How to use def in python

How to Call a Function in Python – Def Syntax Example

Kolade Chris

In Python and other programming languages, you can use functions to avoid repeting yourself and to reuse pieces of code.

To make functions work, you don’t just write them and say goodbye – you have to call them too.

Before you call a function, you need to write it with the def keyword. So in this article, I will not just show you how to call a function, I will also show you how to create it.

What We’ll Cover

How to Define a Function with the def Keyword

To define a function in Python, you type the def keyword first, then the function name and parentheses.

To tell Python the function is a block of code, you specify a colon in front of the function name. What follows is what you want the function to do.

The basic syntax of a function looks like this:

def function_name(): # What you want the function to do 

An example of a function looks like this:

def learn_to_code(): print("You can learn to code for free on freeCodeCamp") 

What we want this function to do is to print the text You can learn to code for free on freeCodeCamp to the terminal.

To make this function run, you have to call it. That’s what we’ll do next.

How to Call a Function in Python

To call a function, you write out the function name followed by a colon.

The syntax for calling a function looks like this:

To call a function we defined earlier, we need to write learn_to_code() :

def learn_to_code(): print("You can learn to code for free on freeCodeCamp") learn_to_code() # Output: You can learn to code for free on freeCodeCamp 

N.B: Make sure you don’t specify the function call inside the function block. It won’t work that way because the call will be treated as a part of the function to run.

You can see the function didn’t print the text to the terminal because I attempted to call it inside the function block.

And here you can see the function runs because I called it outside the function block.

How to Call a Nested Function in Python

It can be confusing to call a nested function, so I want to show you how to do it.

Below is the nested function:

def learn_to_code(): print("You can learn to code for free on freeCodeCamp") def learn_what_language(): print("You can learn any programming language on the freeCodeCamp YouTube channel") 

The learn_what_language function is a part of the learn_to_code function because it is nested inside it.

If you type learn_to_code() and run the code, only the outer function (learn_to_code) gets called:

You can see that only the outer function gets called and the inner function is greyed out.

To call the inner function too, you should type learn_what_language() precisely. But where?

You should look right under the def keyword of the inner function and type the function call there.

But if you do this only, it won’t still work because you have to call the outer function too.

You can see the inner function ( learn_what_language ) still didn’t do what we want it to do.

To make it work, you have to call both functions where necessary:

def learn_to_code(): print("You can learn to code for free on freeCodeCamp") def learn_what_language(): print("You can learn any programming language on the freeCodeCamp YouTube channel") learn_what_language() learn_to_code() """ Output: You can learn to code for free on freeCodeCamp You can learn any programming language on the freeCodeCamp YouTube channel """ 

You can see everything works as expected.

Final Thoughts

I hope this article helps you learn how to properly call a function in Python.

If you want to learn Python more, you can check out the freeCodeCamp Python curriculum. It’s free.

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Python 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:

Example

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()

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»)

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)

Example

This function expects 2 arguments, but gets only 1:

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

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»)

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»)

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:

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»)

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)

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»]

Return Values

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

Example

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

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

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»)
tri_recursion(6)

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