Minor polish to #815

Signed-off-by: Hynek Schlawack <hs@ox.cx>

changelog.d/815.changes.rst

@@ -1,3 +1,4 @@
 ``__match_args__`` are now generated to support Python 3.10's
 `Structural Pattern Matching <https://docs.python.org/3.10/whatsnew/3.10.html#pep-634-structural-pattern-matching>`_.
-This can be controlled by ``match_args`` argument to the class decorators.
+This can be controlled by the ``match_args`` argument to the class decorators on Python 3.10 and later.
+On older versions, it is never added and the argument is ignored.

src/attr/_make.py

@@ -13,6 +13,7 @@
 from . import _config, setters
 from ._compat import (
     PY2,
+    PY310,
     PYPY,
     isclass,
     iteritems,
@@ -1422,10 +1423,11 @@ def attrs(
         fields based on their types.  See `transform-fields` for more details.
 
     :param bool match_args:
-        If `True`, set ``__match_args__`` on the class to support `PEP 634
-        <https://www.python.org/dev/peps/pep-0634/>`_ (Structural Pattern
-        Matching). It is a tuple of all positional-only ``__init__`` parameter
-        names.
+        If `True` (default), set ``__match_args__`` on the class to support
+        `PEP 634 <https://www.python.org/dev/peps/pep-0634/>`_ (Structural
+        Pattern Matching). It is a tuple of all positional-only ``__init__``
+        parameter names on Python 3.10 and later. Ignored on older Python
+        versions.
 
     .. versionadded:: 16.0.0 *slots*
     .. versionadded:: 16.1.0 *frozen*
@@ -1577,7 +1579,11 @@ def wrap(cls):
                     " init must be True."
                 )
 
-        if match_args and not _has_own_attribute(cls, "__match_args__"):
+        if (
+            PY310
+            and match_args
+            and not _has_own_attribute(cls, "__match_args__")
+        ):
             builder.add_match_args()
 
         return builder.build_class()

tests/test_make.py

@@ -21,7 +21,7 @@
 import attr
 
 from attr import _config
-from attr._compat import PY2, ordered_dict
+from attr._compat import PY2, PY310, ordered_dict
 from attr._make import (
     Attribute,
     Factory,
@@ -2328,46 +2328,59 @@ def __setstate__(self, state):
         assert True is i.called
         assert None is getattr(C(), "__getstate__", None)
 
+    @pytest.mark.skipif(PY310, reason="Pre-3.10 only.")
+    def test_match_args_pre_310(self):
+        """
+        __match_args__ is not created on Python versions older than 3.10.
+        """
+
+        @attr.s
+        class C(object):
+            a = attr.ib()
+
+        assert None is getattr(C, "__match_args__", None)
 
+
+@pytest.mark.skipif(not PY310, reason="Structural pattern matching is 3.10+")
 class TestMatchArgs(object):
     """
     Tests for match_args and __match_args__ generation.
     """
 
     def test_match_args(self):
         """
-        __match_args__ generation
+        __match_args__ is created by default on Python 3.10.
         """
 
-        @attr.s()
-        class C(object):
-            a = attr.ib()
+        @attr.define
+        class C:
+            a = attr.field()
 
-        assert C.__match_args__ == ("a",)
+        assert ("a",) == C.__match_args__
 
     def test_explicit_match_args(self):
         """
-        __match_args__ manually set is not overriden.
+        A custom __match_args__ set is not overwritten.
         """
 
         ma = ()
 
-        @attr.s()
-        class C(object):
-            a = attr.ib()
+        @attr.define
+        class C:
+            a = attr.field()
             __match_args__ = ma
 
         assert C(42).__match_args__ is ma
 
     @pytest.mark.parametrize("match_args", [True, False])
     def test_match_args_attr_set(self, match_args):
         """
-        __match_args__ being set depending on match_args.
+        __match_args__ is set depending on match_args.
         """
 
-        @attr.s(match_args=match_args)
-        class C(object):
-            a = attr.ib()
+        @attr.define(match_args=match_args)
+        class C:
+            a = attr.field()
 
         if match_args:
             assert hasattr(C, "__match_args__")
@@ -2376,21 +2389,21 @@ class C(object):
 
     def test_match_args_kw_only(self):
         """
-        kw_only being set doesn't generate __match_args__
-        kw_only field is not included in __match_args__
+        kw_only classes don't generate __match_args__.
+        kw_only fields are not included in __match_args__.
         """
 
-        @attr.s()
-        class C(object):
-            a = attr.ib(kw_only=True)
-            b = attr.ib()
+        @attr.define
+        class C:
+            a = attr.field(kw_only=True)
+            b = attr.field()
 
         assert C.__match_args__ == ("b",)
 
-        @attr.s(match_args=True, kw_only=True)
-        class C(object):
-            a = attr.ib()
-            b = attr.ib()
+        @attr.define(kw_only=True)
+        class C:
+            a = attr.field()
+            b = attr.field()
 
         assert C.__match_args__ == ()
 
@@ -2399,33 +2412,33 @@ def test_match_args_argument(self):
         match_args being False with inheritance.
         """
 
-        @attr.s(match_args=False)
-        class X(object):
-            a = attr.ib()
+        @attr.define(match_args=False)
+        class X:
+            a = attr.field()
 
         assert "__match_args__" not in X.__dict__
 
-        @attr.s(match_args=False)
-        class Y(object):
-            a = attr.ib()
+        @attr.define(match_args=False)
+        class Y:
+            a = attr.field()
             __match_args__ = ("b",)
 
         assert Y.__match_args__ == ("b",)
 
-        @attr.s(match_args=False)
+        @attr.define(match_args=False)
         class Z(Y):
-            z = attr.ib()
+            z = attr.field()
 
         assert Z.__match_args__ == ("b",)
 
-        @attr.s()
-        class A(object):
-            a = attr.ib()
-            z = attr.ib()
+        @attr.define
+        class A:
+            a = attr.field()
+            z = attr.field()
 
-        @attr.s(match_args=False)
+        @attr.define(match_args=False)
         class B(A):
-            b = attr.ib()
+            b = attr.field()
 
         assert B.__match_args__ == ("a", "z")
 
@@ -2435,13 +2448,13 @@ def test_make_class(self):
         """
 
         C1 = make_class("C1", ["a", "b"])
-        assert C1.__match_args__ == ("a", "b")
+        assert ("a", "b") == C1.__match_args__
 
         C1 = make_class("C1", ["a", "b"], match_args=False)
         assert not hasattr(C1, "__match_args__")
 
         C1 = make_class("C1", ["a", "b"], kw_only=True)
-        assert C1.__match_args__ == ()
+        assert () == C1.__match_args__
 
         C1 = make_class("C1", {"a": attr.ib(kw_only=True), "b": attr.ib()})
-        assert C1.__match_args__ == ("b",)
+        assert ("b",) == C1.__match_args__

tests/test_pattern_matching.py

@@ -1,27 +1,30 @@
 # flake8: noqa
-# Python 3.10 issue in flake8 : https://github.com/PyCQA/pyflakes/issues/634
+# Python 3.10 issue in flake8: https://github.com/PyCQA/pyflakes/issues/634
+# Keep this file SHORT, until Black and flake8 can handle it.
 import pytest
 
 import attr
 
-from attr._make import make_class
+from attr import make_class
 
 
-class TestPatternMatching(object):
+class TestPatternMatching:
     """
     Pattern matching syntax test cases.
     """
 
-    def test_simple_match_case(self):
+    @pytest.mark.parametrize("dec", [attr.s, attr.define, attr.frozen])
+    def test_simple_match_case(self, dec):
         """
-        Simple match case statement
+        Simple match case statement works as expected with all class
+        decorators.
         """
 
-        @attr.s()
+        @dec
         class C(object):
             a = attr.ib()
 
-        assert C.__match_args__ == ("a",)
+        assert ("a",) == C.__match_args__
 
         matched = False
         c = C(a=1)
@@ -33,14 +36,14 @@ class C(object):
 
     def test_explicit_match_args(self):
         """
-        Manually set empty __match_args__ will not match.
+        Does not overwrite a manually set empty __match_args__.
         """
 
         ma = ()
 
-        @attr.s()
-        class C(object):
-            a = attr.ib()
+        @attr.define
+        class C:
+            a = attr.field()
             __match_args__ = ma
 
         c = C(a=1)
@@ -53,16 +56,16 @@ class C(object):
 
     def test_match_args_kw_only(self):
         """
-        kw_only being set doesn't generate __match_args__
-        kw_only field is not included in __match_args__
+        kw_only classes don't generate __match_args__.
+        kw_only fields are not included in __match_args__.
         """
 
-        @attr.s()
-        class C(object):
-            a = attr.ib(kw_only=True)
-            b = attr.ib()
+        @attr.define
+        class C:
+            a = attr.field(kw_only=True)
+            b = attr.field()
 
-        assert C.__match_args__ == ("b",)
+        assert ("b",) == C.__match_args__
 
         c = C(a=1, b=1)
         msg = r"C\(\) accepts 1 positional sub-pattern \(2 given\)"
@@ -78,10 +81,10 @@ class C(object):
 
         assert found
 
-        @attr.s(match_args=True, kw_only=True)
-        class C(object):
-            a = attr.ib()
-            b = attr.ib()
+        @attr.define(kw_only=True)
+        class C:
+            a = attr.field()
+            b = attr.field()
 
         c = C(a=1, b=1)
         msg = r"C\(\) accepts 0 positional sub-patterns \(2 given\)"