While this fixes your particular use case - it will break stuff for other people. Now all UUID fields on a model will be an union of str and UUID and client code will need to handle those cases.

I think the right solution here is to extend mypy plugin.
Basically what needs to be done is to hook into get_function_hook and return proper type (Instance with type argument set to str or UUID ) based on as_uuid argument.

I wonder if it would be possible to use an overloaded function with literal types to fix this? Maybe something like subprocess.Popen: https://github.com/python/typeshed/blob/master/stdlib/3/subprocess.pyi#L809

I think we should try what @JukkaL proposes, using a union in this context may cause many false positives.

I wonder if it would be possible to use an overloaded function with literal types to fix this? Maybe something like subprocess.Popen: https://github.com/python/typeshed/blob/master/stdlib/3/subprocess.pyi#L809

It seems I took too long to reply and this line moved? Otherwise I don't see how line 809 of current master relates to this. 😕

I guess this is what you had in mind? https://github.com/python/typeshed/blob/3ad3ed82c75b4fb82f71f5500b50f18a98f12f49/stdlib/3/subprocess.pyi#L809

I just tried the following:

diff --git a/sqlalchemy-stubs/dialects/postgresql/base.pyi b/sqlalchemy-stubs/dialects/postgresql/base.pyi
index d910e5c..2c6917f 100644
--- a/sqlalchemy-stubs/dialects/postgresql/base.pyi
+++ b/sqlalchemy-stubs/dialects/postgresql/base.pyi
@@ -1,8 +1,9 @@
 from ... import schema
 from ...engine import default, reflection
 from ...sql import compiler, expression, sqltypes, type_api
-from typing import Any, Optional, Set, Type, Text, Pattern, Dict
+from typing import Any, Optional, Set, Type, Text, Pattern, Dict, overload
 from datetime import timedelta
+import uuid
 
 from sqlalchemy.types import INTEGER as INTEGER, BIGINT as BIGINT, SMALLINT as SMALLINT, VARCHAR as VARCHAR, \
     CHAR as CHAR, TEXT as TEXT, FLOAT as FLOAT, NUMERIC as NUMERIC, \
@@ -66,12 +67,20 @@ class BIT(sqltypes.TypeEngine[str]):
     def __init__(self, length: Optional[int] = ..., varying: bool = ...) -> None: ...
 PGBit = BIT
 
+@overload
 class UUID(sqltypes.TypeEngine[str]):
     __visit_name__: str = ...
     as_uuid: bool = ...
     def __init__(self, as_uuid: bool = ...) -> None: ...
     def bind_processor(self, dialect: Any): ...
     def result_processor(self, dialect: Any, coltype: Any): ...
+@overload
+class UUID(sqltypes.TypeEngine[uuid.UUID]):
+    __visit_name__: str = ...
+    as_uuid: bool = ...
+    def __init__(self, as_uuid: bool = ...) -> None: ...
+    def bind_processor(self, dialect: Any): ...
+    def result_processor(self, dialect: Any, coltype: Any): ...
 PGUuid = UUID
 
 class TSVECTOR(sqltypes.TypeEngine[str]):

However that doesn't work (same error message as #94). It seems @overload only works for functions and methods, not classes.

You need to overload the constructor, not the class itself.

@ilevkivskyi I made #185 to overload the constructor as per your suggestion. It seems to be pretty quiet here, it almost seems abandoned. Is it time for a community fork or is this just a temporary thing?

Any news?
I have a column uuid = db.Column(UUID(as_uuid=True), primary_key=True, default=uuid4)
When I pass an uuid while instantiating the model I get a warning saying that the uuid is expecting Optional[str].

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Hi, I'm @bochecha in this ticket, but due to $circumstances I had to create a new account under my real name instead of my nickname.

I'm still happy to see people interested in following on this ticket and getting a way to find its way in, but if typing is now maintained by @zzzeek upstream I'm not sure we even need this ticket anymore ?

Sorry about not giving any sign of life this whole time.

($circumstances being my being hospitalized for 2 years and having to give up on all my floss activities during that time 😞)