How to call a function in Python

What are functions and how can we use them?

Calling a function in Python

Let's use a function that's built into Python: Python's sum function.

If we type sum and hit the Enter key, we'll see what the variable sum points to:

>>> numbers = [2, 1, 3, 4, 7, 11, 18]
>>> sum
<built-in function sum>

We're not actually using the function here, we're referring to the function object that the variable sum points to.

To use this function, we need to put parentheses after it. Putting parenthesis after a function will call the function.

>>> sum()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sum() takes at least 1 positional argument (0 given)

When calling this function we got an error because the sum requires at least one argument but we didn't pass it any arguments.

To use the sum function we have to pass it an iterable of numbers as an argument.

Function arguments and return values

To pass an argument to a function, you put the argument inside the parentheses when calling the function.

Arguments are basically the inputs to a function. Functions have inputs (as arguments) and an output as a return value. The integer 46 is the return value of this function:

>>> sum(numbers)
46

We saw 46 printed out at the Python REPL, but this sum function didn't actually print 46, it returned 46.

If we put this function call on the right-hand side of an = sign, we can assign the variable total to whatever the return value of calling sum with numbers was (in this case, it was 46).

>>> total = sum(numbers)
>>> total
46

The default function return value is None

Not all functions have returned values. For example, the print function doesn't have a return value.

If we assign a variable to the return value of a print call:

>>> name = print("Trey")
Trey

We'll see text (Trey) printed out, but we'll also see that the variable name is None:

>>> name
>>> print(name)
None

None is a special value that basically means this function doesn't have a return value.

In Python, we typically assume that functions either perform some action or return something, but not both. The print function performs an action (it prints something to the screen) whereas the sum function returns a value.

Use parentheses to call a function in Python

To use a function, we "call" that function.

To call a function in Python, write the function name followed by parentheses. If the function accepts arguments, pass the arguments inside the parentheses as you call the function.

If that function has a return value, you can capture that return value into a variable or pass that return value straight into another function call.