Add GitHub annotations format for --output

docs/source/command_line.rst

@@ -97,7 +97,7 @@ Optional arguments
 
     Show program's version number and exit.
 
-.. option:: -O FORMAT, --output FORMAT {json}
+.. option:: -O {json,github}, --output {json,github}
 
     Set a custom output format.
 

mypy/error_formatter.py

@@ -34,4 +34,28 @@ def report_error(self, error: "MypyError") -> str:
         )
 
 
-OUTPUT_CHOICES = {"json": JSONFormatter()}
+class GitHubFormatter(ErrorFormatter):
+    """Formatter for GitHub Actions output format."""
+
+    def report_error(self, error: "MypyError") -> str:
+        """Prints out the errors as GitHub Actions annotations."""
+        command = "error" if error.severity == "error" else "notice"
+        title = f"Mypy ({error.errorcode.code})" if error.errorcode is not None else "Mypy"
+
+        message = f"{error.message}."
+
+        if error.hints:
+            message += "%0A%0A"
+            message += "%0A".join(error.hints)
+
+        return (
+            f"::{command} "
+            f"file={error.file_path},"
+            f"line={error.line},"
+            f"col={error.column},"
+            f"title={title}"
+            f"::{message}"
+        )
+
+
+OUTPUT_CHOICES = {"json": JSONFormatter(), "github": GitHubFormatter()}

mypy/main.py

@@ -547,11 +547,7 @@ def add_invertible_flag(
     )
 
     general_group.add_argument(
-        "-O",
-        "--output",
-        metavar="FORMAT",
-        help="Set a custom output format",
-        choices=OUTPUT_CHOICES,
+        "-O", "--output", help="Set a custom output format", choices=OUTPUT_CHOICES
     )
 
     config_group = parser.add_argument_group(

mypy/test/testoutput.py

@@ -56,3 +56,42 @@ def test_output_json(testcase: DataDrivenTestCase) -> None:
     normalized_output = [line.replace(test_temp_dir + json_os_separator, "") for line in output]
 
     assert normalized_output == testcase.output
+
+
+class OutputGitHubsuite(DataSuite):
+    files = ["outputgithub.test"]
+
+    def run_case(self, testcase: DataDrivenTestCase) -> None:
+        test_output_github(testcase)
+
+
+def test_output_github(testcase: DataDrivenTestCase) -> None:
+    """Run Mypy in a subprocess, and ensure that `--output=github` works as intended."""
+    mypy_cmdline = ["--output=github"]
+    mypy_cmdline.append(f"--python-version={'.'.join(map(str, PYTHON3_VERSION))}")
+
+    # Write the program to a file.
+    program_path = os.path.join(test_temp_dir, "main")
+    mypy_cmdline.append(program_path)
+    with open(program_path, "w", encoding="utf8") as file:
+        for s in testcase.input:
+            file.write(f"{s}\n")
+
+    output = []
+    # Type check the program.
+    out, err, returncode = api.run(mypy_cmdline)
+    # split lines, remove newlines, and remove directory of test case
+    for line in (out + err).rstrip("\n").splitlines():
+        if line.startswith(test_temp_dir + os.sep):
+            output.append(line[len(test_temp_dir + os.sep) :].rstrip("\r\n"))
+        else:
+            output.append(line.rstrip("\r\n"))
+
+    if returncode > 1:
+        output.append("!!! Mypy crashed !!!")
+
+    # Remove temp file.
+    os.remove(program_path)
+
+    normalized_output = [line.replace(test_temp_dir + os.sep, "") for line in output]
+    assert normalized_output == testcase.output

test-data/unit/outputgithub.test

@@ -0,0 +1,44 @@
+-- Test cases for `--output=json`.
+-- These cannot be run by the usual unit test runner because of the backslashes
+-- in the output, which get normalized to forward slashes by the test suite on
+-- Windows.
+
+[case testOutputGitHubNoIssues]
+# flags: --output=github
+def foo() -> None:
+    pass
+
+foo()
+[out]
+
+[case testOutputGitHubSimple]
+# flags: --output=github
+def foo() -> None:
+    pass
+
+foo(1)
+[out]
+::error file=main,line=5,col=0,title=Mypy (call-arg)::Too many arguments for "foo".
+
+[case testOutputGitHubWithHint]
+# flags: --output=github
+from typing import Optional, overload
+
+@overload
+def foo() -> None: ...
+@overload
+def foo(x: int) -> None: ...
+
+def foo(x: Optional[int] = None) -> None:
+    ...
+
+reveal_type(foo)
+
+foo('42')
+
+def bar() -> None: ...
+bar('42')
+[out]
+::notice file=main,line=12,col=12,title=Mypy (misc)::Revealed type is "Overload(def (), def (x: builtins.int))".
+::error file=main,line=14,col=0,title=Mypy (call-overload)::No overload variant of "foo" matches argument type "str".%0A%0APossible overload variants:%0A    def foo() -> None%0A    def foo(x: int) -> None
+::error file=main,line=17,col=0,title=Mypy (call-arg)::Too many arguments for "bar".