[Tutor] difference between [[], [], []] and 3*[[]]

[Kent Johnson]

Mitsuo Hashimoto wrote:
> Hello,
> 
> What's the difference between "[[], [], []]" and  "3*[[]]" ?

[[], [], []] makes a list containing references to three different lists.
3*[[]] makes a list with three references to the *same* list. This can cause surprising behavior:

  >>> l=3*[[]]
  >>> l
[[], [], []]
  >>> l[0].append(1)
  >>> l
[[1], [1], [1]]

Because each list in l is the same, changing one changes them all. This is probably not what you want.

> After all, what should I do if I want to initialize a lot of lists ?
> For example a = [], b = [], c = [],...., z = []

Why do you want to do this? Often using a dict is a good solution to this type of question. For example:

  >>> import string
  >>> d={}
  >>> for c in string.ascii_lowercase:
  ...   d[c] = []
  ...
  >>> d
{'a': [], 'c': [], 'b': [], 'e': [], 'd': [], 'g': [], 'f': [], 'i': [], 'h': [], 'k': [], 'j': [], 
'm': [], 'l': [], 'o': [], 'n': [], 'q': [], 'p':
[], 's': [], 'r': [], 'u': [], 't': [], 'w': [], 'v': [], 'y': [], 'x': [], 'z': []}
  >>>

Kent