Amusing Python 1: "*"

Take a look at the three different "lists" below, guess the outputs:

(a)

>>> lists = [ [], [], [] ]
>>> lists
[[], [], []]
>>> lists[1].append("Hi there!")
>>> lists
?

(b)

>>> lists = [ [] for i in range(3) ]
>>> lists
[[], [], []]
>>> lists[1].append("Hi there!")
>>> lists
?

(c)

>>> lists = [ [] * 3 ]
>>> lists
[[], [], []]
>>> lists[1].append("Hi there!")
>>> lists
?

 

All same? Yeah, I do agree with you. However, here are what we actually get:

(a)

[[], ['Hi there!'], []]

(b)

[[], ['Hi there!'], []]

(c)

[['Hi there!'], ['Hi there!'], ['Hi there!']]

 

What has happened is that [[]] is a one-element list containing an empty list, so all three elements of [[]] * 3 are (pointers to) this single empty list. Modifying any of the elements of lists modifies this single list.

See more at Chapter 3.6 - Sequence Types, Python Library Reference .