Functions

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

A function can return data as a result.

Creating a Function

In Python a function is defined using the def keyword:

def my_function():
    print("Hello from a function")

Calling a Function

To call a function, use the function name followed by parenthesis:

my_function()

Arguments

Information can be passed into functions as arguments.

Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want,

just separate them with a comma.

The following example has a function with one argument f_name. When the function is called,

we pass along a first name, which is used inside the function to print the full name:

def my_function(f_name):
    print(f_name + " Refsnes")

my_function("Emil")
my_function("Tobias")
my_function("Linus")

Arguments are often shortened to args in Python documentations.

Parameters or Arguments?

The terms parameter and argument can be used for the same thing: information that are passed into a function.

From a function's perspective:

A parameter is the variable listed inside the parentheses in the function definition.

An argument is the value that is sent to the function when it is called.

Number of Arguments

By default, a function must be called with the correct number of arguments. Meaning that if your function expects 2 arguments, you have to call the function with 2 arguments, not more, and not less.

This function expects 2 arguments, and gets 2 arguments:

def my_function(f_name, l_name):
    print(f_name + " " + l_name)

my_function("Emil", "Refsnes")

If you try to call the function with 1 or 3 arguments, you will get an error.

Keyword Arguments

You can also send arguments with the key = value syntax. This way the order of the arguments does not matter.

def my_function(child3, child2, child1):
    print("The youngest child is " + child3)

my_function(child1="Emil", child2="Tobias", child3="Linus")

The phrase Keyword Arguments are often shortened to kwargs in Python documentations.

Arbitrary Arguments, *args

If you do not know how many arguments that will be passed into your function, add a * before the parameter name in the function definition.

This way the function will receive a tuple of arguments, and can access the items accordingly.

If the number of arguments is unknown, add a * before the parameter name:

def my_function(*kids):
    print("The youngest child is " + kids[2])

my_function("Emil", "Tobias", "Linus")

Arbitrary Arguments are often shortened to *args in Python documentations.

Arbitrary Keyword Arguments, **kwargs

If you do not know how many keyword arguments that will be passed into your function, add two asterisk: ** before the parameter name in the function definition.

This way the function will receive a dictionary of arguments, and can access the items accordingly.

If the number of keyword arguments is unknown, add a double ** before the parameter name:

def my_function(**kid):
    print("His last name is " + kid["l_name"])

my_function(fname="Tobias", lname="Refsnes")

Arbitrary keyword Arguments are often shortened to **kwargs in Python documentations.

def sum_numbers(*args, **kwargs):
    print(kwargs)
    print(args)

sum_numbers(1, 2, 3, a='a', b='b')

Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without argument, it uses the default value:

def my_function(country="Norway"):
    print("I am from " + country)

my_function("Sweden")
my_function("India")
my_function()
my_function("Brazil")

Passing a List as an Argument

You can send any data types of argument to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.

E.g. if you send a List as an argument, it will still be a List when it reaches the function:

def my_function(food):
    for x in food:
        print(x)

fruits = ["apple", "banana", "cherry"]
my_function(fruits)

Passing a function as an argument

def run_function_with_two_args(your_function, a, b):
    print(your_function(a, b))

def sum_numbers(x, y):
    return x + y

run_function_with_two_args(sum_numbers, 11, 11)

Return a function as object

def somthing():
    def sum_two_number(x, y):
        return x + y
    return sum_two_number

a = somthing()
print(a(2, 3))

Return Values

To let a function return a value, use the return statement:

def my_function(x):
    return 5 * x

print(my_function(3))

Return multi value

def calc_sum_sub(num1, num2):
    set_sum = num1 + num2
    set_sub = num1 - num2
    return set_sum, set_sum  # return as tuple

result_sum, result_sub = calc_sum_sub(10, 4)

Pass Statement

function definitions cannot be empty, but if you for some reason have a function definition with no content, put in the pass statement to avoid getting an error.

def myfunction():
    pass

Recursion

Python also accepts function recursion, which means a defined function can call itself.

Recursion is a common mathematical and programming concept. It means that a function calls itself.

This has the benefit of meaning that you can loop through data to reach a result.

The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In this example, tri_recursion() is a function that we have defined to call itself ("recurse").

We use the k variable as the data, which decrements (-1) every time we recurse.

The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.

def plus_one(a):
    if a < 3:
        return plus_one(a + 1)
    return a

print(plus_one(10))

RecursionError

max recursion is 996

maximum recursion depth exceeded while calling a Python object.