TypeVar with constraints causes conflicting type checker errors - redundant cast vs incompatible assignment #2018

hunterhogan · 2025-06-04T20:45:48Z

hunterhogan
Jun 4, 2025

The type checkers are wrong, but I don't really care. I just want any (manageable) syntactic system in which the type checkers will get the typing right.

If the type checkers will work, I'm willing to learn other systems, like protocol or I-don't-know-what.

Mypy states it most clearly: when Mypy looks at the right-hand side, node.annotation d'RHS is type fooT, therefore I'm an idiot because cast is redundant; and when Mypy looks at the left-hand side, node.annotation d'RHS is type ast.expr, therefore I'm an idiot for assigning it to node.annotation d'LHS because it clearly has type ast.expr | None. But maybe my quantum computer is causing this superposition of type states.

One example

from collections.abc import Callable
from typing import cast, TypeVar
import ast

fooT = TypeVar('fooT', ast.expr, ast.expr | None, covariant=True)
barT = TypeVar('barT', ast.AnnAssign, ast.arg)

def foo(param: Callable[[fooT], fooT]) -> Callable[[barT], barT]:
    def bar(node: barT) -> barT:
        node.annotation = param(cast(fooT, cast(barT, node).annotation))
        return node
    return bar

Mypy(assignment) Incompatible types in assignment (expression has type "expr | None", variable has type "expr")
Mypy(redundant-cast) Redundant cast to "expr"
Mypy(redundant-cast) Redundant cast to "expr | None"
Mypy(redundant-cast) Redundant cast to "AnnAssign"
Mypy(redundant-cast) Redundant cast to "arg"
Pylance(reportAttributeAccessIssue) Cannot assign to attribute "annotation" for class "AnnAssign*"
Expression of type "fooT@foo" cannot be assigned to attribute "annotation" of class "AnnAssign"
Type "expr* | None" is not assignable to type "expr"
"None*" is not assignable to "expr"
Pylance(reportUnnecessaryCast) Unnecessary "cast" call; type is already "barT@foo"

What should I do?

Some things that might contribute to a solution:

typing.overload
typing.TypeAlias
Not an elephant
typing.TypeVar
typing.TypeVarTuple
Healthcare
typing.ParamSpec
Not a rhino
typing.Generic
Divine intervention
typing.Protocol
Cross an elephant and a rhino?
Elephino

Repos for masochists

The code is in astToolkit, but most of that package is generated by astToolFactory. Those packages exist because I transform copious amounts of code in mapFolding.

Many tests

from collections.abc import Callable
from typing import cast, overload, TypeAlias, TypeVar
import ast

TypeAlias_AnnAssign_arg: TypeAlias = ast.AnnAssign | ast.arg
TypeVar_AnnAssign_arg = TypeVar('TypeVar_AnnAssign_arg', ast.AnnAssign, ast.arg)
TypeVar_ast_expr_constraints_covariant = TypeVar('TypeVar_ast_expr_constraints_covariant', ast.expr, ast.expr | None, covariant=True)

class Grab:
    @staticmethod # No error
    def test_ast_arg(action: Callable[[ast.expr | None], ast.expr | None]) -> Callable[[ast.arg], ast.arg]: # No error
        def workhorse(node: ast.arg) -> ast.arg: # No error
            node.annotation = action(node.annotation) # No error
            return node # No error
        return workhorse # No error

    @staticmethod
    def testTransitiveExplicitTypeVarTypeVarCastNestCast(action: Callable[[TypeVar_ast_expr_constraints_covariant], TypeVar_ast_expr_constraints_covariant]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
            sherpa: TypeVar_ast_expr_constraints_covariant = action(cast(TypeVar_ast_expr_constraints_covariant, cast(TypeVar_AnnAssign_arg, node).annotation)) # pyright: ignore[reportUnnecessaryCast, reportUnnecessaryCast]
            """
            Mypy says in four messages:
            Redundant cast to "expr"
            Redundant cast to "expr | None"
            Redundant cast to "AnnAssign"
            Redundant cast to "arg"
            """
            node.annotation = sherpa # pyright: ignore[reportAttributeAccessIssue]
            # Mypy(assignment) Incompatible types in assignment (expression has type "expr | None", variable has type "expr")
            return node
        return workhorse

    @staticmethod
    def testTransitiveInferredTypeVarTypeVarCastNestCast(action: Callable[[TypeVar_ast_expr_constraints_covariant], TypeVar_ast_expr_constraints_covariant]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
            sherpa = action(cast(TypeVar_ast_expr_constraints_covariant, cast(TypeVar_AnnAssign_arg, node).annotation)) # pyright: ignore[reportUnnecessaryCast, reportUnnecessaryCast]
            """
            Mypy says in four messages:
            Redundant cast to "expr"
            Redundant cast to "expr | None"
            Redundant cast to "AnnAssign"
            Redundant cast to "arg"
            """
            node.annotation = sherpa # pyright: ignore[reportAttributeAccessIssue]
            # Mypy(assignment) Incompatible types in assignment (expression has type "expr | None", variable has type "expr")
            return node
        return workhorse

    @staticmethod
    def testTypeVarTypeVarCastNestCast(action: Callable[[TypeVar_ast_expr_constraints_covariant], TypeVar_ast_expr_constraints_covariant]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
            node.annotation = action(cast(TypeVar_ast_expr_constraints_covariant, cast(TypeVar_AnnAssign_arg, node).annotation)) # pyright: ignore[reportUnnecessaryCast, reportUnnecessaryCast, reportAttributeAccessIssue]
            """
            Mypy says in four messages:
            Redundant cast to "expr"
            Redundant cast to "expr | None"
            Redundant cast to "AnnAssign"
            Redundant cast to "arg"
            """
            return node
        return workhorse

    # @staticmethod
    # def testTypeVarTypeVarCastLHSCast(action: Callable[[TypeVar_ast_expr_constraints_covariant], TypeVar_ast_expr_constraints_covariant]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
    #     def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
    #         # Ruff: "SyntaxError: Invalid assignment target"
    #         # Mypy(syntax) cannot assign to function call here. Maybe you meant '==' instead of '='?
    #         cast(TypeVar_ast_expr_constraints_covariant, node.annotation) = action(cast(TypeVar_ast_expr_constraints_covariant, node.annotation)) # pyright: ignore[reportGeneralTypeIssues] "Expression cannot be assignment target"
    #         return node
    #     return workhorse

    @staticmethod
    def testTypeVarTypeVarCast(action: Callable[[TypeVar_ast_expr_constraints_covariant], TypeVar_ast_expr_constraints_covariant]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
            node.annotation = action(cast(TypeVar_ast_expr_constraints_covariant, node.annotation)) # pyright: ignore[reportAttributeAccessIssue]
            return node
        return workhorse

    @staticmethod
    def testTypeVarTypeVar(action: Callable[[TypeVar_ast_expr_constraints_covariant], TypeVar_ast_expr_constraints_covariant]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
            node.annotation = action(node.annotation) # pyright: ignore[reportAttributeAccessIssue, reportArgumentType]
            return node
        return workhorse

    @overload
    @staticmethod
    def testOverloadTypeVarTypeVar(action: Callable[[ast.expr], ast.expr]) -> Callable[[ast.AnnAssign], ast.AnnAssign]:... # pyright: ignore[reportOverlappingOverload]
    @overload
    @staticmethod
    def testOverloadTypeVarTypeVar(action: Callable[[ast.expr | None], ast.expr | None]) -> Callable[[ast.arg], ast.arg]:...
    @staticmethod
    def testOverloadTypeVarTypeVar(action: Callable[[TypeVar_ast_expr_constraints_covariant], TypeVar_ast_expr_constraints_covariant]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
            node.annotation = action(node.annotation) # pyright: ignore[reportAttributeAccessIssue, reportArgumentType]
            return node
        return workhorse

    @overload
    @staticmethod
    def testOverloadTypeVar(action: Callable[[ast.expr], ast.expr]) -> Callable[[ast.AnnAssign], ast.AnnAssign]:... # pyright: ignore[reportOverlappingOverload]
    @overload
    @staticmethod
    def testOverloadTypeVar(action: Callable[[ast.expr | None], ast.expr | None]) -> Callable[[ast.arg], ast.arg]:...
    @staticmethod
    def testOverloadTypeVar(action: Callable[[ast.expr], ast.expr] | Callable[[ast.expr | None], ast.expr | None]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg:
            node.annotation = action(node.annotation) # pyright: ignore[reportAttributeAccessIssue, reportArgumentType]
            return node
        return workhorse

    @overload
    @staticmethod
    def testOverloadOverloadTypeVar(action: Callable[[ast.expr], ast.expr]) -> Callable[[ast.AnnAssign], ast.AnnAssign]:... # pyright: ignore[reportOverlappingOverload]
    @overload
    @staticmethod
    def testOverloadOverloadTypeVar(action: Callable[[ast.expr | None], ast.expr | None]) -> Callable[[ast.arg], ast.arg]:...
    @staticmethod
    def testOverloadOverloadTypeVar(action: Callable[[ast.expr], ast.expr] | Callable[[ast.expr | None], ast.expr | None]) -> Callable[[TypeVar_AnnAssign_arg], TypeVar_AnnAssign_arg]:
        @overload
        def workhorse(node: ast.AnnAssign) -> ast.AnnAssign:...
        @overload
        def workhorse(node: ast.arg) -> ast.arg:...
        def workhorse(node: TypeVar_AnnAssign_arg) -> TypeVar_AnnAssign_arg: # pyright: ignore[reportInconsistentOverload]
            node.annotation = action(node.annotation) # pyright: ignore[reportAttributeAccessIssue, reportArgumentType]
            return node
        return workhorse # pyright: ignore[reportReturnType]

    @overload
    @staticmethod
    def annotationAttribute(action: Callable[[ast.expr], ast.expr]) -> Callable[[ast.AnnAssign], ast.AnnAssign]:... # pyright: ignore[reportOverlappingOverload]
    @overload
    @staticmethod
    def annotationAttribute(action: Callable[[ast.expr | None], ast.expr | None]) -> Callable[[ast.arg], ast.arg]:...
    @staticmethod
    def annotationAttribute(action: Callable[[ast.expr], ast.expr] | Callable[[ast.expr | None], ast.expr | None]) -> Callable[[ast.AnnAssign], ast.AnnAssign] | Callable[[ast.arg], ast.arg]:
        @overload
        def workhorse(node: ast.AnnAssign) -> ast.AnnAssign:...
        @overload
        def workhorse(node: ast.arg) -> ast.arg:...
        def workhorse(node: ast.AnnAssign | ast.arg) -> ast.AnnAssign | ast.arg:
            node.annotation = action(node.annotation) # pyright: ignore[reportAttributeAccessIssue, reportArgumentType]
            return node
        return workhorse

hunterhogan · 2025-06-06T16:25:49Z

hunterhogan
Jun 6, 2025
Author

please tell me this is not the solution

@staticmethod
def testAttribute(
    action: Callable[[TypeVar_ast_expr_bound_covariant], TypeVar_ast_expr_bound_covariant]
    # action: Callable[[ast.expr], ast.expr]
    ) -> Callable[[hasDOTtest], hasDOTtest]:
    def workhorse(node: hasDOTtest) -> hasDOTtest:
        setattr(node, 'test', action(getattr(node, 'test')))
        return node
    return workhorse

I hate setattr. It's not semantic. Or, I guess, in the past, there has always been a better, semantic, type-preserving process than setattr. But--I have only done ONE test--it seems to preserve the type across multiple statements, and it does not cause erroneous diagnostic errors.

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TypeVar with constraints causes conflicting type checker errors - redundant cast vs incompatible assignment #2018

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Uh oh!

TypeVar with constraints causes conflicting type checker errors - redundant cast vs incompatible assignment #2018

Uh oh!

hunterhogan Jun 4, 2025

One example

What should I do?

Repos for masochists

Many tests

Replies: 1 comment

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hunterhogan Jun 6, 2025 Author

please tell me this is not the solution

hunterhogan
Jun 4, 2025

hunterhogan
Jun 6, 2025
Author