Add flag for printing reports in colour (#89)

* Remove unused test constant

This was last referenced in 44a7fed.

* Extract logic for writing change reports

Before this change the logic for writing the report to stdout was a part
of ChangeTracker.

By extracting this to a separate class, we allow for other writer
classes to be implemented.

* Add ColorChangeReportWriter

This class can be used to print out change reports in colour.

The colours selected closely match those output by Mypy, with the
exception that links are not highlighted in notes.

The logic is inspired by Mypy's highlighting code.
(See https://github.com/python/mypy/blob/f9e8e0bda5cfbb54d6a8f9e482aa25da28a1a635/mypy/util.py#L761)

This supports only ANSI colours, so I suspect that this will not work on
Windows.

* Add flag for printing change reports in colour

* Print out mypy test results in colour

* Add docstring to DefaultChangeReportWriter

* Update change log with new `--color` flag

* Change spacing around report summary

Before this change an empty line would be printed above the summary when
there were no new errors, and no empty line would be printed when there
were errors.

This change reverses that. An empty line is printed to separate the
summary from the new errors only if there are new errors.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Drop support for Python 3.7
+- Add `parse --color` flag for printing out change reports in color. (Aliases `-c`, `--colour`.)
 
 ## v1.0.4 [2023-05-09]
 

mypy_json_report.py

@@ -31,6 +31,7 @@
     Iterator,
     List,
     Optional,
+    Protocol,
     Union,
     cast,
 )
@@ -87,6 +88,13 @@ def main() -> None:
             """
         ),
     )
+    parse_parser.add_argument(
+        "-c",
+        "--color",
+        "--colour",
+        action="store_true",
+        help="Whether to colorize the diff-report output. Defaults to False.",
+    )
 
     parse_parser.set_defaults(func=_parse_command)
 
@@ -116,7 +124,12 @@ def _parse_command(args: argparse.Namespace) -> None:
     tracker = None
     if args.diff_old_report is not None:
         old_report = cast(ErrorSummary, _load_json_file(args.diff_old_report))
-        tracker = ChangeTracker(old_report)
+        change_report_writer: _ChangeReportWriter
+        if args.color:
+            change_report_writer = ColorChangeReportWriter()
+        else:
+            change_report_writer = DefaultChangeReportWriter()
+        tracker = ChangeTracker(old_report, report_writer=change_report_writer)
         processors.append(tracker)
 
     messages = MypyMessage.from_lines(sys.stdin)
@@ -229,6 +242,102 @@ class DiffReport:
     num_fixed_errors: int
 
 
+class _ChangeReportWriter(Protocol):
+    def write_report(self, diff: DiffReport) -> None:
+        ...
+
+
+class DefaultChangeReportWriter:
+    """Writes an error summary without color."""
+
+    def __init__(self, _write: Callable[[str], Any] = sys.stdout.write) -> None:
+        self.write = _write
+
+    def write_report(self, diff: DiffReport) -> None:
+        new_errors = "\n".join(diff.error_lines)
+        if new_errors:
+            self.write(new_errors + "\n\n")
+        self.write(f"Fixed errors: {diff.num_fixed_errors}\n")
+        self.write(f"New errors: {diff.num_new_errors}\n")
+        self.write(f"Total errors: {diff.total_errors}\n")
+
+
+class ColorChangeReportWriter:
+    """
+    Writes an error summary in color.
+
+    Inspired by the FancyFormatter in mypy.util.
+
+    Ref: https://github.com/python/mypy/blob/f9e8e0bda5cfbb54d6a8f9e482aa25da28a1a635/mypy/util.py#L761
+    """
+
+    _RESET = "\033[0m"
+    _BOLD = "\033[1m"
+    _BOLD_RED = "\033[31;1m"
+    _BOLD_YELLOW = "\033[33;1m"
+    _GREEN = "\033[32m"
+    _YELLOW = "\033[33m"
+    _BLUE = "\033[34m"
+
+    def __init__(self, _write: Callable[[str], Any] = sys.stdout.write) -> None:
+        self.write = _write
+
+    def write_report(self, diff: DiffReport) -> None:
+        new_errors = "\n".join([self._format_line(line) for line in diff.error_lines])
+        if new_errors:
+            self.write(new_errors + "\n\n")
+
+        fixed_color = self._BOLD_YELLOW if diff.num_fixed_errors else self._GREEN
+        error_color = self._BOLD_RED if diff.num_new_errors else self._GREEN
+
+        self.write(self._style(fixed_color, f"Fixed errors: {diff.num_fixed_errors}\n"))
+        self.write(self._style(error_color, f"New errors: {diff.num_new_errors}\n"))
+        self.write(self._style(self._BOLD, f"Total errors: {diff.total_errors}\n"))
+
+    def _style(self, style: str, message: str) -> str:
+        return f"{style}{message}{self._RESET}"
+
+    def _highlight_quotes(self, msg: str) -> str:
+        if msg.count('"') % 2:
+            return msg
+        parts = msg.split('"')
+        out = ""
+        for i, part in enumerate(parts):
+            if i % 2 == 0:
+                out += part
+            else:
+                out += self._style(self._BOLD, f'"{part}"')
+        return out
+
+    def _format_line(self, line: str) -> str:
+        if ": error: " in line:
+            # Separate the location from the message.
+            location, _, message = line.partition(" error: ")
+
+            # Extract the error code from the end of the message if it's there.
+            if message.endswith("]") and "  [" in message:
+                error_msg, _, code = message.rpartition("  [")
+                code = self._style(self._YELLOW, f"  [{code}")
+            else:
+                error_msg = message
+                code = ""
+
+            return (
+                location
+                + self._style(self._BOLD_RED, " error: ")
+                + self._highlight_quotes(error_msg)
+                + code
+            )
+        if ": note: " in line:
+            location, _, message = line.partition(" note: ")
+            return (
+                location
+                + self._style(self._BLUE, " note: ")
+                + self._highlight_quotes(message)
+            )
+        return line
+
+
 class ChangeTracker:
     """
     Compares the current Mypy report against a previous summary.
@@ -239,8 +348,11 @@ class ChangeTracker:
     that are cached in memory.
     """
 
-    def __init__(self, summary: ErrorSummary) -> None:
+    def __init__(
+        self, summary: ErrorSummary, report_writer: _ChangeReportWriter
+    ) -> None:
         self.old_report = summary
+        self.report_writer = report_writer
         self.error_lines: List[str] = []
         self.num_errors = 0
         self.num_new_errors = 0
@@ -282,13 +394,8 @@ def diff_report(self) -> DiffReport:
         )
 
     def write_report(self) -> Optional[ErrorCodes]:
-        write = sys.stderr.write
         diff = self.diff_report()
-        new_errors = "\n".join(diff.error_lines)
-        write(new_errors + "\n")
-        write(f"Fixed errors: {diff.num_fixed_errors}\n")
-        write(f"New errors: {diff.num_new_errors}\n")
-        write(f"Total errors: {diff.total_errors}\n")
+        self.report_writer.write_report(diff)
 
         if diff.num_new_errors or diff.num_fixed_errors:
             return ErrorCodes.ERROR_DIFF