Welcome John
Welcome Denise
Welcome Phoebe
Welcome Adam
Welcome Gryff
Welcome Natalia
"""

Scoping

Function variables are locally scoped by default.

They can access variables that are outer to them and can redefine them within their own scope and within the global scope using the keywords global and nonlocal.

Below a global variable is accessed and changed but only internally within a function scope

max = 100
print('initial value of max:', max)

def print_max():
    global max
    max = max + 1
    print('inside function:', max)


print_max()
print('outside function:', max)

"""
initial value of max: 100
inside function: 101
outside function: 101
"""

Below a higher-scoped variable is redefined from within the lower scope:

def myfunc1():
  x = "John"
  def myfunc2():
    nonlocal x
    x = "hello"
  myfunc2()
  return x

print(myfunc1())

# hello

We cannot however redefine a global variable from a function scope permanently. It will remain whatever it is in global scope, after the function has run.

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