What is a Python function?

A Python 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.

How do we create this functions?

In Python a function is defined using the def keyword:

Example

def my_function():
    print("Enjoy functions")

In-built Functions.

These are defined as functions whose functionality is predefined in python. The python interpreter has several functions that are always readily available for use whenever the need arises. Some examples of these built-n functions are;

Python abs() function.

This function is used to return the absolute value of a number. It only takes one argument and that is the number whose absolute value is to be returned. The argument can either be an integer or a floating point number. If the argument happens to be a complex number, then the abs() function returns its magnitude.
The abs() function, furthermore, ignores a negative sign that precedes a number. This is to mean that if the number is for example -4 abs() returns 4. Returns positive numbers.

# Integer.
a = -34
print(abs(a))
# prints 34.
# floating point.
 b = -45.67
print(abs(b))
#prints 45.67

Python float() function.

This is a built in python function that is used to convert a string or integer to a float point value. The float() function takes one parameter, that is the value you want to convert to a float. Passing on the parameter in this case is optional. The default value is 0.0 .
Converting an integer to a float in python is achieved as in the following example.

a = 45
print(float(a))
# prints 45.0
And converting a string, which is nothing but just a sequence of characters, is don as in the example below.
b = '32'
print(float(b))
# prints 32.0

Python sum() function.

This is a python function that sums up all the numerical values in an iterable such as a list and returns the total of such values. The sum() function calculates the total of both floating point numbers and integers.
The sum() function can take two parameters;
i. The iterable object that you would like to calculate the total of, this parameter is required.
ii. An extra number you want to add to the value you are calculating, this is optional.

a = [1,2,3,4]
total = sum(a)
print(total)
# prints 10
# two parameters.
a = [1,2,3,4]
total = sum(a, 1)
print(total)
# prints 11.

User Defined Functions.

As the name suggests, these are created by the user. These functions are created in a certain way unlike the in-built functions. User defined functions involves the use of keyword. The def keyword is used to create these functions. It is followed by a function name declared by the user. The function name is subject to the rules governing the naming of variables.
The following is a simple syntax to define function;

def hello():
    print("Hello world!")
# prints hello world

The above code is a simple function with no parameters and arguments passed.

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 (fname). When the function is called, we pass along a first name, which is used inside the function to print the full name:

Example

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

my_function("Brian")
my_function("Junior")
my_function("Lin")

Parameters and Arguments? Let's get started.

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

def my_function(fname):
#**fname** represents a parameter

An Argument?

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

my_function("Brian")
#**Brian** represents the argument.

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.

Example

This function expects 2 arguments, and gets 2 arguments:

def my_function(fname, lname):
    print(fname + " " + lname)

my_function("Brian", "lux")

Note:

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

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:

Example

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("John", "Andy", "Lin")

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:

Example:

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:

Example:

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

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

Return Values

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

Example:

def my_function(x):
    return 5 * x

print(my_function(3))
print(my_function(5))
print(my_function(9))

The 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.

Example:

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.

Recursion Example

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

print("\n\nRecursion Example Results")

Return Values

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

def sum(x):
  return 4 + x

print(sum(1)) # 5
print(sum(3)) # 7
print(sum(10)) # 14