Practical Programming, 2nd Edition

Tracing Function Calls in the Memory Model

Read the following code. Can you predict what it will do when we run it?

>>> def f(x):
...     x = 2 * x
...     return x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)

That code is confusing, in large part because x is used all over the place. However, it is pretty short and it only uses Python features that we have seen so far: assignment statements, expressions, function definitions, and function calls. We’re missing some information: Are all the x’s the same variable? Does Python make a new x for each assignment? For each function call? For each function definition?

Here’s the answer: whenever Python executes ...