What does [:] do?

Question

return self.var[:]

What will that return?

Answer 1

Python permits you to "slice" various container types; this is a shorthand notation for taking some subcollection of an ordered collection. For instance, if you have a list

foo = [1,2,3,4,5]

and you want the second, third, and fourth elements, you can do:

foo[1:4]

If you omit one of the numbers in the slice, it defaults to the start of the list. So for instance

foo[1:] == [2,3,4,5]
foo[:4] == [1,2,3,4]

Naturally, if you omit both numbers in the slice you will get the entire list back! However, you will get a copy of the list instead of the original; in fact, this is the standard notation for copying a list. Note the difference:

>>> a = [1,2,3,4]
>>> b = a
>>> b.append(5)
>>> a
[1, 2, 3, 4, 5]
>>>
>>> a = [1,2,3,4]
>>> b = a[:]
>>> b.append(5)
>>> a
[1, 2, 3, 4]

This occurs because b = a tells b to point to the same object as a, so appending to b is the same as appending to a. Copying the list a avoids this. Notice that this only runs one level of indirection deep -- if a contained a list, say, and you appended to that list in b, you would still change a.

By the way, there is an optional third argument to the slice, which is a step parameter -- it lets you move through the list in jumps of greater than 1. So you could write range(100)[0::2] for all the even numbers up to 100.

Answer 2

If self.var is a mutable sequence, it will return a shallow copy of that sequence.

If self.var is an immutable built-in sequence such as a string or a tuple, most implementations will return self.var itself.