Returning tuple with a single item from a function
You need to explicitly make it a tuple (see the official tutorial):
def returns_tuple_of_one(a): return (a, )
This is not a bug, a one-tuple is constructed by val, or (val,). It is the comma and not the parentheses that define the tuple in python syntax.
Your function is actually returning a itself, which is of course not iterable.
To quote sequence and tuple docs:
A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective.
(a) is not a single element tuple, it's just a parenthesized expression. Use (a,).