A long time ago, I wrote some stuff on how generators are not lists (okay, technically it was about iterators), and one of the things that I mentioned is that generators do not have list methods. Well, there's a consequence of that that only struck me recently: you need completely different code to modify a returned generator than to modify a returned list .

Suppose you have a function that returns something that is conceptually a list of items. Further suppose that you have another function that modifies what the first function returns; perhaps you want to add something on the end. If you know you're dealing with a list, you write:

def append(func, extra):
    r = func()
    r.extend(extra)
    return r

If func() is a generator, this code blows up. You have two choices; first, you can forcefully turn the result of func() into a list, and second, you can rewrite append() as a generator (which will work regardless of what func() returns, but may have consequences that make it undesirable):

def append(func, extra):
   for it in (func(), extra):
       for e in it:
           yield e

(Yes, yes, one can write this using itertools.chain() . Then people would have to look it up .)

In either case, you have to actively make a decision about what your function will do. You cannot passively modify whatever you get handed and pass it up to your caller without changing its nature; you must decide that no matter what func() returns, you're either returning a list or an iterator.

(Technically you can, since you can see if you got handed something that follows the iterator protocol or whether it looks sequence-like. But that way lies madness.)