Simplify unions with common base class.

Detects if a `Union` type is redundant. Refs #121.

Includes benchmark change.

multimethod/__init__.py

@@ -29,6 +29,13 @@ def get_mro(cls) -> tuple:  # `inspect.getmro` doesn't handle all cases
     return type.mro(cls) if isinstance(cls, type) else cls.mro()
 
 
+def common_bases(*bases):
+    counts = collections.Counter()
+    for base in bases:
+        counts.update(cls for cls in get_mro(base) if issubclass(abc.ABCMeta, type(cls)))
+    return tuple(cls for cls in counts if counts[cls] == len(bases))
+
+
 class subtype(abc.ABCMeta):
     """A normalized generic type which checks subscripts.
 
@@ -58,12 +65,11 @@ def __new__(cls, tp, *args):
             return origin
         bases = (origin,) if type(origin) in (type, abc.ABCMeta) else ()
         if origin is Literal:
-            bases = (subtype(Union[tuple(map(type, args))]),)
+            bases = (cls(Union[tuple(map(type, args))]),)
         if origin is Union:
-            counts = collections.Counter()
-            for arg in args:
-                counts.update(cls for cls in get_mro(arg) if issubclass(abc.ABCMeta, type(cls)))
-            bases = tuple(cls for cls in counts if counts[cls] == len(args))[:1]
+            bases = common_bases(*args)[:1]
+            if bases[0] in args:
+                return bases[0]
         if origin is Callable and args[:1] == (...,):
             args = args[1:]
         namespace = {'__origin__': origin, '__args__': args}

tests/test_dispatch.py

@@ -36,7 +36,6 @@ def _(left, right):
     return 'paper covers rock'
 
 
-@pytest.mark.benchmark
 def test_roshambo():
     assert roshambo.__name__ == 'roshambo'
     r, p, s = rock(), paper(), scissors()

tests/test_methods.py

@@ -36,6 +36,7 @@ def join(seq: tree, sep: object):
     return join(seq.walk(), sep)
 
 
+@pytest.mark.benchmark
 def test_join():
     sep = '<>'
     seq = [0, tree([1]), 2]
@@ -94,7 +95,7 @@ def test_signature():
     assert (type,) - signature([object]) == (1,)
     # using EnumMeta because it is a standard, stable, metaclass
     assert signature([enum.EnumMeta]) - signature([object]) == (2,)
-    assert signature([Union[type, enum.EnumMeta]]) - signature([object]) == (2,)
+    assert signature([Union[type, enum.EnumMeta]]) - signature([object]) == (1,)
 
 
 class namespace:

tests/test_subscripts.py

@@ -34,6 +34,7 @@ def func(arg: Literal['a', 0]):
 def test_union():
     assert issubclass(int, subtype(int | float))
     assert issubclass(subtype(int | float), subtype(int | float | None))
+    assert subtype(Iterable | Mapping | Sequence) is Iterable
 
 
 @pytest.mark.skipif(sys.version_info < (3, 12), reason="Type aliases added in 3.12")