2 ms·

Seems like a very verbose way to explain the situation to the original poster... Lists are mutable, strings aren't. When you reassign one of the dictionary val

• arundelo 17y ago
  The distinction between mutable and immutable is important, but more important here is the difference between >>> dict['a']=3 and >>> dict['a'].append(3) The first rebinds dict['a'] (and in the process mutates the object dict is bound to). The second mutates the object dict['a'] is bound to (and in the process binds dict['a'][1] to 3 [EDIT: not really -- 1 is not a name and so doesn't have a binding]). I think looking at it purely in terms of mutable versus immutable makes it more likely for a newbie to expect the assignment above to work differently depending on the mutability of dict['a']'s old or new value. (I have seen this type of error.) EDIT 2: I think a better way of making my point is that this: > When you reassign one of the dictionary values > pointing to an existing string, it can't change that > string, so it creates a new one. can be taken (incorrectly) to imply that reassigning a dictionary value pointing to an existing list will mutate that list.