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Python is a convenient language that’s often used for scripting, data science, and web development.

In this article, we’ll look at how to define and use Python functions.

Functions

A function is a piece of code that we can invoke repeatedly in different places.

It’s defined with the def keyword as follows:

def greet(name):  
  print('Hello', name)

The greet function above has one parameter. We can pass in an argument to set the value of the parameter when we call it.

Functions always start with the def keyword, then the function name, then parentheses and zero or more parameters inside. Then the first line ends with a colon.

The code for the function is indented inside the function block.

For example, we can call greet as follows:

greet('Joe')

Then we see:

Hello Joe

displayed on the screen since we passed in 'Joe' as the argument of the greet function call.

Functions can call other functions. For example, we can write the following code to make our greet function call another function:

def greet(first_name, last_name):  
  print('Hello', full_name(first_name, last_name))

def full_name(first_name, last_name):  
  return '%s %s' % (first_name, last_name)

greet('Jane', 'Smith')

In the code above, our greet function calls the full_name function which returns the full name constructed by combining first_name and last_name .

In the full_name function, we use the return keyword to return the computed result of combining the first_name and last_name parameters into one string.

The function ends whenever we use the return keyword. Nothing below it will run.

Therefore, we can use it as follows to return the values that we want to return conditionally by using multiple return statements as follows:

import random

def crystal_ball(num):  
  if num == 1:  
    return 'It is a great day'  
  elif num == 2:  
    return 'It is a lucky day'  
  elif num == 3:  
    return 'It is an auspicious day'

r = random.randint(1, 4)  
fortune = crystal_ball(r)  
print(fortune)

In the code above, we have if statements to return something whenever the value of num is 1, 2, or 3.

If num is 1, then crystal_ball returns 'It is a great day' .

If num is 2, then crystal_ball returns 'It is a lucky day' .

If num is 3, then crystal_ball returns ‘It is an auspicious day’ .

Once something is returned, crystal_ball function stops running.

We can then assigned the returned value to another variable for storage as we did by writing:

fortune = crystal_ball(r)

Then we printed out the value of fortune .

The None Value

In Python, we have the value None to represent no value. None has the type NoneType .

None has the capital N .

We can use it as a return value of something that shouldn’t have a value.

For example, the print returns None since there’s nothing to return. It just prints a value on the screen.

If we write:

foo = print('Hello!')  
print(None == foo)

In the code above, we should see:

Hello!  
True

printed on the screen since print returns None , so the value that’s assigned to foo would be None .

Therefore, None == foo returns True .

Keyword Arguments

Python functions can have named arguments. This way, we know what values the arguments are set when we’re passing them.

For example, we can pass in named parameters as follows:

def full_name(first_name, last_name):
  return '%s %s' % (first_name, last_name)

print(full_name(first_name='Jane', last_name='Smith'))

In the code, we called full_name by writing:

full_name(first_name='Jane', la_name='Smith')

Now we know that we’re passing 'Jane' as the value of the first_name parameter and 'Smith' as the value of the last_name parameter.

The Call Stack

The call stack is a data structure that tells us what function we’ve called in the order they’re called.

The earliest called function is at the bottom of the stack, and the later ones are higher in the stack.

For example, in the example we have earlier:

def greet(first_name, last_name):
  print('Hello', full_name(first_name, last_name))

def full_name(first_name, last_name):
  return '%s %s' % (first_name, last_name)

greet('Jane', 'Smith')

Our call stack would have the greet function at the bottom and full_name at the top.

Local and Global Scope

Variables that are inside a function has a local scope. It’s only available inside the function and nested function inside it.

They can’t be referenced outside the function.

Anything at the top level of a Python file has global scope.

Variables with global scope can be accessed inside anything located in a lower function like a block or function.

For example, if we have:

x = 1

def show_x():
  print(x)

show_x()

Then we can reference x inside the show_x function as x has global scope.

On the other hand, if we have:

def show_x():  
  x = 1

print(x)

We’ll get an error saying x isn’t defined.

Having scopes lets us narrow down the scope of the code that can cause a bug. We don’t have to search through the whole program to troubleshoot everything.

Conclusion

Functions are used to organize code into reusable pieces. They take parameters and we call them by passing in arguments.

They can return something with the return keyword. Returned values from a function can be assigned to other variables.

Variables inside functions have a local scope. They can’t be accessed from outside the function.

On the other hand, variables at the top level have global scope, which can be accessed inside functions.