Forbids type reuse, closes #219

Author: sobolevn

classes/contrib/mypy/features/typeclass.py

@@ -105,17 +105,19 @@ def typeclass_def_return_type(ctx: MethodContext) -> MypyType:
     assert isinstance(ctx.default_return_type, Instance)
     assert isinstance(ctx.context, Decorator)
 
+    instance_args.mutate_typeclass_def(
+        ctx.default_return_type,
+        ctx.context.func.fullname,
+        ctx,
+    )
+
     if isinstance(ctx.default_return_type.args[2], Instance):
         validate_associated_type.check_type(
             associated_type=ctx.default_return_type.args[2],
+            typeclass=ctx.default_return_type,
             ctx=ctx,
         )
 
-    instance_args.mutate_typeclass_def(
-        ctx.default_return_type,
-        ctx.context.func.fullname,
-        ctx,
-    )
     return ctx.default_return_type
 
 

classes/contrib/mypy/validation/validate_associated_type.py

@@ -10,9 +10,14 @@
     'Single direct subclass of "{0}" required; got "{1}"'
 )
 
+_TYPE_REUSE_MSG: Final = (
+    'AssociatedType "{0}" must not be reused, originally associated with "{1}"'
+)
+
 
 def check_type(
     associated_type: Instance,
+    typeclass: Instance,
     ctx: MethodContext,
 ) -> bool:
     """
@@ -24,7 +29,7 @@ def check_type(
     """
     return all([
         _check_base_class(associated_type, ctx),
-        # TODO: check_type_reuse
+        _check_type_reuse(associated_type, typeclass, ctx),
         # TODO: check_body
         # TODO: check_generics_match_definition
         # TODO: we also need to check type vars used on definition:
@@ -50,3 +55,26 @@ def _check_base_class(
             ctx.context,
         )
     return has_correct_base
+
+
+def _check_type_reuse(
+    associated_type: Instance,
+    typeclass: Instance,
+    ctx: MethodContext,
+) -> bool:
+    fullname = getattr(typeclass.args[3], 'value', None)
+    metadata = associated_type.type.metadata.setdefault('classes', {})
+
+    has_reuse = (
+        fullname is not None and
+        'typeclass' in metadata and
+        metadata['typeclass'] != fullname
+    )
+    if has_reuse:
+        ctx.api.fail(
+            _TYPE_REUSE_MSG.format(associated_type.type.fullname, fullname),
+            ctx.context,
+        )
+
+    metadata['typeclass'] = fullname
+    return has_reuse

typesafety/test_associated_type/test_validation/test_base_classes.yml

@@ -0,0 +1,31 @@
+- case: typeclass_definied_by_wrong_type
+  disable_cache: false
+  main: |
+    from classes import typeclass
+
+    class ToJson(object):
+        ...
+
+    @typeclass(ToJson)
+    def to_json(instance, verbose: bool = False) -> str:
+        ...
+  out: |
+    main:6: error: Single direct subclass of "classes._typeclass.AssociatedType" required; got "main.ToJson"
+
+
+- case: typeclass_definied_by_multiple_parents
+  disable_cache: false
+  main: |
+    from classes import typeclass, AssociatedType
+
+    class A(object):
+        ...
+
+    class ToJson(AssociatedType, A):
+        ...
+
+    @typeclass(ToJson)
+    def to_json(instance, verbose: bool = False) -> str:
+        ...
+  out: |
+    main:9: error: Single direct subclass of "classes._typeclass.AssociatedType" required; got "main.ToJson"

typesafety/test_associated_type/test_validation/test_reuse.yml

@@ -0,0 +1,17 @@
+- case: associated_type_reuse
+  disable_cache: false
+  main: |
+    from classes import typeclass, AssociatedType
+
+    class ToJson(AssociatedType):
+        ...
+
+    @typeclass(ToJson)
+    def to_json(instance) -> str:
+        ...
+
+    @typeclass(ToJson)
+    def from_json(instance) -> str:
+        ...
+  out: |
+    main:10: error: AssociatedType "main.ToJson" must not be reused, originally associated with "main.from_json"

typesafety/test_typeclass/test_typeclass.yml

@@ -55,39 +55,6 @@
     main:10: error: Instance callback is incompatible "def (instance: builtins.str) -> builtins.int"; expected "def (instance: builtins.str, verbose: builtins.bool =) -> builtins.str"
 
 
-- case: typeclass_definied_by_wrong_type
-  disable_cache: false
-  main: |
-    from classes import typeclass
-
-    class ToJson(object):
-        ...
-
-    @typeclass(ToJson)
-    def to_json(instance, verbose: bool = False) -> str:
-        ...
-  out: |
-    main:6: error: Single direct subclass of "classes._typeclass.AssociatedType" required; got "main.ToJson"
-
-
-- case: typeclass_definied_by_multiple_parents
-  disable_cache: false
-  main: |
-    from classes import typeclass, AssociatedType
-
-    class A(object):
-        ...
-
-    class ToJson(AssociatedType, A):
-        ...
-
-    @typeclass(ToJson)
-    def to_json(instance, verbose: bool = False) -> str:
-        ...
-  out: |
-    main:9: error: Single direct subclass of "classes._typeclass.AssociatedType" required; got "main.ToJson"
-
-
 - case: typeclass_definied_by_literal
   disable_cache: false
   main: |