functools.singledispatch fails when "not BaseClass" is True

[SergioPascual]

BPO	23572
Nosy	@pfmoore, @ambv, @serhiy-storchaka, @1st1
Files	singledispatch-test.py: A class that breaks singledispatch issue23572.stoneleaf.01.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ambv'
closed_at = <Date 2015-08-18.18:24:22.035>
created_at = <Date 2015-03-03.12:17:09.172>
labels = ['library']
title = 'functools.singledispatch fails when "not BaseClass" is True'
updated_at = <Date 2015-09-04.17:23:56.610>
user = 'https://bugs.python.org/SergioPascual'

bugs.python.org fields:

activity = <Date 2015-09-04.17:23:56.610>
actor = 'lukasz.langa'
assignee = 'lukasz.langa'
closed = True
closed_date = <Date 2015-08-18.18:24:22.035>
closer = 'yselivanov'
components = ['Library (Lib)']
creation = <Date 2015-03-03.12:17:09.172>
creator = 'Sergio Pascual'
dependencies = []
files = ['38314', '39398']
hgrepos = []
issue_num = 23572
keywords = ['patch']
message_count = 9.0
messages = ['237126', '242291', '242381', '243374', '248685', '248782', '248783', '248784', '249792']
nosy_count = 7.0
nosy_names = ['paul.moore', 'lukasz.langa', 'python-dev', 'serhiy.storchaka', 'yselivanov', 'Drekin', 'Sergio Pascual']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'patch review'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue23572'
versions = ['Python 3.4', 'Python 3.5', 'Python 3.6']

[SergioPascual]

I admit this case is rather convoluted, but I have been debugging a few hours so I think I should share my findings.

I have a metaclass MetaA that provides to the classes constructed with it have a dictionary interface. Of all the functions only __len__ is important. In some cases, the result of __len__ is 0 and that is what triggers the error

class MetaA(type):
    def __len__(self):
        return 0

class A(metaclass=MetaA):
    pass

class AA(A):
    pass

Now, I construct a function with single dispatch and register the case of class A but not the class AA

@singledispatch
def fun(a):
    print('base case')

@fun.register(A)
def _(a):
    print('fun A')

And then, call fun with an object of class AA

fun(AA())

This should call the function for the class up in the hierarchy, A
Instead it raises an exception

RuntimeError: Inconsistent hierarchy

in function functools._c3_merge
because in the line

if not candidate:
    raise RuntimeError("Inconsistent hierarchy")

"not candidate" evaluates to True due to __len__ returning 0

This can be avoided by:

• adding __nonzero__ to MetaA

• not adding the map interface to a class in the first place

• changing the code in _c3_merge

I'm not really sure, but instead of:

if not candidate:
    raise RuntimeError("Inconsistent hierarchy")

would this work?

if candidate is None:
    raise RuntimeError("Inconsistent hierarchy")

I attach a test case

[ambv]

Yes, this needs addressing.

[serhiy-storchaka]

An example is Enum.

[ethanfurman]

Attached is patch and test case.

[Drekin]

I was also bitten by this via Enum. Is there any chance this will be fixed in Python 3.5?

[python-dev]

New changeset 73984e665bf5 by Yury Selivanov in branch '3.5':
Issue bpo-23572: Fixed functools.singledispatch on classes with falsy metaclasses.
https://hg.python.org/cpython/rev/73984e665bf5

New changeset 94d0c219d46f by Yury Selivanov in branch 'default':
Merge 3.5 (issue bpo-23572)
https://hg.python.org/cpython/rev/94d0c219d46f

[1st1]

I was also bitten by this via Enum. Is there any chance this will be fixed in Python 3.5?

It will be fixed in 3.5.1.

Thanks for the patch, Ethan!

[python-dev]

New changeset 586195685aaf by Yury Selivanov in branch '3.4':
Issue bpo-23572: Fixed functools.singledispatch on classes with falsy metaclasses.
https://hg.python.org/cpython/rev/586195685aaf

[ambv]

Thank you for fixing this and sorry for not being responsive sooner!