All computer programming languages allow developers to seperate code into reuseable pieces of code called functions. Functions are critical because they allow us to generalize pieces of work into a block of code and reuse that code as many times as needed. When designed well, functions improve code readability by cutting down on the length of the code. We can also debug our code easier because we only have to look in on place for a bug rather than several places.


Let’s begin with a function demonstration.

# A function
def nested_func():
    print('Inside of nested_func()')
    return 'value'

# A function
def func():
    print('Inside of func()')

    val = nested_func()
    print('Back in func(). nest_func() returned {}'.format(val))

# Not a function
if __name__ == '__main__':
    print('Outside of all functions. Calling func()')
    print('Back from our functions')

This is a block of code that creates two functions. When we run the code, we get this output.

Outside of all functions. Calling func()
Inside of func()
Inside of nested_func()
Back in func(). nest_func() returned value
Back from our functions

Computer programs normally run from top to bottom one line at a time. In the case of this program, the program doesn’t start until we reach if __name__ == '__main__':. This is because Python runtime isn’t going to execute the code inside of nest_func() or func() until the functions are called.

When the code reaches 17, it executes the print statement. The next line, 18, is our first call to a function. We named our function func() in this case. Calling func() causes the program’s execution to jump up to line 9. Once we are at line 9, Python executes the print statement and then moves onto the next line in the function, line 11.

Line 11 creates a variable called val and then calls our next function, nested_func(). The nested_func() function is a function that returns a value. Program execution moves to line 3. Line 3 executes the print statement, and then line 4 returns a String value. The program execution returns back to line 11.

At this point, the variable val has a value stored in it. The program’s execution goes to line 12 and the print statement is executed. Now the program exits the func() function and control returns to line 19. The program executes the final print statement found on line 19 and then exits.

Defining a function

You create functions in Python by using the def keyword followed by the name of the function. After the name of the function, you have an opening parentheses ( followed by a closing parenthese ). You can place any number of variables inside of the parentheses. Here is an example

# Function with arguments
def func(val, val2):

# Calling the function
func('Hello', 'World')

The name of this function is func. It has two arguments, val and val2. After the colon, you can include any number of statements you would like inside of the function. The function is now a seperate unit of code at this point. This function get’s called by func('Hello', 'World'). Anytime the Python interpreter something like this, it will execute all of the statements inside of func. We do not have to use ‘Hello’ or ‘World’ as the argument either. It’s perfectly ok to do something like func(47, 'Thunderbiscuit').

Optional Arguments

We can specify default values to our functions.

def some_func(arg1='Mickey'):

# Prints 'Mouse'

# Prints 'Mickey'

Since this function has optional arguments, we can either pass it our own argument, or we can just use the default. The first call passes ‘Mouse’ to some_func, in which case arg1 = ‘Mouse’. The second call does not specify a value, so arg1 gets the default ‘Mickey’ value.


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