Question about idioms for clearing a list

[Steven Watanabe]

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.
--
Steven.