fix: don't reset context.scratch between files (#1151)

#453 fixed scratch leaking between files by setting it to empty, but that drops all the scratch space that was set up before the codemod runs (e.g. in the transformer's constructor)

This PR improves the fix by preserving the initial scratch.

libcst/codemod/_cli.py

@@ -14,6 +14,7 @@
 import sys
 import time
 import traceback
+from copy import deepcopy
 from dataclasses import dataclass, replace
 from multiprocessing import cpu_count, Pool
 from pathlib import Path
@@ -214,6 +215,7 @@ def _execute_transform(  # noqa: C901
     transformer: Codemod,
     filename: str,
     config: ExecutionConfig,
+    scratch: Dict[str, object],
 ) -> ExecutionResult:
     for pattern in config.blacklist_patterns:
         if re.fullmatch(pattern, filename):
@@ -251,7 +253,7 @@ def _execute_transform(  # noqa: C901
         transformer.context = replace(
             transformer.context,
             filename=filename,
-            scratch={},
+            scratch=deepcopy(scratch),
         )
 
         # determine the module and package name for this file
@@ -634,6 +636,7 @@ def parallel_exec_transform_with_prettyprint(  # noqa: C901
                 "transformer": transform,
                 "filename": filename,
                 "config": config,
+                "scratch": transform.context.scratch,
             }
             for filename in files
         ]

libcst/tests/test_e2e.py

@@ -2,7 +2,7 @@
 import os
 from pathlib import Path
 from tempfile import TemporaryDirectory
-from typing import Generator
+from typing import Dict, Generator
 from unittest import TestCase
 
 from libcst import BaseExpression, Call, matchers as m, Name
@@ -16,7 +16,14 @@
 
 
 class PrintToPPrintCommand(VisitorBasedCodemodCommand):
+    def __init__(self, context: CodemodContext, **kwargs: Dict[str, object]) -> None:
+        super().__init__(context, **kwargs)
+        self.context.scratch["PPRINT_WAS_HERE"] = True
+
     def leave_Call(self, original_node: Call, updated_node: Call) -> BaseExpression:
+        if not self.context.scratch["PPRINT_WAS_HERE"]:
+            raise AssertionError("Scratch space lost")
+
         if m.matches(updated_node, m.Call(func=m.Name("print"))):
             AddImportsVisitor.add_needed_import(
                 self.context,