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max() over itertools.ifilter() is broken in 0.18.0 #510

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@xeron

Description

@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.

Activity

changed the title [-]max() over ifilter() is broken in 0.18.0[/-] [+]max() over itertools.ifilter() is broken in 0.18.0[/+] on Oct 16, 2019
jmadler

jmadler commented on Oct 17, 2019

@jmadler
Contributor

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

xeron

xeron commented on Oct 17, 2019

@xeron
Author

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

gazpachoking

gazpachoking commented on Oct 17, 2019

@gazpachoking
Contributor

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.

added a commit that references this issue on Oct 18, 2019
03f94a3
megies

megies commented on Oct 21, 2019

@megies
Contributor

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

jmadler commented on Oct 24, 2019

@jmadler
Contributor

Thanks megies for the confirmation!

Thanks all for helping make this software better :)

xeron

xeron commented on Oct 24, 2019

@xeron
Author

Fix has not been merged yet, right?

megies

megies commented on Oct 25, 2019

@megies
Contributor

It's been merged now :)

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        Issue actions

          max() over itertools.ifilter() is broken in 0.18.0 · Issue #510 · PythonCharmers/python-future