Understanding functions in python

Understanding Functions in Python

In this Python tutorial, we will discuss, what is a function in Python, Python function syntax, and how to define a function in Python. Also, I will show you how to call a function in Python.

What is a Function in Python?

In Python, a function is a reusable block of code that performs a specific task. Functions allow you to break down your program into smaller, modular chunks. This results in cleaner, more readable code that is easier to maintain and debug.

Syntax of a Function in Python

To define a function in Python, you use the def keyword, followed by the function name, parentheses, and a colon. The code block within a function is indented.

def function_name(parameters): # code return # optional return statement 

• def: Keyword that informs Python that you’re defining a function.
• function_name: The name you assign to your function.
• parameters: Optional. Values you can pass into the function.
• return: Optional. Use this statement to pass a value back from the function.

How to Define a Function in Python

Let’s create a simple function named greet that prints a greeting message:

def greet(name): print(f"Hello, !") 

Here, greet is the function name, and name is the parameter it accepts.

How to Call a Function in Python

After defining a function, you can call it by using its name followed by parentheses. You can pass values (arguments) inside these parentheses if the function expects parameters.

Let us see, how we can call the above function.

greet("Alice") # This will output "Hello, Alice!" 

Return Statement

A function can return a value using the return statement. Once a return statement is executed, the function terminates and passes the specified value back to the caller.

def add(a, b): sum = a + b return sum result = add(2, 3) # result will be 5 print(result) 

Real Example of a Python Function

Let us check a complete example of how to define and call a function in Python.

In this example, let’s define a function called greet_names that accepts a list of names and returns a list of personalized greeting messages.

def greet_names(names): """ Create greeting messages for a list of names. :param names: A list of names. :return: A list of personalized greeting messages. """ # List to store the greeting messages greetings = [] # Loop through each name in the names list for name in names: # Create a personalized greeting message greeting = f"Hello, ! Welcome to the USA!" # Append the greeting message to the greetings list greetings.append(greeting) # Return the list of greeting messages return greetings 

Now, let’s call this function with a list of names and print the personalized greeting messages.

# Define a list of names names = ["Alice", "Bob", "Charlie", "Diana"] # Call the function and store the returned greeting messages greeting_messages = greet_names(names) # Print each greeting message for message in greeting_messages: print(message) 

Now, when you run the above code, you can see the output like in the below screenshot.

Common Built-in Functions

Python provides a host of built-in functions. Here’s a table with some of the commonly used ones:

Function	Description
len(s)	Returns the length of an object s .
max(iterable)	Returns the largest item in an iterable.
min(iterable)	Returns the smallest item in an iterable.
sum(iterable)	Returns the sum of all items in an iterable.
type(obj)	Returns the type of an object obj .
sorted(iterable)	Returns a sorted list of the specified iterable.
round(number, ndigits)	Rounds a number to a specified number of decimals.
print(*objects, sep=’ ‘, end=’\n’, file=sys.stdout, flush=False)	Prints the specified message to the screen.
input(prompt)	Allows user input.

Conclusion

In this Python tutorial, I have explained, how to define a function in Python, and how to call a function in Python. Also, we saw a few examples of built-in Python functions.

I am Bijay Kumar, a Microsoft MVP in SharePoint. Apart from SharePoint, I started working on Python, Machine learning, and artificial intelligence for the last 5 years. During this time I got expertise in various Python libraries also like Tkinter, Pandas, NumPy, Turtle, Django, Matplotlib, Tensorflow, Scipy, Scikit-Learn, etc… for various clients in the United States, Canada, the United Kingdom, Australia, New Zealand, etc. Check out my profile.

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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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