How Function Calls Work

Function calls have been hiding in plain sight. You used print() to display messages and input() to collect answers before you knew their shared pattern. Let us give that pattern a name.

A function has a name and a job

A function is a named operation with a particular job. Calling a function means asking it to do that job now:

print("Hello")

This is a function call. It has three visible parts:

The value inside the parentheses is an argument. It gives the function information for this particular call. The same function can receive a different argument next time:

print("Good morning")
print("Good evening")

Same job, different message.

A function can return a value

Some functions produce a value for the rest of the program to use. That result is the function’s return value.

You have already used a returned value here:

name = input("What is your name? ")

If the person types Ada, the call can be pictured like this:

input("What is your name? ") -> "Ada"

The arrow is an explanation, not Python syntax. It shows the value produced by the call.

Assignment stores the returned value

Python completes the right side of an assignment before attaching its result to the name on the left. For the line above, Python:

1. calls input();

2. supplies "What is your name? " as the argument;

3. waits for the returned string;

4. assigns that string to name.

The assignment has to wait for Enter because it needs the function’s return value. No mystery here, just four small steps in order.

The diagram below follows that same journey: argument into the call, return value out, then assignment to a variable.

Trace one complete trip

The playground keeps the pattern short:

answer = input("Type anything: ")
print(f"You entered: {answer}")

Press Run and enter any answer. On the first line, the prompt is the argument, your typed text is the return value, and assignment stores it in answer. On the second line, the finished f-string is the argument supplied to print().

In the next lesson, int() will use the same pattern: give the function text as an argument, receive an integer as its return value, and store that value in a variable.