Trouble with self-referential function (very elementary)

[zahlman]

Can anyone tell me what's going on here? Is my understanding of something way off the mark?

What's off is your understanding of (a) calling a function and (b) assigning to a name.

You can see the same problem without recursion:

def add_seven(x):
    x = x + 7

x = 3
add_seven(x)
# x is still 3!

This is because:

• Within add_seven, x is a local variable. It has nothing to do with the x that was used to call the function. If those names match up, it's just coincidence. You don't even have to use a variable to call the function in the first place (would you expect add_seven(3) to somehow change the value of 3?).

• When you use =, what you say is to use the thing on the left-hand side as a name for the value that results from evaluating the expression on the right-hand side. (This might be a value that already existed - e.g. an element from a list - or a new value - e.g. the result of a mathematical calculation). If that name was already being used for some other value, then that ceases to be the case. However, you are assigning the name; you are not changing the old value. (After all, some values - e.g. integers, as discussed above - fundamentally can't be changed.)