[New-bugs-announce] [issue18244] singledispatch: When virtual-inheriting ABCs at distinct points in MRO, composed MRO is dependent on haystack ordering

[Edward Catmur]

New submission from Edward Catmur:

Suppose we have a class C with MRO (C, B, A, object).  C virtual-inherits an ABC V, while B virtual-inherits an unrelated ABC W:

       object
      /   |  \
      A   W  |
      | .`  /
      B`  V
      | .`
      C`

Recalling that per PEP 443 singledispatch prefers concrete bases to virtual bases, we would expect the following composed MRO:

    C, B, V, A, W, object

However what actually happens is the composed MRO depends on the order of the haystack; if W is processed first the result is correct, but if V is processed first then (because V does not subclass W) W is inserted in the MRO *before* V:

    C, B, A, object
    C, B, V, A, object
    C, B, W, V, A, object

This results in ambiguity between V and W.

Suggested fix is a slight change to the MRO composition algorithm, considering whether the items already placed in the MRO are concrete base classes.

----------
components: Extension Modules
hgrepos: 200
messages: 191350
nosy: ecatmur
priority: normal
severity: normal
status: open
title: singledispatch: When virtual-inheriting ABCs at distinct points in MRO, composed MRO is dependent on haystack ordering
versions: Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18244>
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