How to Define a Function in Python? (2023)

What is a Function?

In programming, a piece of code that executes a specific task or a group of related operations is known as a function.

What does Python Functions Do?

If you frequently reuse the same code block, it is preferred to write a comparable function. Next time when you require this piece of code, call the function.

User-defined functions have several significant advantages:

  • When you need to complete the same task regularly, functions come in handy. Instead of repeatedly entering or copying and pasting the same code snippet, you may just call a function once it has been defined. Additionally, if you choose to alter a code block, you must alter the portion where the function is defined. Anywhere the function is invoked, this modification will take effect.
  • You can divide complicated programs into smaller steps by defining and using functions. Each stage may serve a distinct purpose in solving a particular job.
  • Finally, your code is typically simpler to understand when using user-defined functions. Each code block's function is readily visible, indicating which task it completes. This is especially true if functions are defined in accordance with the best practises later in the article.

Python Function Definition: Syntax and Samples

The following is the syntax for declaring a function in Python.


def function_name(arguments) block of code

The syntax is described as follows:

  • To tell Python that a new function is being defined, we begin by using the def keyword.
  • Next, we give our function a name that makes sense.
  • The function's required arguments or parameters are then listed within parentheses. There is no restriction on how many factors you can use. By creating a function without parameters, the parentheses may also be omitted.
  • The colon character indicates a transition from the function's header to its body ":"
  • We write the code block required to complete the task after the indentation.
  • We leave an empty line after the code block to indicate that the function definition is complete.

You should practice function definition now that you are familiar with the syntax.

Works With Just One Parameter

Let us start with a straightforward function that accepts a name as an argument and prints out the first line of the introduction. How to define this function is as follows:

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def introduction(name):“ “ “ Introduces a person given their name” “ “print('Hello! My name is, name)

The introduction is a function that takes a single argument, name. Only one line in the function's body prints the message along with the name. Additionally, we added a note that outlines the goal of our function.

Let's call it now and evaluate the results:



Hello! My name is, rohan

Because the name is a string, we must supply it to our function in quotation marks. The output is the welcome message.

If a function is used frequently, even a simple one-line function can save us time. So, consider the advantages of lengthier code blocks!

Without Parameter Functions

You should define functions without any arguments in various circumstances. Creating a function that displays a message telling the user if their passwords is incorrect will serve as an example.

def error_message_password():"Prints out an error message about the incorrect password" print('Sorry. We couldn’t log you in. The password you entered didn’t match our records.')

There are no parameters necessary for this function. Therefore, we define it without using any parenthesis. This function is invoked by writing its name followed by a pair of empty parentheses:


(Video) Defining a Function in Python | Python Tutorial for Beginners


Sorry. We couldn’t log you in. The password you entered didn’t match our records

Multiple Parameter Functions

Let's now define a function with more parameters that are more complicated. This time, depending on a person's birth date, we want to develop a function that would estimate their age in years (birth year, birth month, and birthday).

From datetime import datedef get_age(birth_year, birth_month, birth_day): "Calculates age (in years) based on birth year, birth month, and birthday.""" birth_date = date(birth_year, birth_month, birth_day) age = - birth_date age_years = age.days / 365.2425 return age_years

We begin by importing the date class from the datetime module to work with dates conveniently.

The definition of our get age function is now complete. Be aware that the birth year, birth month, and birthday inputs are all required for this function. The birth date is initially defined in the get age function's body using the supplied data.

The age in days is then determined by deducting the birthdate from the current date. The age in years is finally determined by dividing the age in days by 365.2425, the typical number of days in a year.

The function specification ends with a return statement. In the previous instances, the print() function, which let us see each function's output in our earlier examples, was used. But most of the time, you need the function to return a specific value rather than necessarily print it out.

The return statement is then applicable. Use the return keyword to specify the return value, that can be nearly any Python object (such as an integer, text, list, tuple, dictionary, set, etc.).

Let's now invoke our get age() function with several inputs. Regarding passing the arguments, we have two choices:

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Positional justifications. This entails passing the arguments in the order that the function definition's parameters appear:




Keyword Arguments: The arguments are passed in an arbitrary order, and each parameter is specified by name:

get_age(birth_day=2, birth_month=9, birth_year=1987)



The order of the arguments is not important when using keyword arguments, but you must type the exact names of every parameter.

Also, remember that the number of arguments and parameters in the function definition must match regardless of the option you select.

You can see from the output above that the getting age() function responds to the return statement's request for the age in years. But perhaps we should assign this value to a variable rather than just print it out.

kate_age = get_age(1987, 9, 2)print(kate_age)


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The number produced by the getting age() function is now stored in the Kate age variable.

Functions That Produce No Output

Every Python function produces a value. If you don't use the return statement explicitly in Python, it will be provided implicitly with nothing at all as the return value.

In some cases, you might like your function to perform certain tasks without returning any data.This was evident in the first two instances. We were printing the function's output without explicitly returning a value.

Let's now assign the output of our error message password() method to a variable and examine its value:

error = error_message_password()print(error)



As you've seen, the error variable actually stores the value None instead of the error message that you might expect. This is due to the fact that we printed the following error rather than explicitly returning its value in our password() function's error message.

Therefore, pay great attention and make sure the return statement is always present if you would like your procedure to return a value other than None.

Best Practices for Developing Python Functions

Let’s see some best practices for Python function definition:

(Video) Python Programming #12 - Defining and Creating Functions

  • Give your function names some meaning. Your functions ought to be named in a way that accurately describes the expected duties. This makes your code easier for you to read when you return to it later, as well as for other programmers who are working with it. Additionally, adhere to the Python conventions for function names, which only allow lowercase letters and underscores between words.
  • Give each function just one job to do. One task should ideally be handled by one function. This promotes clean code organization and increases readability.
  • Specify your thoughts on the function task. Right beneath the heading, add a succinct description of what the function does. This comment should be included in triple-double quotes ("") and is known as a docstring.
  • You should follow these guidelines to make your code look neat and polished.


How do you define a function in Python? ›

The four steps to defining a function in Python are the following: Use the keyword def to declare the function and follow this up with the function name. Add parameters to the function: they should be within the parentheses of the function. End your line with a colon. Add statements that the functions should execute.

How can we define a function? ›

A function is defined as a relation between a set of inputs having one output each. In simple words, a function is a relationship between inputs where each input is related to exactly one output. Every function has a domain and codomain or range.

What is the correct way to define a function in Python 3? ›

A function is defined by using the def keyword, followed by a name of your choosing, followed by a set of parentheses which hold any parameters the function will take (they can be empty), and ending with a colon.

How do I create a function in #define? ›

A function consist of two parts:
  1. Declaration: the function's name, return type, and parameters (if any)
  2. Definition: the body of the function (code to be executed)

How do you define a function in a variable? ›

Definition: A function is a mathematical relationship in which the values of a single dependent variable are determined by the values of one or more independent variables. Function means the dependent variable is determined by the independent variable(s).

Can we define our own functions in Python? ›

Functions that readily comes with Python are called built-in functions. Python provides built-in functions like print(), etc. but we can also create your own functions. These functions are known as user defines functions.

How to call a function Python? ›

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

How do you define a function in Pycharm? ›

Extracting a method
  1. In the editor, select a block of code to be transformed into a method or a function. ...
  2. From the main menu or from the context menu, select Refactor | Extract | Method or press Ctrl+Alt+M .
  3. In the Extract Method dialog that opens, specify the name of the new function.
Sep 28, 2022

What are the three steps to define the function? ›

It demonstrates all three steps: declare a function, define it, and call it.

Can I use #define in a function? ›

You can use #define anywhere you want. It has no knowledge of functions and is not bound by their scope.

How do I create a custom function? ›

How to create a custom function in Excel
  1. Open VBE by pressing Alt+F11 on a PC or FN+ALT+F11 on a Mac.
  2. Locate "Insert."
  3. Select "Module."
  4. Type "Function," then specify what function you want to use.
  5. Confirm Excel automatically included "End Function."
  6. Update the code with any arguments and value specifications.
Apr 25, 2022

How do you define a function in pandas Python? ›

You define a pandas UDF using the keyword pandas_udf as a decorator and wrap the function with a Python type hint.
Iterator of Series to Iterator of Series UDF
  1. The Python function. ...
  2. The length of the entire output in the iterator should be the same as the length of the entire input.
Nov 15, 2022

How to define a variable in Python? ›

Python has no command for declaring a variable.
Thus, declaring a variable in Python is very simple.
  1. Just name the variable.
  2. Assign the required value to it.
  3. The data type of the variable will be automatically determined from the value assigned, we need not define it explicitly.
Mar 10, 2021

How do you call a function as a variable in Python? ›

To use functions in Python, you write the function name (or the variable that points to the function object) followed by parentheses (to call the function). If that function accepts arguments (as most functions do), then you'll pass the arguments inside the parentheses as you call the function.

What are the 4 types of functions in Python? ›

The following are the different types of Python Functions:
  • Python Built-in Functions.
  • Python Recursion Functions.
  • Python Lambda Functions.
  • Python User-defined Functions.
Sep 23, 2022

How do you define a function in Python with default value concept? ›

Python has a different way of representing syntax and default values for function arguments. Default values indicate that the function argument will take that value if no argument value is passed during function call. The default value is assigned by using assignment (=) operator.

What is function in Python with example? ›

In Python, standard library functions are the built-in functions that can be used directly in our program. For example, print() - prints the string inside the quotation marks. sqrt() - returns the square root of a number. pow() - returns the power of a number.

How do you call a function in code? ›

You call the function by typing its name and putting a value in parentheses. This value is sent to the function's parameter. e.g. We call the function firstFunction(“string as it's shown.”);

How do I call a function? ›

The most common way is simply to use the function name followed by parentheses. If you have a function stored in a variable, you can call it by using the variable name followed by parentheses. You can also call functions using the apply() or call() methods.

How do you define a function in Numpy Python? ›

Define the function as usually using the def keyword. Add this function to numpy library using frompyfunc() method.
Example :
  1. Create function with name fxn that takes one value and also return one value.
  2. The inputs are array elements one by one.
  3. The outputs are modified array elements using some logic.
Nov 2, 2021

What is the definition of function with example? ›

A special relationship where each input has a single output. It is often written as "f(x)" where x is the input value. Example: f(x) = x/2 ("f of x equals x divided by 2") It is a function because each input "x" has a single output "x/2": • f(2) = 1.

Which keyword is used to define a function *? ›

The def keyword is used to create, (or define) a function.

Why do we need to define a function? ›

Quite often when carrying out a project, some set of tasks will need to be repeated with different input. To ensure that the analyses are carried out the same way each time, it is important to wrap the code into a named function that is called each time it's needed.

How do you create a function in DAX? ›

How to write a DAX formula in Power BI? DAX formulas are entered into the formula bar just below the ribbon in Power BI. Start by giving the formula a name and then follow with the equal-to sign (“=”). Then write in your formula using functions, constants, or strings.

How do you create define a function in JavaScript? ›

A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses (). Function names can contain letters, digits, underscores, and dollar signs (same rules as variables). The parentheses may include parameter names separated by commas: (parameter1, parameter2, ...)

How do I create a user-defined function in R? ›

Creating a User-Defined Function
  1. userDefinedFunction <- function(x){
  2. result <- x * 10.
  3. print(result)
  4. }
  5. x <- 385.
  6. userDefinedFunction(x)
Dec 24, 2021

How do you declare define and call a function? ›

Function Declarations

The actual body of the function can be defined separately. int max(int, int); Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.


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