From f72951fbe8ef488123e6e531bd80ee43b69f7415 Mon Sep 17 00:00:00 2001
From: Jukka Lehtosalo <jukka.lehtosalo@iki.fi>
Date: Sat, 6 Nov 2021 15:28:11 +0000
Subject: [PATCH] Only read the stdlib versions dict once per run

This was being read repeatedly, and it was bad enough to
show up in a performance profile.
---
 mypy/modulefinder.py | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/mypy/modulefinder.py b/mypy/modulefinder.py
index 4469533835e3d..8a0eb3a71a0d3 100644
--- a/mypy/modulefinder.py
+++ b/mypy/modulefinder.py
@@ -13,7 +13,7 @@
 from enum import Enum, unique
 
 from typing import Dict, Iterator, List, NamedTuple, Optional, Set, Tuple, Union
-from typing_extensions import Final
+from typing_extensions import Final, TypeAlias as _TypeAlias
 
 from mypy.fscache import FileSystemCache
 from mypy.options import Options
@@ -33,6 +33,9 @@
 OnePackageDir = Tuple[str, bool]
 PackageDirs = List[OnePackageDir]
 
+# Minimum and maximum Python versions for modules in stdlib as (major, minor)
+StdlibVersions: _TypeAlias = Dict[str, Tuple[Tuple[int, int], Optional[Tuple[int, int]]]]
+
 PYTHON_EXTENSIONS: Final = [".pyi", ".py"]
 
 PYTHON2_STUB_DIR: Final = "@python2"
@@ -126,7 +129,8 @@ class FindModuleCache:
     def __init__(self,
                  search_paths: SearchPaths,
                  fscache: Optional[FileSystemCache],
-                 options: Optional[Options]) -> None:
+                 options: Optional[Options],
+                 stdlib_py_versions: Optional[StdlibVersions] = None) -> None:
         self.search_paths = search_paths
         self.fscache = fscache or FileSystemCache()
         # Cache for get_toplevel_possibilities:
@@ -139,7 +143,9 @@ def __init__(self,
         custom_typeshed_dir = None
         if options:
             custom_typeshed_dir = options.custom_typeshed_dir
-        self.stdlib_py_versions = load_stdlib_py_versions(custom_typeshed_dir)
+        self.stdlib_py_versions = (
+            stdlib_py_versions or load_stdlib_py_versions(custom_typeshed_dir)
+        )
         self.python_major_ver = 3 if options is None else options.python_version[0]
 
     def clear(self) -> None:
@@ -261,7 +267,12 @@ def _can_find_module_in_parent_dir(self, id: str) -> bool:
         of the current working directory.
         """
         working_dir = os.getcwd()
-        parent_search = FindModuleCache(SearchPaths((), (), (), ()), self.fscache, self.options)
+        parent_search = FindModuleCache(
+            SearchPaths((), (), (), ()),
+            self.fscache,
+            self.options,
+            stdlib_py_versions=self.stdlib_py_versions
+        )
         while any(file.endswith(("__init__.py", "__init__.pyi"))
                   for file in os.listdir(working_dir)):
             working_dir = os.path.dirname(working_dir)
@@ -791,8 +802,7 @@ def compute_search_paths(sources: List[BuildSource],
                        typeshed_path=tuple(lib_path))
 
 
-def load_stdlib_py_versions(custom_typeshed_dir: Optional[str]
-                            ) -> Dict[str, Tuple[Tuple[int, int], Optional[Tuple[int, int]]]]:
+def load_stdlib_py_versions(custom_typeshed_dir: Optional[str]) -> StdlibVersions:
     """Return dict with minimum and maximum Python versions of stdlib modules.
 
     The contents look like