Defining and Using Functions in Python

Learn about functions in Python, including how they run when called, accept parameters, and return results. Discover the basics of creating functions using the def keyword with practical examples to help you master function definition and usage.

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:

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

Calling a Function

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

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

my_function()
            

Hello from a 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 (name). When the function is called, we pass along a name, which is used inside the function to print a greeting:

def my_function(name):
  print("Hello, " + name)

my_function("Alice")
my_function("Bob")
my_function("Charlie")
            

Hello, Alice
Hello, Bob
Hello, Charlie
            

Parameters or Arguments?

The terms parameter and argument can be used for the same thing: information that is 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.

def my_function(first_name, last_name):
  print(first_name + " " + last_name)

my_function("Alice", "Smith")
            

Alice Smith
            

If you try to call the function with 1 or 3 arguments, you will get an error:

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

my_function("Emil")
          

Traceback (most recent call last):
  File "demo_function_args_error.py", line 4, in 
    my_function("Emil")
TypeError: my_function() missing 1 required positional argument: 'lname'
        

Arbitrary Arguments, *args

If you do not know how many arguments 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:

def my_function(*names):
  print("The second name is " + names[1])

my_function("Alice", "Bob", "Charlie")
            

The second name is Bob
            

Keyword Arguments

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

This way the order of the arguments does not matter.

def my_function(child1, child2, child3):
  print("The oldest child is " + child1)

my_function(child1 = "Alice", child2 = "Bob", child3 = "Charlie")
            

The oldest child is Alice
            

Arbitrary Keyword Arguments, **kwargs

If you do not know how many keyword arguments will be passed into your function, add two asterisks: ** before the parameter name in the function definition.

This way the function will receive a dictionary of arguments, and can access the items accordingly:

def my_function(**kid):
  print("His last name is " + kid["last_name"])

my_function(first_name = "Charlie", last_name = "Brown")
            

His last name is Brown
            

Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without an argument, it uses the default value:

def my_function(country = "USA"):
  print("I am from " + country)

my_function("Canada")
my_function("India")
my_function()
my_function("Brazil")
            

I am from Canada
I am from India
I am from USA
I am from Brazil
            

Passing a List as an Argument

You can send any data type 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:

def my_function(food):
  for x in food:
    print(x)

fruits = ["kiwi", "pear", "peach"]

my_function(fruits)
            

kiwi
pear
peach
            

Return Values

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

def my_function(x):
  return 7 * x

print(my_function(2))
print(my_function(4))
print(my_function(6))
            

14
28
42
            

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.

def my_function():
  pass
            

Positional-Only Arguments

You can specify that a function can have ONLY positional arguments, or ONLY keyword arguments.

To specify that a function can have only positional arguments, add , / after the arguments:

def my_function(x, /):
  print(x)

my_function(3)
            

3
            

Keyword-Only Arguments

To specify that a function can have only keyword arguments, add *, before the arguments:

def my_function(*, x):
  print(x)

my_function(x = 3)
            

3
            

Combine Positional-Only and Keyword-Only

You can combine the two argument types in the same function.

Any argument before the /, are positional-only, and any argument after the *, are keyword-only.

def my_function(a, b, /, *, c, d):
  print(a + b + c + d)

my_function(2, 3, c = 5, d = 7)
            

17
            

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 n 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, the best way to find out is by testing and modifying it.

def tri_recursion(n):
  if(n > 0):
    result = n + tri_recursion(n - 1)
    print(result)
  else:
    result = 0
  return result

print("\n\nRecursion Example Results")
tri_recursion(4)
            

1
3
6
10