Functions

def my_first_function():
    print("Hello world!")


print(f"type: {type(my_first_function)}")

my_first_function()  # Calling a function

type: <class 'function'>
Hello world!

Arguments

def greet_us(name1, name2):
    print(f"Hello {name1} and {name2}!")


greet_us("John Doe", "Superman")

Hello John Doe and Superman!

# Function with return value
def strip_and_lowercase(original):
    modified = original.strip().lower()
    return modified


uggly_string = "  MixED CaSe "
pretty = strip_and_lowercase(uggly_string)
print(f"pretty: {pretty}")

pretty: mixed case

Keyword arguments

def my_fancy_calculation(first, second, third):
    return first + second - third


print(my_fancy_calculation(3, 2, 1))

print(my_fancy_calculation(first=3, second=2, third=1))

# With keyword arguments you can mix the order
print(my_fancy_calculation(third=1, first=3, second=2))

# You can mix arguments and keyword arguments but you have to start with arguments
print(my_fancy_calculation(3, third=1, second=2))

4
4
4
4

Default arguments

def create_person_info(name, age, job=None, salary=300):
    info = {"name": name, "age": age, "salary": salary}

    # Add 'job' key only if it's provided as parameter
    if job:
        info.update(dict(job=job))

    return info


person1 = create_person_info("John Doe", 82)  # use default values for job and salary
person2 = create_person_info("Lisa Doe", 22, "hacker", 10000)
print(person1)
print(person2)

{'name': 'John Doe', 'age': 82, 'salary': 300}
{'name': 'Lisa Doe', 'age': 22, 'salary': 10000, 'job': 'hacker'}

Don't use mutable objects as default arguments!

def append_if_multiple_of_five(number, magical_list=[]):
    if number % 5 == 0:
        magical_list.append(number)
    return magical_list


print(append_if_multiple_of_five(100))
print(append_if_multiple_of_five(105))
print(append_if_multiple_of_five(123))
print(append_if_multiple_of_five(123, []))
print(append_if_multiple_of_five(123))

[100]
[100, 105]
[100, 105]
[]
[100, 105]

Here's how you can achieve desired behavior:

def append_if_multiple_of_five(number, magical_list=None):
    if not magical_list:
        magical_list = []
    if number % 5 == 0:
        magical_list.append(number)
    return magical_list


print(append_if_multiple_of_five(100))
print(append_if_multiple_of_five(105))
print(append_if_multiple_of_five(123))
print(append_if_multiple_of_five(123, []))
print(append_if_multiple_of_five(123))

[100]
[105]
[]
[]
[]

Docstrings

Strings for documenting your functions, methods, modules and variables.

def print_sum(val1, val2):
    """Function which prints the sum of given arguments."""
    print(f"sum: {val1 + val2}")


print(help(print_sum))

Help on function print_sum in module __main__:

print_sum(val1, val2)
    Function which prints the sum of given arguments.

None

def calculate_sum(val1, val2):
    """This is a longer docstring defining also the args and the return value.

    Args:
        val1: The first parameter.
        val2: The second parameter.

    Returns:
        The sum of val1 and val2.

    """
    return val1 + val2


print(help(calculate_sum))

Help on function calculate_sum in module __main__:

calculate_sum(val1, val2)
    This is a longer docstring defining also the args and the return value.
    
    Args:
        val1: The first parameter.
        val2: The second parameter.
    
    Returns:
        The sum of val1 and val2.

None

pass statement

pass is a statement which does nothing when it's executed. It can be used e.g. a as placeholder to make the code syntatically correct while sketching the functions and/or classes of your application. For example, the following is valid Python.

def my_function(some_argument):
    pass


def my_other_function():
    pass