max() over itertools.ifilter() is broken in 0.18.0

[xeron]

Python 2.7.16.
macOS 10.14.6.

Reproducible with:

import builtins
import operator

from itertools import ifilter
from random import shuffle


class Backend(object):
    def __init__(self, priority):
        super(Backend, self).__init__()
        self.priority = priority


def max(*args, **kwargs):
    """
    Add support for 'default' kwarg.

    >>> max([], default='res')
    'res'

    >>> max(default='res')
    Traceback (most recent call last):
    ...
    TypeError: ...

    >>> max('a', 'b', default='other')
    'b'
    """
    missing = object()
    default = kwargs.pop('default', missing)
    try:
        mmax = builtins.max(*args, **kwargs)
        return mmax
    except ValueError as exc:
        if 'empty sequence' in str(exc) and default is not missing:
            return default
        raise


by_priority = operator.attrgetter('priority')

backends = [Backend(5), Backend(1), Backend(0.5), Backend(0.6)]

while True:
    shuffle(backends)
    filtered_backends = ifilter(None, backends)
    max_object = max(filtered_backends, key=by_priority)
    print(max_object.priority)
    if max_object.priority != 5:
        exit(1)

Can't reproduce with 0.17.1.

This affects older versions of keyring library using similar code to select keychain backend where it wants max priority backend but returns the wrong one.

Also it looks like it depends on the order of the list.

[jmadler]

Can you submit a bugfix please? I'll include it in a release in the next few days.

[xeron]

Sorry, I have no idea how to fix this. We've just downgraded to 0.17.1 for now.

[gazpachoking]

I think I'm running into a related issue. The new min and new max function consume the first item of an iterator before passing to the underlying min and max functions. I'll take a stab at a PR if I get time tonight.

[megies]

I'm running into problems with with min/max of generators as well. Can't tell what the above code is doing, but it sounds related.

For me specifically, the problem seems to be min() or max() over a generator with only a single item in it. Here is a minimal example:

from future.builtins import *
min(x for x in [1])

I bisected this and it starts failing on master with commit f141395.

I checked current state of #514 and can confirm that it fixes this issue. Thanks @gazpachoking

[jmadler]

Thanks megies for the confirmation!

Thanks all for helping make this software better :)

[xeron]

Fix has not been merged yet, right?

[megies]

It's been merged now :)