Names Are Not Boxes

Run this, and predict the output before you check it:

a = [1, 2, 3]b = ab.append(4)print(a)

Try itAfter you check, change line 2 to b = a.copy() and run it again.

If a variable were a box, a would still read [1, 2, 3] — you only changed b. But a prints [1, 2, 3, 4]. Line 2 did not copy the list into a second box. It pointed a second name at the same list. There was only ever one list in this program.

This only ever shows up with values you can change after they exist, like a list. Numbers, strings, and tuples cannot be edited in place at all — later lessons call that immutable — so pointing two names at one of those never produces a surprise like this. The box picture happens to work for those values by accident, which is exactly what makes it dangerous: it stops working the moment you touch something mutable, and nothing warns you in advance which kind you are holding.

Now the case that actually reassures people. If a and b both point at 3, and you write a = 5, does b change too?

a = 3b = aa = 5print(b)# 3

It does not, and the toggle above shows why. a = 5 does not reach into the number 3 and edit it — numbers cannot be edited. It points the name a at a different value entirely and leaves b exactly where it was. Assignment always moves an arrow. It only looks like mutation when the arrow happens to point at something, like a list, that can be changed from where it sits.

Two names can point at separate values that happen to look identical. Python has two different operators for the two different questions this raises, and treating them as interchangeable is a quiet source of bugs.

$ >>> a = [1, 2, 3]$ >>> b = [1, 2, 3]$ >>> a == b$ True$ >>> a is b$ False

a and b here are two separate lists that happen to hold the same three numbers — equal, but not the same object. Compare that with what you already saw above: assign b = a directly, and there is only one list on two names.

$ >>> a = [1, 2, 3]$ >>> b = a$ >>> a is b$ True

A name can contain letters, digits, and underscores, but cannot start with a digit, and cannot be one of Python's reserved words — if, for, class, and about thirty others that mean something fixed to the language itself.

$ 2nd_place = "Alice"  File "<stdin>", line 1    2nd_place = "Alice"    ^$ SyntaxError: invalid decimal literal

Names are also case-sensitive, which trips people up in a quieter way than a SyntaxError does — Python treats score and Score as two entirely unrelated names, not two spellings of the same one.

$ >>> score = 90$ >>> Score$ NameError: name 'Score' is not defined