Fix edge cases in pretty formatting (#20809)

This fixes few edge cases I discovered accidentally:
* Caret position on imports in try/except gets set at column 1 (before
the actual word `import`)
* Code lines with a symbol `^` get all red (this is a silly one)
* If the exact text `: error:` appears in a code line, it gets
highlighted as a mypy error

Author: ilevkivskyi

mypy/errors.py

@@ -1063,9 +1063,10 @@ def format_messages_default(
                 if severity == "error" and source_lines and line > 0:
                     source_line = source_lines[line - 1]
                     source_line_expanded = source_line.expandtabs()
-                    if column < 0:
+                    min_column = len(source_line) - len(source_line.lstrip())
+                    if column < min_column:
                         # Something went wrong, take first non-empty column.
-                        column = len(source_line) - len(source_line.lstrip())
+                        column = min_column
 
                     # Shifts column after tab expansion
                     column = len(source_line[:column].expandtabs())

mypy/util.py

@@ -35,6 +35,7 @@
 ENCODING_RE: Final = re.compile(rb"([ \t\v]*#.*(\r\n?|\n))??[ \t\v]*#.*coding[:=][ \t]*([-\w.]+)")
 
 DEFAULT_SOURCE_OFFSET: Final = 4
+CODE_START: Final = " " * DEFAULT_SOURCE_OFFSET
 DEFAULT_COLUMNS: Final = 80
 
 # At least this number of columns will be shown on each side of
@@ -729,19 +730,27 @@ def style(
             start += self.DIM
         return start + self.colors[color] + text + self.NORMAL
 
+    def is_marker_line(self, line: str) -> bool:
+        s_line = line.lstrip()
+        return (
+            line.startswith(CODE_START)
+            and s_line.startswith("^")
+            and set(s_line).issubset({"^", "~"})
+        )
+
     def fit_in_terminal(
         self, messages: list[str], fixed_terminal_width: int | None = None
     ) -> list[str]:
         """Improve readability by wrapping error messages and trimming source code."""
         width = fixed_terminal_width or get_terminal_width()
         new_messages = messages.copy()
         for i, error in enumerate(messages):
-            if ": error:" in error:
+            # TODO: detecting source code highlights through an indent can be surprising.
+            if not error.startswith(CODE_START) and ": error:" in error:
                 loc, msg = error.split("error:", maxsplit=1)
                 msg = soft_wrap(msg, width, first_offset=len(loc) + len("error: "))
                 new_messages[i] = loc + "error:" + msg
-            if error.startswith(" " * DEFAULT_SOURCE_OFFSET) and "^" not in error:
-                # TODO: detecting source code highlights through an indent can be surprising.
+            elif error.startswith(CODE_START) and not self.is_marker_line(error):
                 # Restore original error message and error location.
                 error = error[DEFAULT_SOURCE_OFFSET:]
                 marker_line = messages[i + 1]
@@ -768,7 +777,12 @@ def fit_in_terminal(
 
     def colorize(self, error: str) -> str:
         """Colorize an output line by highlighting the status and error code."""
-        if ": error:" in error:
+        # TODO: detecting source code highlights through an indent can be surprising.
+        if error.startswith(CODE_START):
+            if not self.is_marker_line(error):
+                return self.style(error, "none", dim=True)
+            return self.style(error, "red")
+        elif ": error:" in error:
             loc, msg = error.split("error:", maxsplit=1)
             if self.hide_error_codes:
                 return (
@@ -790,11 +804,6 @@ def colorize(self, error: str) -> str:
             loc, msg = error.split("note:", maxsplit=1)
             formatted = self.highlight_quote_groups(self.underline_link(msg))
             return loc + self.style("note:", "blue") + formatted
-        elif error.startswith(" " * DEFAULT_SOURCE_OFFSET):
-            # TODO: detecting source code highlights through an indent can be surprising.
-            if "^" not in error:
-                return self.style(error, "none", dim=True)
-            return self.style(error, "red")
         else:
             return error
 

test-data/unit/cmdline.test

@@ -738,6 +738,20 @@ expected "str"
     test_between(1  +       1)
                  ^~~~~~~~~~~~
 
+[case testErrorMarkerSkipsLeftSpace]
+# cmd: mypy --pretty --show-error-codes imp.py
+[file imp.py]
+try:
+    import foobar
+except ValueError:
+    pass
+[out]
+imp.py:2: error: Cannot find implementation or library stub for module named
+"foobar"  [import-not-found]
+        import foobar
+        ^
+imp.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+
 [case testErrorMessageWhenOpenPydFile]
 # cmd: mypy a.pyd
 [file a.pyd]