[issue40397] Refactor typing._GenericAlias

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

@Serhiy can you look at this? A possible fix might be:

diff --git a/Lib/typing.py b/Lib/typing.py
index 681ab6d21e..adcef1e82b 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -701,7 +701,8 @@ class _GenericAlias(_BaseGenericAlias, _root=True):
                 arg = subst[arg]
             elif isinstance(arg, (_BaseGenericAlias, GenericAlias)):
                 subargs = tuple(subst[x] for x in arg.__parameters__)
-                arg = arg[subargs]
+                if subargs:
+                    arg = arg[subargs]
             new_args.append(arg)
         return self.copy_with(tuple(new_args))
 

The immediate cause of the problem seems to be that we call arg[subargs] with subargs being (). So arg is a _GenericAlias but its __parameters__ is (). I'm not sure what path was taken previously. :-(

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40397>
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