In Python, a name is a sticker, not a box

Understand it one step at a time

The short runs these in order in about 24 seconds. Here they are written out — pick any step to jump the short straight to it.

1. 1

   You changed b, a changed too

   A list assigned to a second name is not a copy. Both names sit on one object.

   python REPL idle

2. 2

   b = a copies the sticker

   The list is built once in memory. The second line just points another name at it.

   binding names

3. 3

   A list is mutable

   append edits the object itself. The id never changes, so both names show the new value.

   mutating in place

4. 4

   A string is immutable

   It cannot be edited, so += builds a new object and moves only that one name.

   rebinding s

5. 5

   Which types are which

   Numbers, strings and tuples cannot change. Lists, dicts and sets can.

   the two families

6. 6

   This is the classic bug

   A mutable default argument is created once, then quietly reused by every call.

   default argument trap

7. 7

   The fix is a fresh object

   Default to None and build the list inside, or copy before you mutate.

   safe patterns

8. 8

   Mutate or rebind

   Every confusing line in Python comes down to which of these two it is doing.

   the whole rule