Basics of Python functions
Functions are the next step after you've learned Python's programming basics. In its simplest form, a function contains code that always returns a value (or values). In some cases, a function also has optional or required inputs.
When you start writing code that duplicates other parts of the program, it becomes a perfect opportunity to extract the code into a function. Although sharing common code through functions is useful, you can also limit the size of code by extracting parts out into smaller (more readable) functions.
Programs that avoid duplication and prevent large functions by using smaller functions are more readable and maintainable. They're also easier to debug when things aren't working right.
Several rules about function inputs are critical for you to fully take advantage of everything that functions have to offer.
Important
Although we're using the term input to describe what functions take in, these elements are usually called arguments or parameters. For consistency in this module, we'll refer to inputs as arguments.
Functions with no arguments
To create a function, you use the def keyword followed by a name, parentheses, and then the body with the function code:
def rocket_parts():
print("payload, propellant, structure")
In this case, rocket_parts is the name of the function. That name is followed by the empty parentheses, which indicate that no arguments are needed. Last is the code, indented with four spaces. To use the function, you must call it by its name by using parentheses:
rocket_parts()
payload, propellant, structure
The rocket_parts() function doesn't take any arguments and prints a statement. If you need to use a value that a function is returning, you can assign the function output to a variable:
output = rocket_parts()
payload, propellant, structure
output is None
True
It might seem surprising that the value for the output variable is None. This is because the rocket_parts() function didn't explicitly return a value. In Python, if a function doesn't explicitly return a value, it implicitly returns None. Updating the function to return the string instead of printing it causes the output variable to have a different value:
def rocket_parts():
return "payload, propellant, structure"
output = rocket_parts()
output
payload, propellant, structure
If you need to use the value of a function, that function must return explicitly. Otherwise, None is returned.
Note
You don't need to always assign the return of a function. In most cases where a function doesn't return a value (or values) explicitly, it means that you don't need to assign or use the implicit None that's returned.
Required and optional arguments
In Python, several built-in functions require arguments. Some built-in functions make arguments optional. Built-in functions are immediately available, so you don't need to import them explicitly.
An example of a built-in function that requires an argument is any(). This function takes an iterable (for example, a list) and returns True if any item in the iterable is True. Otherwise, it returns False.
any([True, False, False])
True
any([False, False, False])
False
If you call any() without any arguments, a helpful exception is raised. The error message explains that you need at least one argument:
any()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: any() takes exactly one argument (0 given)
You can verify that some functions allow the use of optional arguments by using another built-in function called str(). This function creates a string from an argument. If no argument is passed in, it returns an empty string:
str()
''
str(15)
'15'