MyPy does not enforce Hashability

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#2,455 opened on Nov 15, 2016 4 comments 3 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 35/100

Issue type: Bug
Clarity: Mostly clear
Activity status: Stale
Tech stack: python
Domain: compilers, devtools

Research direction

The report points to checker.py; start by tracing how mypy validates set elements and whether existing Protocol checks are handled there. Reproduce the Python 3 example and compare it with the expected rejection of Foo in a set. Done means the checker reports the unhashable class while preserving valid hashable cases.

Written by the indexing model from the issue text.

Description

feature priority-1-normal

For example, the following code is illegal in Python 3 (implementing __eq__ shadows your __hash__ implementation with None in Python 3)

class Foo(object):
  def __init__(self, x):
    self.x = x
  def __eq__(self, other):
    if not isinstance(other, Foo):
      return NotImplemented
    return self.x == other.x

set_of_foo = {Foo(1), Foo(2)}

Poking around the code, AFAICT mypy doesn't do any Hashable checks. I don't see any other Protocols being checked in checker.py. Is there any existing tickets around supporting Protocols in mypy?

Dominant language: Python
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