Python __getitem__ and in operator result in strange behavior

If an object doesn’t have a __contains__ implementation, in falls back on a default that basically works like this:

def default__contains__(self, element):
    for thing in self:
        if thing == element:
            return True
    return False

And if an object doesn’t have an __iter__ implementation, for falls back on a default that basically works like this:

def default__iter__(self):
    i = 0
    try:
        while True:
            yield self[i]
            i += 1
    except IndexError:
        pass

These defaults are used even if the object is not intended to be a sequence.

Your 1 in f and 5 in f tests are using the default fallbacks for in and for, leading to the observed behavior. 1 in f finds 1 immediately, but your __getitem__ never returns 5, so 5 in f runs forever.

(Well, actually, on the reference implementation of Python, the default __iter__ fallback stores the index in a C-level variable of type Py_ssize_t, so if you wait long enough, that variable maxes out and Python raises an OverflowError. If you saw that, you must be on a 32-bit Python build. Computers haven’t existed long enough for anyone to hit that on a 64-bit Python.)