12.1. Function Definition

12.1.1. Rationale

  • Reuse code

  • Improves code readability

  • Clean-up code

  • Allows for easier refactoring

12.1.2. Syntax

def <name>():
    <do something>

12.1.3. Definition

>>> def say_hello():
...     print('My name... José Jiménez')
>>>
>>>
>>> say_hello()
My name... José Jiménez
>>> say_hello()
My name... José Jiménez
>>> say_hello()
My name... José Jiménez

12.1.4. Convention

  • Add underscore (_) at the end of name when name collide

  • System functions names starts and ends with 'dunder' - double underscore: __

Do not use camelCase or PascalCase names.

The camelCase name is c/c++/Java/JavaScript convention. It is not good to mix conventions from different languages. If you write C code, use C conventions. If you program in Python, use Python conventions. Remember, there are different communities around both of those languages:

>>> def sayHello():
...     pass

The PascalCase name has completely different meaning in Python - it is used for classes. Using such name convention will mistake others.

>>> def SayHello():
...     pass

Use snake_case names in Python. It is easy to remember. Python looks like a snake, and sounds like a snake ;) This is double internal joke, because Python name came from Monty Python, of which Guido van Rossum was a big fun. The other reference is to duck typing (dynamic typing) - "If it walks like a duck and it quacks like a duck, then it must be a duck":

This is snake_case() name. It is Pythonic way:

>>> def say_hello():
...     pass

Use better names, rather than comments:

>>> def cal_var(data, m):
...     # Calculate variance
...     return sum((Xi-m) ** 2 for Xi in data) / len(data)

>>> def calculate_variance(data, m):
...     return sum((Xi-m) ** 2 for Xi in data) / len(data)

Add underscore (_) at the end of name when name collide. Although prefer naming it differently:

>>> def print_(text):
...     # Add underscore (``_``) at the end of name when name collide.
...     print(f'<strong>{text}</strong>')

>>> def print_html(text):
...     # Although prefer naming it differently.
...     print(f'<strong>{text}</strong>')

System functions names starts and ends with 'dunder' - double underscore: __:

>>> def __import__(module_name):
...     pass

12.1.5. Assignments

Code 12.1. Solution
"""
* Assignment: Function Definition Print
* Required: yes
* Complexity: easy
* Lines of code: 5 lines
* Time: 3 min

English:
    1. Define function `call` without parameters
    2. Function appends `Beetlejuice` to `results`
    3. Call function three times
    4. Run doctests - all must succeed

Polish:
    1. Zdefiniuj funkcję `call` bez parametrów
    2. Funkcja dodaje `Beetlejuice` do `result`
    3. Wywołaj funkcję trzy razy
    4. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isfunction

    >>> isfunction(call)
    True
    >>> result
    ['Beetlejuice', 'Beetlejuice', 'Beetlejuice']
"""

result = []