PEP 653: Change semantics to be closer to PEP 634, while keeping the important features of PEP 653. (#1868)
* Change name of `MATCH_CLASS` to `MATCH_POSITIONAL`. * Drop `__attributes__` and use `__match_args__` instead. * For `MATCH_DEFAULT`, `__match_args__` acts a map from positions to names. * For `MATCH_POSITIONAL`, `__match_args__` acts a map from names to positions. * Change semantics of `MATCH_DEFAULT` when match class patterns to those of PEP 634. * Update translations. * Further expand examples
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Mark Shannon
GitHub
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pep-0653.rst
310
pep-0653.rst
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@ -14,14 +14,16 @@ Abstract
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This PEP proposes a semantics for pattern matching that respects the general concept of PEP 634,
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but is more precise, easier to reason about, and should be faster.
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The object model will be extended with three special (dunder) attributes to support pattern matching:
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The object model will be extended with two special (dunder) attributes,
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in addition to the ``__match_args__`` attribute from PEP 634, to support pattern matching.
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* A ``__match_kind__`` attribute. Must be an integer.
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* An ``__attributes__`` attribute. Only needed for those classes wanting to customize matching the class pattern.
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* A ``__match_args__`` attribute. Only needed for those classes wanting to customize matching the class pattern.
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If present, it must be a tuple of strings.
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* A ``__deconstruct__()`` method. Only needed if ``__attributes__`` is present.
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* A ``__deconstruct__()`` method. Only needed for customizing matching of class patterns with positional arguments.
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Returns an iterable over the components of the deconstructed object.
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With this PEP:
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* The semantics of pattern matching will be clearer, so that patterns are easier to reason about.
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@ -107,7 +109,7 @@ This allows the kind of a value to be determined once and in a efficient fashion
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To deconstruct an object, pre-existing special methods can be used for sequence and mapping patterns, but something new is needed for class patterns.
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PEP 634 proposes using ad-hoc attribute access, disregarding the possibility of side-effects.
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This could be problematic should the attributes of the object be dynamically created or consume resources.
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By adding the ``__attributes__`` attribute and ``__deconstruct__()`` method, objects can control how they are deconstructed,
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By adding the ``__deconstruct__()`` method, objects can control how they are deconstructed,
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and patterns with a different set of attributes can be efficiently rejected.
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Should deconstruction of an object make no sense, then classes can define ``__match_kind__`` to reject class patterns completely.
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@ -131,7 +133,7 @@ bitwise ``or``\ ed with exactly one of these::
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0
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MATCH_DEFAULT
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MATCH_CLASS
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MATCH_POSITIONAL
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MATCH_SELF
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.. note::
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@ -139,19 +141,20 @@ bitwise ``or``\ ed with exactly one of these::
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Symbolic constants will be provided both for Python and C, and once defined they will
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never be changed.
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Classes inheriting from ``object`` will inherit ``__match_kind__ = MATCH_DEFAULT``.
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Classes inheriting from ``object`` will inherit ``__match_kind__ = MATCH_DEFAULT`` and ``__match_args__ = ()``
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Classes which define ``__match_kind__ & MATCH_CLASS`` to be non-zero must
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implement one additional special attribute, and one special method:
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Classes which define ``__match_kind__ & MATCH_POSITIONAL`` to be non-zero must
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implement ``__deconstruct__()`` and should consider redefining ``__match_args__``.
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* ``__attributes__``: should hold a tuple of strings indicating the names of attributes that are to be considered for matching; it may be empty for postional-only matches.
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* ``__match_args__``: should hold a tuple of strings indicating the names of attributes that are to be considered for matching; it may be empty for positional-only matches.
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* ``__deconstruct__()``: should return a sequence which contains the parts of the deconstructed object.
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.. note::
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``__attributes__`` and ``__deconstruct__`` will be automatically generated for dataclasses and named tuples.
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``__match_args__`` will be automatically generated for dataclasses, as specified in PEP 634.
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``__match_args__`` and ``__deconstruct__`` will be automatically generated for named tuples.
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The pattern matching implementation is *not* required to check that ``__attributes__`` and ``__deconstruct__`` behave as specified.
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If the value of ``__attributes__`` or the result of ``__deconstruct__()`` is not as specified, then
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The pattern matching implementation is *not* required to check that ``__match_args__`` and ``__deconstruct__`` behave as specified.
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If the value of ``__match_args__`` or the result of ``__deconstruct__()`` is not as specified, then
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the implementation may raise any exception, or match the wrong pattern.
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Of course, implementations are free to check these properties and provide meaningful error messages if they can do so efficiently.
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@ -339,8 +342,6 @@ Class pattern with no arguments::
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translates to::
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if $kind & (MATCH_CLASS | MATCH_DEFAULT) == 0:
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FAIL
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if not isinstance($value, ClsName):
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FAIL
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@ -364,46 +365,34 @@ Positional-only class pattern::
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translates to::
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if $kind & MATCH_CLASS == 0:
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FAIL
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if not isinstance($value, ClsName):
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FAIL
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if $kind & MATCH_POSITIONAL:
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if $items is None:
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$items = type($value).__deconstruct__($value)
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# $VARS is a meta-variable.
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if len($items) != len($VARS):
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if len($items) < len($VARS):
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FAIL
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$VARS = $items
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elif $kind & MATCH_DEFAULT:
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if $attrs is None:
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$attrs = type($value).__match_args__
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if len($attr) < len($VARS):
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FAIL
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try:
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for i, $VAR in enumerate($VARS):
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$VAR = getattr($value, $attrs[i])
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except AttributeError:
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FAIL
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else:
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FAIL
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Example: [6]_
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.. note::
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``__attributes__`` is not checked when matching positional-only class patterns,
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this allows classes to match only positional-only patterns by setting ``__attributes__`` to ``()``.
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Class patterns with keyword patterns::
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case ClsName($VARS, $KEYWORD_PATTERNS):
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translates to::
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if $kind & MATCH_CLASS == 0:
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FAIL
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if not isinstance($value, ClsName):
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FAIL
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if $attrs is None:
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$attrs = type($value).__attributes__
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if $items is None:
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$items = type($value).__deconstruct__($value)
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$right_attrs = attrs[len($VARS):]
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if not set($right_attrs) >= set($KEYWORD_PATTERNS):
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FAIL
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$VARS = items[:len($VARS)]
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for $KEYWORD in $KEYWORD_PATTERNS:
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$index = $attrs.index(QUOTE($KEYWORD))
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$KEYWORD_PATTERNS[$KEYWORD] = $items[$index]
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Example: [6]_
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``__match_args__`` is not checked when matching positional-only class patterns,
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this allows classes to match only positional-only patterns by leaving ``__match_args__`` set to the default value of ``()``.
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Class patterns with all keyword patterns::
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@ -411,22 +400,89 @@ Class patterns with all keyword patterns::
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translates to::
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if $kind & MATCH_CLASS:
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As above with $VARS == ()
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elif $kind & MATCH_DEFAULT:
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if not isinstance($value, ClsName):
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FAIL
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if not hasattr($value, "__dict__"):
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FAIL
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if not $value.__dict__.keys() >= set($KEYWORD_PATTERNS):
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FAIL
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if $kind & MATCH_POSITIONAL:
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if $items is None:
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$items = type($value).__deconstruct__($value)
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if $attrs is None:
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$attrs = type($value).__match_args__
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$kwname_tuple = tuple(QUOTE($KEYWORD) for $KEYWORD in $KEYWORD_PATTERNS)
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$indices = multi_index($attrs, $kwname_tuple, 0)
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if $indices is None:
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raise TypeError(...)
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try:
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for $KEYWORD, $index in zip($KEYWORD_PATTERNS, indices):
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$KEYWORD_PATTERNS[$KEYWORD] = $items[$index]
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except IndexError:
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raise TypeError(...)
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elif $kind & MATCH_DEFAULT:
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try:
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for $KEYWORD in $KEYWORD_PATTERNS:
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$KEYWORD_PATTERNS[$KEYWORD] = $value.__dict__[QUOTE($KEYWORD)]
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$tmp = getattr($value, QUOTE($KEYWORD))
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$KEYWORD_PATTERNS[$KEYWORD] = $tmp
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except AttributeError:
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FAIL
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else:
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FAIL
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Where the helper function ``multi_index(t, values, min)`` returns a tuple of indices of ``values`` into ``t``,
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or ``None`` if any value is not present in ``t`` or the index of the value is less than ``min``.
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Examples::
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multi_index(("a", "b", "c"), ("a", "c"), 0) == (0,2)
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multi_index(("a", "b", "c"), ("a", "c"), 1) is None
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multi_index(("a", "b", "c"), ("a", "d"), 0) is None
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Example: [7]_
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Class patterns with positional and keyword patterns::
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case ClsName($VARS, $KEYWORD_PATTERNS):
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translates to::
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if not isinstance($value, ClsName):
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FAIL
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if $kind & MATCH_POSITIONAL:
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if $items is None:
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$items = type($value).__deconstruct__($value)
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if $attrs is None:
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$attrs = type($value).__match_args__
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if len($items) < len($VARS):
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FAIL
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$VARS = $items[:len($VARS)]
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$kwname_tuple = tuple(QUOTE($KEYWORD) for $KEYWORD in $KEYWORD_PATTERNS)
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$indices = multi_index($attrs, $kwname_tuple, len($VARS))
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if $indices is None:
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raise TypeError(...)
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try:
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for $KEYWORD, $index in zip($KEYWORD_PATTERNS, indices):
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$KEYWORD_PATTERNS[$KEYWORD] = $items[$index]
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except IndexError:
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raise TypeError(...)
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elif $kind & MATCH_DEFAULT:
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if $attrs is None:
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$attrs = type($value).__match_args__
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if len($attr) < len($VARS):
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raise TypeError(...)
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$positional_names = $attrs[:len($VARS)]
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try:
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for i, $VAR in enumerate($VARS):
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$VAR = getattr($value, $attrs[i])
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for $KEYWORD in $KEYWORD_PATTERNS:
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$name = QUOTE($KEYWORD)
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if $name in $positional_names:
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raise TypeError(...)
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$KEYWORD_PATTERNS[$KEYWORD] = getattr($value, $name)
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except AttributeError:
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FAIL
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else:
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FAIL
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Example: [8]_
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Nested patterns
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'''''''''''''''
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@ -448,14 +504,8 @@ translates to::
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FAIL
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$value_0, $value_1 = $list
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#Now match on temporary values
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$kind_0 = type($value_0).__match_kind__
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if $kind_0 & (MATCH_CLASS | MATCH_DEFAULT) == 0:
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FAIL
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if not isinstance($value_0, int):
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FAIL
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$kind_1 = type($value_1).__match_kind__
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if $kind_1 & (MATCH_CLASS | MATCH_DEFAULT) == 0:
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FAIL
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if not isinstance($value_1, str):
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FAIL
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@ -480,12 +530,12 @@ All classes should ensure that the the value of ``__match_kind__`` follows the s
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Therefore, implementations can assume, without checking, that all the following are true::
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(__match_kind__ & (MATCH_SEQUENCE | MATCH_MAPPING)) != (MATCH_SEQUENCE | MATCH_MAPPING)
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(__match_kind__ & (MATCH_SELF | MATCH_CLASS)) != (MATCH_SELF | MATCH_CLASS)
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(__match_kind__ & (MATCH_SELF | MATCH_POSITIONAL)) != (MATCH_SELF | MATCH_POSITIONAL)
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(__match_kind__ & (MATCH_SELF | MATCH_DEFAULT)) != (MATCH_SELF | MATCH_DEFAULT)
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(__match_kind__ & (MATCH_DEFAULT | MATCH_CLASS)) != (MATCH_DEFAULT | MATCH_CLASS)
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(__match_kind__ & (MATCH_DEFAULT | MATCH_POSITIONAL)) != (MATCH_DEFAULT | MATCH_POSITIONAL)
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Thus, implementations can assume that ``__match_kind__ & MATCH_SEQUENCE`` implies ``(__match_kind__ & MATCH_MAPPING) == 0``, and vice-versa.
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Likewise for ``MATCH_SELF``, ``MATCH_CLASS`` and ``MATCH_DEFAULT``.
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Likewise for ``MATCH_SELF``, ``MATCH_POSITIONAL`` and ``MATCH_DEFAULT``.
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If ``__match_kind__`` does not follow the specification,
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then implementations may treat any of the expressions of the form ``$kind & MATCH_...`` above as having any value.
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@ -509,7 +559,7 @@ For common builtin classes ``__match_kind__`` will be:
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* ``tuple``: ``MATCH_SEQUENCE | MATCH_SELF``
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* ``dict``: ``MATCH_MAPPING | MATCH_SELF``
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Named tuples will have ``__match_kind__`` set to ``MATCH_SEQUENCE | MATCH_CLASS``.
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Named tuples will have ``__match_kind__`` set to ``MATCH_SEQUENCE | MATCH_POSITIONAL``.
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* All other standard library classes for which ``issubclass(cls, collections.abc.Mapping)`` is true will have ``__match_kind__`` set to ``MATCH_MAPPING``.
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* All other standard library classes for which ``issubclass(cls, collections.abc.Sequence)`` is true will have ``__match_kind__`` set to ``MATCH_SEQUENCE``.
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@ -530,7 +580,8 @@ to treat the following functions and methods as pure:
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* ``dict.__contains__()``
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* ``dict.__getitem__()``
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Implementations are also allowed to freely replace ``isinstance(obj, cls)`` with ``issubclass(type(obj), cls)`` and vice-versa.
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Implementations are allowed to freely replace ``isinstance(obj, cls)`` with ``issubclass(type(obj), cls)`` and vice-versa.
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Implementations are also allowed to elide repeated tests of ``isinstance(obj, cls)``.
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Security Implications
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=====================
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@ -662,19 +713,33 @@ The changes to the semantics can be summarized as:
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* Selecting the kind of pattern uses ``cls.__match_kind__`` instead of
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``issubclass(cls, collections.abc.Mapping)`` and ``issubclass(cls, collections.abc.Sequence)``
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and allows classes control over which kinds of pattern they match.
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* Class matching is via the ``__attributes__`` attribute and ``__deconstruct__`` method,
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rather than the ``__match_args__`` method, and allows classes more control over how
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they are deconstructed.
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* The default behavior when matching a class pattern with keyword patterns is changed.
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Only the instance dictionary is used. This is to avoid unintended capture of bound-methods.
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* Class matching is controlled by the ``__match_kind__`` attribute,
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and the ``__deconstruct__`` method allows classes more control over how they are deconstructed.
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* The default behavior when matching a class pattern with keyword patterns is more precisely defined,
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but is broadly unchanged.
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There are no changes to syntax.
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There are no changes to syntax. All examples given in the PEP 636 tutorial should continue to work as they do now.
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Rejected Ideas
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==============
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None, as yet.
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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_kind__ == MATCH_DEFAULT``.
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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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class C:
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def __init__(self):
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self.a = "a"
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@property
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def p(self):
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...
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def m(self):
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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_DEFAULT``.
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Classes may override this behavior if needed by using ``__match_kind__ == MATCH_POSITIONAL`` or ``__match_args__``.
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Open Issues
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===========
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@ -696,8 +761,7 @@ Code examples
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::
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class Basic:
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__match_kind__ = MATCH_CLASS
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__attributes__ = ()
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__match_kind__ = MATCH_POSITIONAL
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def __deconstruct__(self):
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return self._args
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@ -777,24 +841,30 @@ translates to::
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This::
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match ClsName(x, a=y):
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match ClsName(x, y):
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translates to::
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if $kind & MATCH_CLASS == 0:
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FAIL
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if not isinstance($value, ClsName):
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FAIL
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if $attrs is None:
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$attrs = type($value).__attributes__
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if $kind & MATCH_POSITIONAL:
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if $items is None:
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$items = type($value).__deconstruct__($value)
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$right_attrs = $attrs[1:]
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if not "a" in $right_attrs:
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if len($items) < 2:
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FAIL
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x, y = $items
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elif $kind & MATCH_DEFAULT:
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if $attrs is None:
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$attrs = type($value).__match_args__
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if len($attr) < 2:
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FAIL
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try:
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x = getattr($value, $attrs[0])
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y = getattr($value, $attrs[1])
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except AttributeError:
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FAIL
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else:
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FAIL
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$y_index = $attrs.index("a")
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x = $items[0]
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y = $items[$y_index]
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.. [7]
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@ -804,36 +874,72 @@ This::
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||||
translates to::
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|
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if $kind & MATCH_CLASS:
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||||
if not isinstance($value, ClsName):
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FAIL
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if $attrs is None:
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$attrs = type($value).__attributes__
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if $kind & MATCH_POSITIONAL:
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if $items is None:
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$items = type($value).__deconstruct__($value)
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if not "a" in $attrs:
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FAIL
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if not "b" in $attrs:
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FAIL
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$x_index = $attrs.index("a")
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x = $items[$x_index]
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$y_index = $attrs.index("b")
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y = $items[$y_index]
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if $attrs is None:
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$attrs = type($value).__match_args__
|
||||
$indices = multi_index($attrs, ("a", "b"), 0)
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if $indices is None:
|
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raise TypeError(...)
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||||
try:
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||||
x = $items[$indices[0]]
|
||||
y = $items[$indices[1]]
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||||
except IndexError:
|
||||
raise TypeError(...)
|
||||
elif $kind & MATCH_DEFAULT:
|
||||
if not isinstance($value, ClsName):
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||||
try:
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||||
x = $value.a
|
||||
y = $value.b
|
||||
except AttributeError:
|
||||
FAIL
|
||||
if not hasattr($value, "__dict__"):
|
||||
FAIL
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||||
$obj_dict = $value.__dict__
|
||||
if not "a" in $attrs:
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||||
FAIL
|
||||
if not "b" in $attrs:
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||||
FAIL
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||||
x = $obj_dict["a"]
|
||||
y = $obj_dict["b"]
|
||||
else:
|
||||
FAIL
|
||||
|
||||
.. [8]
|
||||
|
||||
This::
|
||||
|
||||
match ClsName(x, a=y):
|
||||
|
||||
translates to::
|
||||
|
||||
|
||||
if not isinstance($value, ClsName):
|
||||
FAIL
|
||||
if $kind & MATCH_POSITIONAL:
|
||||
if $items is None:
|
||||
$items = type($value).__deconstruct__($value)
|
||||
if $attrs is None:
|
||||
$attrs = type($value).__match_args__
|
||||
if len($items) < 1:
|
||||
FAIL
|
||||
x = $items[0]
|
||||
$indices = multi_index($attrs, ("a",), 1)
|
||||
if $indices is None:
|
||||
raise TypeError(...)
|
||||
$index = $indices[0]
|
||||
try:
|
||||
y = $items[$index]
|
||||
except IndexError:
|
||||
raise TypeError(...)
|
||||
elif $kind & MATCH_DEFAULT:
|
||||
if $attrs is None:
|
||||
$attrs = type($value).__match_args__
|
||||
if len($attr) < 1:
|
||||
raise TypeError(...)
|
||||
$positional_names = $attrs[:1]
|
||||
try:
|
||||
x = getattr($value, $attrs[0])
|
||||
if "a" in $positional_names:
|
||||
raise TypeError(...)
|
||||
y = $value.a
|
||||
except AttributeError:
|
||||
FAIL
|
||||
else:
|
||||
FAIL
|
||||
|
||||
Copyright
|
||||
=========
|
||||
|
|
|
|||
Loading…
Reference in New Issue