Make the space in error comments optional

mypy/test/data.py

@@ -533,9 +533,9 @@ def expand_errors(input: list[str], output: list[str], fnam: str) -> None:
 
     for i in range(len(input)):
         # The first in the split things isn't a comment
-        for possible_err_comment in input[i].split(" # ")[1:]:
+        for possible_err_comment in re.split(r"(?!\\)#\s*(?=[ENW]\s*:)", input[i])[1:]:
             m = re.search(
-                r"^([ENW]):((?P<col>\d+):)? (?P<message>.*)$", possible_err_comment.strip()
+                r"^([ENW])\s*:((?P<col>\d+):)?(?P<message>.*)$", possible_err_comment.strip()
             )
             if m:
                 if m.group(1) == "E":
@@ -545,12 +545,14 @@ def expand_errors(input: list[str], output: list[str], fnam: str) -> None:
                 elif m.group(1) == "W":
                     severity = "warning"
                 col = m.group("col")
-                message = m.group("message")
-                message = message.replace("\\#", "#")  # adds back escaped # character
+                message = m.group(
+                    "message"
+                )  # Message may, and probably does, include leading spaces
+                message = message.replace(r"\#", "#")  # adds back escaped # character
                 if col is None:
-                    output.append(f"{fnam}:{i + 1}: {severity}: {message}")
+                    output.append(f"{fnam}:{i + 1}: {severity}:{message}")
                 else:
-                    output.append(f"{fnam}:{i + 1}:{col}: {severity}: {message}")
+                    output.append(f"{fnam}:{i + 1}:{col}: {severity}:{message}")
 
 
 def fix_win_path(line: str) -> str:

mypy/test/update_data.py

@@ -64,7 +64,7 @@ def _iter_fixes(
         fix_lines = []
         for lineno, source_line in enumerate(source_lines, start=1):
             reports = reports_by_line.get((file_path, lineno))
-            comment_match = re.search(r"(?P<indent>\s+)(?P<comment># [EWN]: .+)$", source_line)
+            comment_match = re.search(r"(?P<indent>\s+)(?P<comment>#\s*[EWN]\s*:.+)$", source_line)
             if comment_match:
                 source_line = source_line[: comment_match.start("indent")]  # strip old comment
             if reports:

test-data/unit/README.md

@@ -31,7 +31,7 @@ with text "abc..."
 - note a space after `E:` and `flags:`
 - `# E:12` adds column number to the expected error
 - use `\` to escape the `#` character and indicate that the rest of the line is part of
-the error message
+the error message (note that there is no support for using `\\` to escape a backslash itself)
 - repeating `# E: ` several times in one line indicates multiple expected errors in one line
 - `W: ...` and `N: ...` works exactly like `E: ...`, but report a warning and a note respectively
 - lines that don't contain the above should cause no type check errors