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Note that Python 3.6.10 cannot be used on Windows XP or earlier. No files for this release. Python 3.5.9 - Nov. Note that Python 3.5.9 cannot be used on Windows XP or earlier. No files for this release. Python 3.5.8 - Oct. Note that Python 3.5.8 cannot be used on Windows XP or earlier. No files for this release. Start learning Python with the w3schools course and lay the foundations of your Programming skills. Python is a popular object oriented programming language. This is a structured and interactive version of the w3schools Python Tutorial together with the w3schools certification.

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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:

Example

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

Calling a Function

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

Example

def my_function():
print('Hello from a function')
my_function()
Try it Yourself »

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('Emil')
my_function('Tobias')
my_function('Linus')
Try it Yourself »

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.

Example

This function expects 2 arguments, and gets 2 arguments:

def my_function(fname, lname):
print(fname + ' ' + lname)
my_function('Emil', 'Refsnes')
Try it Yourself » If you try to call the function with 1 or 3 arguments, you will get an error:

Example

This function expects 2 arguments, but gets only 1:

def my_function(fname, lname):
print(fname + ' ' + lname)
my_function('Emil')
Try it Yourself »

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('Emil', 'Tobias', 'Linus')
Try it Yourself »

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

Keyword Arguments

You can also send arguments with the key = value syntax.

This way the order of the arguments does not matter.

Example

def my_function(child3, child2, child1):
print('The youngest child is ' + child3)
my_function(child1 = 'Emil', child2 = 'Tobias', child3 = 'Linus')
Try it Yourself »

The phrase Keyword Arguments are often shortened to kwargs 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:

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Example

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['lname'])
my_function(fname = 'Tobias', lname = 'Refsnes')
Try it Yourself »

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

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')
Try it Yourself »

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)
Try it Yourself »

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))
Try it Yourself »

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

Try it Yourself »

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.

Example

Recursion Example

def tri_recursion(k):
if(k > 0):
result = k + tri_recursion(k - 1)
print(result)
else:
result = 0
return result
print('nnRecursion Example Results')
tri_recursion(6)
Try it Yourself »

Python is a programming language.

Python can be used on a server to create web applications.

Start learning Python now »

Learning by Examples

With our 'Try it Yourself' editor, you can edit Python code and view the result.

Example

Try it Yourself »

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Click on the 'Try it Yourself' button to see how it works.

Python File Handling

In our File Handling section you will learn how to open, read, write, and delete files.

Python Database Handling

In our database section you will learn how to access and work with MySQL and MongoDB databases:

Python Exercises

Python Examples

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Learn by examples! This tutorial supplements all explanations with clarifying examples.

Python Quiz

Test your Python skills with a quiz.

Python Reference

You will also find complete function and method references:

Download Python

Download Python from the official Python web site: https://python.org


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