Improve error message for mismatched copies in code blocks (#31535)

improve error message for mismatched code blocks

utils/check_copies.py

@@ -169,7 +169,6 @@
     },
 }
 
-
 # This is to make sure the transformers module imported is the one in the repo.
 transformers_module = direct_transformers_import(TRANSFORMERS_PATH)
 
@@ -185,7 +184,7 @@ def _should_continue(line: str, indent: str) -> bool:
     return line.startswith(indent) or len(line.strip()) == 0 or _is_definition_header_ending_line(line)
 
 
-def _sanity_check_splits(splits_1, splits_2, is_class):
+def _sanity_check_splits(splits_1, splits_2, is_class, filename):
     """Check the two (inner) block structures of the corresponding code block given by `split_code_into_blocks` match.
 
     For the case of `class`, they must be of one of the following 3 cases:
@@ -246,11 +245,12 @@ def g(x):
             ["block_without_name", "block_with_name"],
         ]:
             raise ValueError(
-                "For a class, it must have a specific structure. See the docstring of `_sanity_check_splits` in the file `utils/check_copies.py`"
+                f"""Class defined in {filename} doesn't have the expected stucture.
+                See the docstring of `_sanity_check_splits` in the file `utils/check_copies.py`""",
             )
 
     if block_names_1 != block_names_2:
-        raise ValueError("The structures in the 2 code blocks differ.")
+        raise ValueError(f"In {filename}, two code blocks expected to be copies have different structures.")
 
 
 def find_block_end(lines: List[str], start_index: int, indent: int) -> int:
@@ -661,11 +661,8 @@ def is_copy_consistent(filename: str, overwrite: bool = False, buffer: dict = No
     diffs = []
     line_index = 0
     # Not a for loop cause `lines` is going to change (if `overwrite=True`).
+    search_re = _re_copy_warning_for_test_file if filename.startswith("tests") else _re_copy_warning
     while line_index < len(lines):
-        search_re = _re_copy_warning
-        if filename.startswith("tests"):
-            search_re = _re_copy_warning_for_test_file
-
         search = search_re.search(lines[line_index])
         if search is None:
             line_index += 1
@@ -718,7 +715,7 @@ def is_copy_consistent(filename: str, overwrite: bool = False, buffer: dict = No
 
         is_class = lines[start_index].startswith(f"{' ' * (len(indent) - 4)}class ")
         # sanity check
-        _sanity_check_splits(theoretical_code_splits, observed_code_splits, is_class=is_class)
+        _sanity_check_splits(theoretical_code_splits, observed_code_splits, is_class=is_class, filename=filename)
 
         # observed code in a structured way (a dict mapping block names to blocks' code)
         observed_code_blocks = OrderedDict()