How to call a function in Python

Series: Functions
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Trey Hunner
2 minute read Python 3.7—3.10
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What are functions and how can we use them?

Calling a function

Let's use a function that's built into Python: the built-in 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 function 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.

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)

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

The default 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")

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

>>> name
>>> print(name)

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.


To use functions in Python, you write the function name (or the variable that points to the function object) followed by parentheses (to call the function). If that function accepts arguments (as most functions do), then you'll 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.

Series: Functions

Python, like many programming languages, has functions. A function is a block of code you can call to run that code.

Python's functions have a lot of "wait I didn't know that" features. Functions can define default argument values, functions can be called with keyword arguments, and functions can be written to accept any number of arguments.

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