[New-bugs-announce] [issue46373] TypedDict and NamedTuple do not evaluate cross-module ForwardRef in all cases

Andreas H. report at bugs.python.org
Fri Jan 14 04:06:20 EST 2022

New submission from Andreas H. <ahangauer at gmx.net>:

TypedDict does not resolve cross-module ForwardRefs when the ForwardRef is not a direct one. 

In other words the fix GH-27017 (issue 41249) for TypedDict seems incomplete.

The same issue seem to exist for NamedTuple.


   TD = typing.TypedDict("TD", {'test': typing.List[typing.Optional['Y']]})
   class Y:

   # other module
   class TDSub(module.TD):
       a: int
   # -> Exception   NameError: Y not found

On the other hand, with direct ForwardRef, as e.g. in 
   TD = typing.TypedDict("TD", {'test':  'Y' } )

it works (that was indeed fixed by GH-27017)

Same issue exists for NamedTuple. There neither of the above works, i.e. cross-module ForwardRefs are never resolve (but they could - als NamedTuple has the __module__ member set with to calling module). I am not sure if inheritance for NamedTuple is supported so I do not know if it is really a bug. 

The problem in the code is that in TypedDict the `module` parameter is passed only onto the immediate ForwardRef. One option could be to recursively walk the type and search for unpatched ForwardRefs and set the module parameter.
On the other hand, the retroactive patching of forward refs is problematic as it may mess with the caching mechanism im typing.py. There may be a type with ForwardRef already in cache (and used by more than one user), but only for one user the `module` is to be updated. So probably a correct implementation is tricky, or some other way has to be found to update the `module` of ForwardRefs (e.g. by copying the type tree).

For NamedTuple the whole mechanism of passing the `module` parameter to the ForwardRefs is not done (not even for direct ForwardRef ones).

Not sure how important this (likely not very) is as I do not use TypedDict and NamedTuple. This is just to report it.

components: Library (Lib)
messages: 410547
nosy: AlexWaygood, Jelle Zijlstra, andreash, gvanrossum, kj, kumaraditya303, sobolevn
priority: normal
severity: normal
status: open
title: TypedDict and NamedTuple do not evaluate cross-module ForwardRef in all cases
type: behavior
versions: Python 3.10, Python 3.11

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