gh-100039: enhance __signature__ to work with str and callables (GH-100168)

Callables should be either class- or static-methods.
Enum now uses the classmethod version to greatly improve the help
given for enums and flags.

1 files changed, 9 insertions, 1 deletions

diff --git a/Lib/inspect.py b/Lib/inspect.py
index e165937..e92c355 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -2443,10 +2443,18 @@ def _signature_from_callable(obj, *,
         pass
     else:
         if sig is not None:
+            # since __text_signature__ is not writable on classes, __signature__
+            # may contain text (or be a callable that returns text);
+            # if so, convert it
+            o_sig = sig
+            if not isinstance(sig, (Signature, str)) and callable(sig):
+                sig = sig()
+            if isinstance(sig, str):
+                sig = _signature_fromstr(sigcls, obj, sig)
             if not isinstance(sig, Signature):
                 raise TypeError(
                     'unexpected object {!r} in __signature__ '
-                    'attribute'.format(sig))
+                    'attribute'.format(o_sig))
             return sig
 
     try: