Optimization to avoid stat() calls from is_module()

mypy/build.py

@@ -604,8 +604,15 @@ def correct_rel_imp(imp: Union[ImportFrom, ImportAll]) -> str:
 
         return res
 
+    _all_modules = None  # type: Optional[Set[str]]
+
+    def set_all_modules(self, all_modules: Optional[Set[str]]) -> None:
+        self._all_modules = all_modules
+
     def is_module(self, id: str) -> bool:
         """Is there a file in the file system corresponding to module id?"""
+        if self._all_modules is not None:
+            return id in self._all_modules
         return self.find_module_cache.find_module(id) is not None
 
     def parse_file(self, id: str, path: str, source: str, ignore_errors: bool) -> MypyFile:

mypy/dmypy_server.py

@@ -375,6 +375,7 @@ def fine_grained_increment(self,
             changed, removed = self.update_changed(sources,
                                                    add or [], remove or [], update or [])
         manager.search_paths = compute_search_paths(sources, manager.options, manager.data_dir)
+        manager.set_all_modules(set(s.module for s in sources))
         t1 = time.time()
         manager.log("fine-grained increment: find_changed: {:.3f}s".format(t1 - t0))
         messages = self.fine_grained_manager.update(changed, removed)