Clear List (python) - the confusion of slice-assignment vs. slice selection

clear_list.py

def clear(alist):
    '''clear(alist)

    when you want the contents of a list cleared,
    so that all references to that list are affected.

    python3 has a clear() method on lists;
    there are at least 3 other ways to accomplish the same,
    two of them syntactically tricky, and obscure.

    The trickiness is slice notation:
    - if on lhs of expression, slice assignment calls
       the class's  __setitem__ and does iteration in
       the background;
    - if on rhs of expression, a copy of the slice is returned;
    These are very different operations for the same notation.

    This function encapsulates that trickiness.

    Here are the ways:

     del alist[:]
     - uses slice assignment to clear the list
     - fastest: {python2.7.10: ~60ns; python3.5: ~85ns}
     - see https://docs.python.org/2/tutorial/datastructures.html#the-del-statement

     alist[:] = []
     - slice assignment;
     - {python2: ~90ns; python3: ~110ns}

     while len(alist) > 0: alist.pop()
     - explicitly clears list's contents
     - syntactically as clear as python3's alist.clear()
     - ~120ns in either python

     alist.clear()
     - python3 only: ~95ns

     using this function:
     clear(alist)
     - ~180ns

     the 'del' form is fastest, if most syntactically obscure.
     we'll use that here
    '''
    del alist[:]