Plugin development: Use of `get_dynamic_class_hook` causes deserialization of types to fail from `.mypy_cache`

Hello folks. I'm working on a plugin that uses get_dynamic_class_hook to construct type constructors (TypeInfo) and class definitions (ClassDef) that don't exist in a module. I add these definitions to a MypyFile. Finally, I add this MypyFile to the modules dictionary provided by the SemanticAnalyzerPluginInterface API to convince mypy of my "fake" module.

This works wonderfully until a second run of my plugin when the .mypy_cache has been constructed. Before semantic analysis, mypy seems to be trying to read my fake module, because in the cache names in real modules have a target that refers to the fake module. For instance,

{
  ".class": "MypyFile",
  "_fullname": "RealModule",
   ...
  "names": {
    ".class": "SymbolTable",
    ...
    "SomeType": {
      ".class": "SymbolTableNode",
      "kind": "Gdef",
      "node": {
        ".class": "TypeAlias",
        ...
        "fullname": "RealModule.SomeType",
        ...
        "target": "FakeModule.SomeType"
      }
    },
    ...

The stacktrace is below. mypy fails to resolve FakeModule.SomeType because FakeModule doesn't exist in its modules dictionary. I don't think my use of get_dynamic_class_hook is unusual, so I think there's a hole in my understanding of this hook.

...
-> process_graph(graph, manager)
  /usr/lib/python3.9/site-packages/mypy/build.py(3333)process_graph()
-> process_fresh_modules(graph, prev_scc, manager)
  /usr/lib/python3.9/site-packages/mypy/build.py(3414)process_fresh_modules()
-> graph[id].fix_cross_refs()
  /usr/lib/python3.9/site-packages/mypy/build.py(2110)fix_cross_refs()
-> fixup_module(self.tree, self.manager.modules, self.options.use_fine_grained_cache)
  /usr/lib/python3.9/site-packages/mypy/fixup.py(52)fixup_module()
-> node_fixer.visit_symbol_table(tree.names, tree.fullname)
  /usr/lib/python3.9/site-packages/mypy/fixup.py(156)visit_symbol_table()
-> self.visit_type_info(value.node)
  /usr/lib/python3.9/site-packages/mypy/fixup.py(115)visit_type_info()
-> self.current_info = save_info
  /usr/lib/python3.9/site-packages/mypy/fixup.py(158)visit_symbol_table()
-> value.node.accept(self)
  /usr/lib/python3.9/site-packages/mypy/nodes.py(1037)accept()
-> return visitor.visit_var(self)
  /usr/lib/python3.9/site-packages/mypy/fixup.py(211)visit_var()
-> v.type.accept(self.type_fixer)
  /usr/lib/python3.9/site-packages/mypy/types.py(1444)accept()
-> return visitor.visit_instance(self)
  /usr/lib/python3.9/site-packages/mypy/fixup.py(230)visit_instance()
-> inst.type = lookup_fully_qualified_typeinfo(
  /usr/lib/python3.9/site-packages/mypy/fixup.py(367)lookup_fully_qualified_typeinfo()
-> stnode = lookup_fully_qualified(name, modules, raise_on_missing=not allow_missing)
  /usr/lib/python3.9/site-packages/mypy/lookup.py(31)lookup_fully_qualified()
-> assert "." in head, f"Cannot find module for {name}"

Inizia con l’API get_dynamic_class_hook e il percorso di deserializzazione della cache in build.py, quindi segui fix_cross_refs attraverso fixup.py e lookup.py usando lo stack trace riportato. Riproduci la seconda esecuzione con .mypy_cache e un modulo fittizio e verifica che i tipi creati dai plugin possano essere deserializzati senza una ricerca del modulo non risolta.