Simplifications of PEP 653: (#1919)
* Make 0 the defualt for __match_class__ and drop ability to not match class patterns * Use equality not bitwise ands to check for matches * Add to rejected and deferred sections, explaining the above
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Mark Shannon
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pep-0653.rst
96
pep-0653.rst
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@ -16,7 +16,7 @@ but is more precise, easier to reason about, and should be faster.
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The object model will be extended with two special (dunder) attributes, ``__match_container__`` and
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``__match_class__``, in addition to the ``__match_args__`` attribute from PEP 634, to support pattern matching.
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Both of these new attributes must be integers and ``__match_args__`` is required to be a tuple.
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Both of these new attributes must be integers and ``__match_args__`` is required to be a tuple of unique strings.
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With this PEP:
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@ -114,29 +114,34 @@ The ``__match_container__ ``and ``__match_class__`` attributes will be added to
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``__match_container__`` must be an integer and should be exactly one of these::
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0
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MATCH_SEQUENCE
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MATCH_MAPPING
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MATCH_SEQUENCE = 1
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MATCH_MAPPING = 2
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``MATCH_SEQUENCE`` is used to indicate that instances of the class can match sequence patterns.
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``MATCH_MAPPING`` is used to indicate that instances of the class can match mapping patterns.
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``__match_class__`` must be an integer and should be exactly one of these::
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0
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MATCH_ATTRIBUTES
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MATCH_SELF
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MATCH_SELF = 8
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``MATCH_SELF`` is used to indicate that for a single positional argument class pattern, the subject will be used and not deconstructed.
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.. note::
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It does not matter what the actual values are. We will refer to them by name only.
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Symbolic constants will be provided both for Python and C, and once defined they will
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In the rest of this document, we will refer to the above values by name only.
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Symbolic constants will be provided both for Python and C, and the values will
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never be changed.
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``object`` will have the following values for the special attributes::
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__match_container__ = 0
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__match_class__= MATCH_ATTRIBUTES
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__match_class__= 0
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__match_args__ = ()
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These special attributes will be inherited as normal.
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If ``__match_args__`` is overridden, then it is required to hold a tuple of strings. It may be empty.
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If ``__match_args__`` is overridden, then it is required to hold a tuple of unique strings. It may be empty.
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.. note::
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``__match_args__`` will be automatically generated for dataclasses and named tuples, as specified in PEP 634.
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@ -153,7 +158,6 @@ In the following, all variables of the form ``$var`` are temporary variables and
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They may be visible via introspection, but that is an implementation detail and should not be relied on.
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The psuedo-statement ``FAIL`` is used to signify that matching failed for this pattern and that matching should move to the next pattern.
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If control reaches the end of the translation without reaching a ``FAIL``, then it has matched, and following patterns are ignored.
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All the translations below include guards. If no guard is present, simply substitute the guard ``if True`` when translating.
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Variables of the form ``$ALL_CAPS`` are meta-variables holding a syntactic element, they are not normal variables.
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So, ``$VARS = $items`` is not an assignment of ``$items`` to ``$VARS``,
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@ -241,7 +245,7 @@ A pattern not including a star pattern::
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translates to::
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$kind = type($value).__match_container__
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if $kind & MATCH_SEQUENCE == 0:
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if $kind != MATCH_SEQUENCE:
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FAIL
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if len($value) != len($VARS):
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FAIL
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@ -256,7 +260,7 @@ A pattern including a star pattern::
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translates to::
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$kind = type($value).__match_container__
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if $kind & MATCH_SEQUENCE == 0:
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if $kind != MATCH_SEQUENCE:
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FAIL
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if len($value) < len($VARS):
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FAIL
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@ -275,7 +279,7 @@ translates to::
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$sentinel = object()
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$kind = type($value).__match_container__
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if $kind & MATCH_MAPPING == 0:
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if $kind != MATCH_MAPPING:
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FAIL
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# $KEYWORD_PATTERNS is a meta-variable mapping names to variables.
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for $KEYWORD in $KEYWORD_PATTERNS:
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@ -293,7 +297,7 @@ A pattern including a double-star pattern::
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translates to::
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$kind = type($value).__match_container__
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if $kind & MATCH_MAPPING == 0:
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if $kind != MATCH_MAPPING:
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FAIL
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# $KEYWORD_PATTERNS is a meta-variable mapping names to variables.
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$tmp = dict($value)
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@ -317,12 +321,6 @@ translates to::
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if not isinstance($value, ClsName):
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FAIL
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.. note::
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``case ClsName():`` is the only class pattern that can succeed if
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``($kind & (MATCH_SELF|MATCH_ATTRIBUTES)) == 0``
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Class pattern with a single positional pattern::
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case ClsName($VAR):
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@ -330,7 +328,7 @@ Class pattern with a single positional pattern::
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translates to::
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$kind = type($value).__match_class__
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if $kind & MATCH_SELF:
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if $kind == MATCH_SELF:
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if not isinstance($value, ClsName):
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FAIL
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$VAR = $value
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@ -346,8 +344,6 @@ translates to::
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if not isinstance($value, ClsName):
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FAIL
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$kind = type($value).__match_class__
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if $kind & MATCH_ATTRIBUTES:
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$attrs = ClsName.__match_args__
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if len($attr) < len($VARS):
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raise TypeError(...)
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@ -356,8 +352,6 @@ translates to::
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$VAR = getattr($value, $attrs[i])
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except AttributeError:
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FAIL
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else:
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FAIL
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Example: [6]_
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@ -369,16 +363,12 @@ translates to::
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if not isinstance($value, ClsName):
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FAIL
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$kind = type($value).__match_class__
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if $kind & MATCH_ATTRIBUTES:
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try:
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for $KEYWORD in $KEYWORD_PATTERNS:
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$tmp = getattr($value, QUOTE($KEYWORD))
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$KEYWORD_PATTERNS[$KEYWORD] = $tmp
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except AttributeError:
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FAIL
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else:
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FAIL
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Example: [7]_
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@ -390,8 +380,6 @@ translates to::
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if not isinstance($value, ClsName):
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FAIL
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$kind = type($value).__match_class__
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if $kind & MATCH_ATTRIBUTES:
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$attrs = ClsName.__match_args__
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if len($attr) < len($VARS):
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raise TypeError(...)
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@ -406,8 +394,6 @@ translates to::
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$KEYWORD_PATTERNS[$KEYWORD] = getattr($value, $name)
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except AttributeError:
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FAIL
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else:
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FAIL
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Example: [8]_
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@ -425,7 +411,7 @@ For example, the pattern::
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translates to::
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$kind = type($value).__match_class__
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if $kind & MATCH_SEQUENCE == 0:
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if $kind != MATCH_SEQUENCE:
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FAIL
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if len($value) != 2:
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FAIL
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@ -457,11 +443,10 @@ All classes should ensure that the the values of ``__match_container__``, ``__ma
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and ``__match_args__`` follow the specification.
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Therefore, implementations can assume, without checking, that the following are true::
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(__match_container__ & (MATCH_SEQUENCE | MATCH_MAPPING)) != (MATCH_SEQUENCE | MATCH_MAPPING)
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(__match_class__ & (MATCH_SELF | MATCH_ATTRIBUTES)) != (MATCH_SELF | MATCH_ATTRIBUTES)
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__match_container__ == 0 or __match_container__ == MATCH_SEQUENCE or __match_container__ == MATCH_MAPPING
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__match_class__ == 0 or __match_class__ == MATCH_SELF
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Thus, implementations can assume that ``__match_container__ & MATCH_SEQUENCE`` implies ``(__match_container__ & MATCH_MAPPING) == 0``, and vice-versa.
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Likewise for ``__match_class__``, ``MATCH_SELF`` and ``MATCH_ATTRIBUTES``.
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and that ``__match_args__`` is a tuple of unique strings.
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Values of the special attributes for classes in the standard library
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--------------------------------------------------------------------
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@ -518,9 +503,9 @@ Implementations are allowed to make the following assumptions:
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* ``isinstance(obj, cls)`` can be freely replaced with ``issubclass(type(obj), cls)`` and vice-versa.
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* ``isinstance(obj, cls)`` will always return the same result for any ``(obj, cls)`` pair and repeated calls can thus be elided.
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* Reading any of ``__match_container__``, ``__match_class__`` or ``__match_args__`` is a pure operation, and may be cached.
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* Sequences, that is any class for which ``__match_container__&MATCH_SEQUENCE`` is not zero, are not modified by iteration, subscripting or calls to ``len()``.
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* Sequences, that is any class for which ``__match_container__ == MATCH_SEQUENCE`` is not zero, are not modified by iteration, subscripting or calls to ``len()``.
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Consequently, those operations can be freely substituted for each other where they would be equivalent when applied to an immutable sequence.
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* Mappings, that is any class for which ``__match_container__&MATCH_MAPPING`` is not zero, will not capture the second argument of the ``get()`` method.
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* Mappings, that is any class for which ``__match_container__ == MATCH_MAPPING`` is not zero, will not capture the second argument of the ``get()`` method.
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So, the ``$sentinel`` value may be freely re-used.
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In fact, implementations are encouraged to make these assumptions, as it is likely to result in signficantly better performance.
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@ -668,7 +653,7 @@ Rejected Ideas
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Using attributes from the instance's dictionary
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-----------------------------------------------
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An earlier version of this PEP only used attributes from the instance's dictionary when matching a class pattern with ``MATCH_ATTRIBUTES``.
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An earlier version of this PEP only used attributes from the instance's dictionary when matching a class pattern with ``__match_class__`` was the default value.
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The intent was to avoid capturing bound-methods and other synthetic attributes. However, this also mean that properties were ignored.
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For the class::
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@ -683,7 +668,7 @@ For the class::
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...
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Ideally we would match the attributes "a" and "p", but not "m".
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However, there is no general way to do that, so this PEP now follows the semantics of PEP 634 for ``MATCH_ATTRIBUTES``.
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However, there is no general way to do that, so this PEP now follows the semantics of PEP 634.
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Lookup of ``__match_args__`` on the subject not the pattern
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-----------------------------------------------------------
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@ -703,6 +688,7 @@ An earlier version of this PEP combined ``__match_class__`` and ``__match_contai
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Using a single value has a small advantage in terms of performance,
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but is likely to result in unintended changes to container matching when overriding class matching behavior, and vice versa.
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Deferred Ideas
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==============
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@ -722,6 +708,12 @@ but is tricky. With these additional features it can be implemented easily [9]_.
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This idea will feature in a future PEP for 3.11.
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However, it is too late in the 3.10 development cycle for such a change.
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Having a separate value to reject all class matches
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---------------------------------------------------
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In an earlier version of this PEP, there was a distinct value for ``__match_class__`` that allowed classes to not match any class
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pattern that would have required deconstruction. However, this would become redundant once ``MATCH_POSITIONAL`` is introduced, and
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complicates the specification for an extremely rare case.
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References
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==========
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@ -748,7 +740,7 @@ This::
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translates to::
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$kind = type($value).__match_container__
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if $kind & MATCH_SEQUENCE == 0:
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if $kind != MATCH_SEQUENCE:
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FAIL
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if len($value) != 2:
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FAIL
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@ -765,7 +757,7 @@ This::
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translates to::
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$kind = type($value).__match_container__
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if $kind & MATCH_SEQUENCE == 0:
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if $kind != MATCH_SEQUENCE:
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FAIL
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if len($value) < 2:
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FAIL
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@ -780,7 +772,7 @@ This::
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translates to::
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$kind = type($value).__match_container__
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if $kind & MATCH_MAPPING == 0:
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if $kind != MATCH_MAPPING:
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FAIL
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$tmp = $value.get("x", $sentinel)
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if $tmp is $sentinel:
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@ -802,7 +794,7 @@ This::
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translates to::
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$kind = type($value).__match_container__
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if $kind & MATCH_MAPPING == 0:
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if $kind != MATCH_MAPPING:
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FAIL
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$tmp = dict($value)
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if not $tmp.keys() >= {"x", "y"}:
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@ -821,8 +813,6 @@ translates to::
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if not isinstance($value, ClsName):
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FAIL
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$kind = type($value).__match_class__
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if $kind & MATCH_ATTRIBUTES:
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$attrs = ClsName.__match_args__
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if len($attr) < 2:
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FAIL
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@ -831,8 +821,6 @@ translates to::
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y = getattr($value, $attrs[1])
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except AttributeError:
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FAIL
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else:
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FAIL
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.. [7]
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@ -844,15 +832,11 @@ translates to::
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if not isinstance($value, ClsName):
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FAIL
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$kind = type($value).__match_class__
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lif $kind & MATCH_ATTRIBUTES:
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try:
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x = $value.a
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y = $value.b
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except AttributeError:
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FAIL
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else:
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FAIL
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.. [8]
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@ -865,8 +849,6 @@ translates to::
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if not isinstance($value, ClsName):
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FAIL
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$kind = type($value).__match_class__
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if $kind & MATCH_ATTRIBUTES:
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$attrs = ClsName.__match_args__
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if len($attr) < 1:
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raise TypeError(...)
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@ -878,8 +860,6 @@ translates to::
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y = $value.a
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except AttributeError:
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FAIL
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else:
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FAIL
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.. [9]
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