Make merge_pyi not to overwrite existing type annotations. When we as…

…sume that the existing type annotations are correct, we should totally not overwrite it as the same thing can be named in different ways depending on how you import it. But it's not a good idea to change the way how the users have written the import already.

The former change was introduced in #1644, but it seems that respecting the original code best suits what we think this tool should behave.

PiperOrigin-RevId: 678640645

pytype/tools/merge_pyi/merge_pyi.py

@@ -26,7 +26,7 @@ def _merge_csts(*, py_tree, pyi_tree):
   vis.store_stub_in_context(context, pyi_tree)
   return vis(
       context,
-      overwrite_existing_annotations=True,
+      overwrite_existing_annotations=False,
       strict_posargs_matching=False,
       strict_annotation_matching=True,
   ).transform_module(py_tree)
@@ -80,7 +80,7 @@ class Mode(enum.Enum):
   OVERWRITE = 3
 
 
-def _get_diff(a, b):
+def _get_diff(a, b) -> str:
   a, b = a.split("\n"), b.split("\n")
   diff = difflib.Differ().compare(a, b)
   return "\n".join(diff)
@@ -90,16 +90,25 @@ def merge_files(
     *, py_path: str, pyi_path: str, mode: Mode, backup: Optional[str] = None
 ) -> bool:
   """Merges a .py and a .pyi (experimental: or a pickled pytd) file."""
-
-  with open(py_path) as f:
-    py_src = f.read()
   _, ext = os.path.splitext(pyi_path)
   if re.fullmatch(r"\.pickled(-\d+)?", ext):
     with open(pyi_path, "rb") as file:
       pyi_src = pytd_utils.Print(pickle_utils.DecodeAst(file.read()).ast)
   else:
     with open(pyi_path) as f:
       pyi_src = f.read()
+  return merge_files_src(py_path, pyi_src, mode, backup)
+
+
+def merge_files_src(
+    py_path: str,
+    pyi_src: str,
+    mode: Mode,
+    backup: Optional[str] = None,
+) -> bool:
+  """Merges annotations from pyi_src content into the .py file py_path."""
+  with open(py_path) as f:
+    py_src = f.read()
   annotated_src = merge_sources(py=py_src, pyi=pyi_src)
   changed = annotated_src != py_src
   if mode == Mode.PRINT:

pytype/tools/merge_pyi/test_data/defaults.pep484.py

@@ -26,7 +26,7 @@ def f6(x:e6=int) -> r6:
     pass
 
 # static analysis would give error here
-def f7(x:e7=int) -> r7:
+def f7(x : int=int):
     pass
 
 def f8(x:e8={1:2}) -> r8:

pytype/tools/merge_pyi/test_data/partial.pep484.py

@@ -1,7 +1,7 @@
-def f1(a: e1, b: e2) -> r1:
+def f1(a, b : int):
     pass
 
-def f2(a: e1, b: e2) -> r2:
+def f2(a, b) -> int:
     pass
 
 def f3(a) -> r3: