Question about idioms for clearing a list

I know that the standard idioms for clearing a list are:

 (1) mylist[:] = []
 (2) del mylist[:]

I guess I'm not in the "slicing frame of mind", as someone put it, but can someone explain what the difference is between these and:

 (3) mylist = []

Why are (1) and (2) preferred? I think the first two are changing the list in-place, but why is that better? Isn't the end result the same?

Thanks in advance.
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Steven.
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