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Change definition of __match__ (#1477) 

It should never raise if an attribute is missing.

Also fix one last mention of the exception for a single positional argument.

See https://github.com/gvanrossum/patma/issues/111#issuecomment-650874268
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Guido van Rossum 2020-06-29 09:32:21 -07:00 committed by GitHub
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pep-0622.rst
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@ -580,7 +580,7 @@ The procedure is as following:
``ImpossibleMatchError`` is raised. ``ImpossibleMatchError`` is raised.
* If the ``__match_args__`` attribute is absent on the matched class, * If the ``__match_args__`` attribute is absent on the matched class,
but more than one positional item appears in a match, and one or more positional item appears in a match,
``ImpossibleMatchError`` is also raised. We don't fall back on ``ImpossibleMatchError`` is also raised. We don't fall back on
using ``__slots__`` or ``__annotations__`` -- "In the face of ambiguity, using ``__slots__`` or ``__annotations__`` -- "In the face of ambiguity,
refuse the temptation to guess." refuse the temptation to guess."
@ -588,17 +588,7 @@ The procedure is as following:
* If there are any match-by-keyword items the keywords are looked up * If there are any match-by-keyword items the keywords are looked up
as attributes on the proxy. If the lookup succeeds the value is as attributes on the proxy. If the lookup succeeds the value is
matched against the corresponding sub-pattern. If the lookup fails, matched against the corresponding sub-pattern. If the lookup fails,
two cases are distinguished: the match fails.
* If an attribute is missing on the proxy and the class being matched
has no ``__match_args__`` attribute, the match
fails. This allows one to write ``case object(name=_)`` to
implement a check for the presence of a given attribute, or ``case
object(name=var)`` to check for its presence and extract its value.
* If an attribute is missing and the class has a ``__match_args__``,
the match fails if the attribute name is in
``__match_args__``, else the match raises ``ImpossibleMatchError``.
Such a protocol favors simplicity of implementation over flexibility and Such a protocol favors simplicity of implementation over flexibility and
performance. For other considered alternatives, see `extended matching`_. performance. For other considered alternatives, see `extended matching`_.