Difference between arr[:]=[] and arr[]=[] in python3.x?

Question

Is there any difference between these two lines:

arr[:] = []
arr = []

I know both of them clear list.

Answer 1

For the second one, I think you meant arr = [].

What that does differently is that it has arr point to a new empty list and just decrements the refcount on the existing list.

The difference is only important if something else is pointing to the original list.

>>> orig = [10, 20, 30]
>>> arr = orig              # Second reference to the same list.
>>> arr[:] = []             # Clears the one list, so that arr and orig are empty
>>> orig
[]

Contrast that with:

>>> orig = [10, 20, 30]
>>> arr = orig              # Second reference to the same list.
>>> arr = []                # Points arr to a new list, leaving orig unchanged
>>> orig
[10, 20, 30]