Functions

Anonymous contributor's avatar
Anonymous contributor
Published May 6, 2021Updated May 13, 2025
Contribute to Docs

A function in Python is a block of code that allows tasks to be performed multiple times within a program without having to be rewritten. It helps in reducing code duplication, improving readability and organization, dividing a program into smaller chunks, and facilitate debugging and testing.

Defining a Function

Python functions are defined with the def keyword followed by the name of the function and parentheses:

# Define a function
def hello():
  print("Hello, world!")


# Call the function
hello()

In this example, the hello() function prints the given message to the console.

Here is the output:

Hello, world!

Defining a Function with Parameters

Python functions can be defined with parameters, which help in passing data to the function:

# Define a function
def hello(name):
  print(f"Hello, {name}!")


# Call the function
hello("Robin")

In this example, a name is passed to the function as an argument, which is stored in the name parameter (variable).

Here is the output:

Hello, Robin!

Note: Function names in Python are written in snake_case.

Returning Values from a Function

In Python, a value can be returned from a function in two ways:

  • Using the return keyword
  • Using the yield keyword

Returning Values Using return

The return keyword is used to return a value from a Python function. The value returned can then be stored in a variable and used in the program.

In this example, the check_leap_year() function returns a string that indicates if the given year is a leap year or not:

# Define a function
def check_leap_year(year):
  if year % 4 == 0:
    return str(year) + " is a leap year."
  else:
    return str(year) + " is not a leap year."


# Store the year to check in a variable
year_to_check = 2018


# Call the function
returned_value = check_leap_year(year_to_check)


print(returned_value)

The resulting output will look like this:

2018 is not a leap year.

Returning Values Using yield

A function can also return values with the yield keyword. Like return, yield suspends the function’s execution and returns the value specified. Unlike return, the yield statement retains the state of the function and will resume where it left off on the next function call (i.e., execution resumes after the last yield statement). This way, the function can produce a number of values over time.

Functions using yield rather than return are known as generator functions. Such a function can be used as an iterator.

This example will automatically generate successive Fibonacci numbers:

# Function to produce infinite Fibonacci numbers
def fibonacci():
  # Generate first number
  a = 1
  yield a


  # Generate second number
  b = 1
  yield b


  # Infinite loop
  while True:
    # Return sum of a + b
    c = a + b
    yield c


    # Function resumes loop here on next call
    a = b
    b = c


# Iterate through the Fibonacci sequence until a limit is reached
for num in fibonacci():
  if num > 50:
    break


  print(num)

Here is the output:

1
1
2
3
5
8
13
21
34

Python Library Functions

Python library functions are built-in functions that are always available for use without needing to define them explicitly. These functions are a part of the Python standard library and help perform common tasks like mathematical operations, type conversions, string manipulations, and more.

Some examples for Python library functions are:

Example: Using the len() Function

The len() function is a commonly used library function in Python that returns the total number of elements in an object like a string, list, tuple, etc.

# Define a list
my_list = [10, 20, 30, 40]


# Return the number of items in the list
print(len(my_list))

Here is the output:

4

Higher-Order Functions

In Python, functions are treated as first-class objects. This means that they can be assigned to variables, stored in data structures, and passed to or returned from other functions.

Functions are considered to be higher-order values because they can be used as parameters or return values for other functions.

Example: Using the filter() Function

The built-in filter() function in Python is used to filter elements from an iterable (like lists, tuples, or sets) that satisfy a specific condition:

# Function that returns True if n is a perfect square, and False otherwise
def is_perfect_square(n):
  return (n ** 0.5).is_integer()


# Store the numbers to check in a variable
numbers = [3, 4, 37, 9, 7, 32, 25, 81, 79, 100]


# Call the function
perfect_squares = filter(is_perfect_square, numbers)


print(list(perfect_squares))

filter() takes a predicate (a function that returns a boolean value) and an iterable, and returns a new iterable containing all elements of the first one that makes the predicate True.

Here is the output:

[4, 9, 25, 81, 100]

Codebyte Example: Calculate the Area of a Rectangle

This codebyte example defines a function to calculate the area of a rectangle with the help of its length and width:

Code
Output
Loading...


Frequently Asked Questions


1. What’s the difference between a parameter and an argument in a Python function?


    

  • A parameter is a variable used in the function definition.
    • 

  • An argument is the value passed to the function when it’s called.
    • 



2. What happens if you call a function before defining it?


Python will raise a NameError because the function must be defined before it is called.


3. What is a default parameter in a Python function?


A default parameter in a Python function is a value that is used if no argument is provided:


def hello(name="Guest"):
  print(f"Hello, {name}!")




Here, the given value will be used for the name parameter (variable) if no argument is passed to the function.


Functions
Anonymous Functions
Defines a function without a name using the lambda keyword.
Arguments/Parameters
Supplies data to a defined function when it is called in a program.
All contributors
Contribute to Docs
Learn Python on Codecademy