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.

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1. Define function (def) and Call function

In Python a function is defined using the def keyword. To call a function, use the function name followed by parenthesis.

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

# Calling the function
my_function()

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2. Arguments (positional, keyword)

Information can be passed into functions as arguments. Arguments are specified after the function name, inside the parentheses.

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

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

Keyword Arguments (kwargs)

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")

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3. Default arguments

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") # Prints Sweden
my_function()         # Prints Norway (default)

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4. Variable-length arguments (*args, **kwargs)

If you do not know how many arguments that will be passed into your function, you can use variable-length arguments.

Arbitrary Arguments, *args

Add a * before the parameter name. The function will receive a tuple of arguments.

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

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

Arbitrary Keyword Arguments, **kwargs

Add two asterisks ** before the parameter name. The function will receive a dictionary of arguments.

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

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

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5. Return values

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

def my_function(x):
    return 5 * x

print(my_function(3)) # 15
print(my_function(5)) # 25

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6. Pass statement in functions

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

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7. 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.

def tri_recursion(k):
    if(k > 0):
        result = k + tri_recursion(k - 1)
        print(result)
    else:
        result = 0
    return result

print("Recursion Example Results")
tri_recursion(6)