bpo-41905: Add abc.update_abstractmethods() (GH-22485)

This function recomputes `cls.__abstractmethods__`.
Also update `@dataclass` to use it.

1 files changed, 38 insertions, 0 deletions

diff --git a/Lib/abc.py b/Lib/abc.py
index 431b640..276ef9a 100644
--- a/Lib/abc.py
+++ b/Lib/abc.py
@@ -122,6 +122,44 @@ else:
             _reset_caches(cls)
 
 
+def update_abstractmethods(cls):
+    """Recalculate the set of abstract methods of an abstract class.
+
+    If a class has had one of its abstract methods implemented after the
+    class was created, the method will not be considered implemented until
+    this function is called. Alternatively, if a new abstract method has been
+    added to the class, it will only be considered an abstract method of the
+    class after this function is called.
+
+    This function should be called before any use is made of the class,
+    usually in class decorators that add methods to the subject class.
+
+    Returns cls, to allow usage as a class decorator.
+
+    If cls is not an instance of ABCMeta, does nothing.
+    """
+    if not hasattr(cls, '__abstractmethods__'):
+        # We check for __abstractmethods__ here because cls might by a C
+        # implementation or a python implementation (especially during
+        # testing), and we want to handle both cases.
+        return cls
+
+    abstracts = set()
+    # Check the existing abstract methods of the parents, keep only the ones
+    # that are not implemented.
+    for scls in cls.__bases__:
+        for name in getattr(scls, '__abstractmethods__', ()):
+            value = getattr(cls, name, None)
+            if getattr(value, "__isabstractmethod__", False):
+                abstracts.add(name)
+    # Also add any other newly added abstract methods.
+    for name, value in cls.__dict__.items():
+        if getattr(value, "__isabstractmethod__", False):
+            abstracts.add(name)
+    cls.__abstractmethods__ = frozenset(abstracts)
+    return cls
+
+
 class ABC(metaclass=ABCMeta):
     """Helper class that provides a standard way to create an ABC using
     inheritance.