Inconsistent type of *args

Question

I'm trying to write a generic memoization function in python 2.7 (no issue in 3.x)

def init_memoize():
    fns = dict()
    def memoize(fn, *args):
        if fn not in fns:
            fns[fn] = dict()
        print type(args)       # prints "<type 'tuple'>"
        print args in fns[fn]  # throws the error
        if args not in fns[fn]:
            fns[fn][args] = fn(*args)
        return fns[fn][args]
    return memoize
memoize = init_memoize()

memoize(sorted, range(5))
memoize(sorted, range(10))

When I try and run this code, I get an error TypeError: unhashable type: 'list' at the noted line, but immediately before, I verify that the type of args is a hashable tuple.

Can someone clarify why the type of args changes?

Answer 1

A tuple with a list inside it is still unhashable for the same reason that a list is unhashable.

>>> args = ([],)
>>> type(args)
<type 'tuple'>
>>> d = {args: None}
TypeError: unhashable type: 'list'
>>> hash(args)
TypeError: unhashable type: 'list'