Help genericising a class

[OddBloke]

I have the following class:

class DictWrapper:

    def __init__(self, dict_type: Type[MutableMapping]) -> None: ...

    def get_internal_thing(self) -> MutableMapping[str, str]: ...

which seems to work fine. However, if I want to be able to treat the return value of get_internal_thing as the type that was passed in to __init__ (e.g. DictWrapper(OrderedDict).get_internal_thing().move_to_end('a')) then I (unsurprisingly) run in to problems.

I've tried genericising it like this:

T = TypeVar('T')


class GenericDictWrapper(Generic[T]):

    def __init__(self, dict_type: Type[T]) -> None: ...

    def get_internal_thing(self) -> T: ...

As I've dropped all reference to str, it's not especially surprising that it doesn't work;

GenericDictWrapper(OrderedDict).get_internal_thing().move_to_end('a')

gives:

test.pyi:22: error: Argument 1 to "move_to_end" of "OrderedDict" has incompatible type "str"; expected "_KT"

I'm not really sure where to proceed from here; it really feels like I'm missing something. Can anyone help me out?

[OddBloke]

Another thing that occurred to me as a possibility was bounding my type variable:

S = TypeVar('S', bound=MutableMapping[str, str])


class BoundGenericDictWrapper(Generic[S]):

    def __init__(self, dict_type: Type[S]) -> None: ...

    def get_internal_thing(self) -> S: ...

but this still doesn't work:

BoundGenericDictWrapper(OrderedDict).get_internal_thing().move_to_end('a')

gives:

test.pyi:35: error: Value of type variable "S" of "BoundGenericDictWrapper" cannot be OrderedDict[_KT, _VT]
test.pyi:35: error: Argument 1 to "move_to_end" of "OrderedDict" has incompatible type "str"; expected "_KT"

[OddBloke]

I have just noticed that passing an OrderedDict[str, str] in does cause both GenericDictWrapper and BoundGenericDictWrapper to work as I expect, but existing callers of the code just specify what dict implementation they want to use without typing information, and I'd like to retain that.

[ilevkivskyi]

I think this is a bug in how Type[t] is implemented. It should never complain about _KT because these type variables should be substituted with Any.

[OddBloke]

I've spent a while digging in to this, and the following diff solves my case and doesn't break any existing tests.

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index 4ac17c37..5658b7b0 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -154,6 +154,10 @@ class ExpressionChecker(ExpressionVisitor[Type]):
         elif isinstance(node, TypeInfo):
             # Reference to a type object.
             result = type_object_type(node, self.named_type)
+            if isinstance(self.type_context[-1], TypeType):
+                # This is the type in a Type[] expression, so substitute type
+                # variables with Any.
+                result = erasetype.erase_typevars(result)
         elif isinstance(node, MypyFile):
             # Reference to a module object.
             try:

[ilevkivskyi]

Actually, this seems to be the right idea. Would you like to make a PR (with tests)?

[OddBloke]

For sure!