PythonCharmers/python-future

futurize `__bool__` not implemented for subclasses of dict

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#282 opened on May 5, 2017

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0.20bughelp wanted

Description

If a run futurize on python 2 code containing a subclass of the built in dict, e.g.

class A(dict):
    __nonzero__(self):
    return False

it is translated to

class A(dict):
    __bool__(self):
    return False

However, the __bool__ method is not called in python 2. Seems some other kind of trickery is needed.

For now I'm working around the issue by doing

class A(dict):
    __bool__(self):
    return False
    if sys.version_info[0] == 2:
        __nonzero__ = __bool__

Contributor guide

Tech stack
python
Domain
toolingbackend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Python 2/3 compatibility conceptsfamiliarity with special methods
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
Investigate the futurize source code, likely in the `libfuturize` or `futurize` directory, for the transformation that converts ` nonzero ` to ` bool `. The issue indicates that ` bool ` is not called in Python 2 for dict subclasses. Examine the implementation and consider adding a fallback that also defines ` nonzero ` for Python 2 compatibility. No linked PRs or comments exist.