What does colon at assignment for list[:] = […] do in Python [duplicate]

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This syntax is a slice assignment. A slice of [:] means the entire list. The difference between nums[:] = and nums = is that the latter doesn’t replace elements in the original list. This is observable when there are two references to the list

>>> original = [1, 2, 3]
>>> other = original
>>> original[:] = [0, 0] # changes the contents of the list that both
                         # original and other refer to 
>>> other # see below, now you can see the change through other
[0, 0]

To see the difference just remove the [:] from the assignment above.

>>> original = [1, 2, 3]
>>> other = original
>>> original = [0, 0] # original now refers to a different list than other
>>> other # other remains the same
[1, 2, 3]

Note: vincent thorpe’s comments below are either incorrect or irrelevant to the question. This is not a matter of value vs reference semantics, nor whether you apply the operator to an lvalue or rvalue.

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