Add GitHub annotations format for --output

mypy/error_formatter.py

@@ -34,4 +34,20 @@ 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"
+        code = f"(`{error.errorcode.code}`) " if error.errorcode is not None else ""
+
+        result = f"::{command} file={error.file_path},line={error.line},col={error.column}::{code}{error.message}"
+        if len(error.hints) > 0:
+            # TODO: Add hints to the output?
+            pass
+
+        return result
+
+
+OUTPUT_CHOICES = {"json": JSONFormatter(), "github": GitHubFormatter()}

mypy/test/testoutput.py

@@ -56,3 +56,46 @@ 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)
+
+    # JSON encodes every `\` character into `\\`, so we need to remove `\\` from windows paths
+    # and `/` from POSIX paths
+    json_os_separator = os.sep.replace("\\", "\\\\")
+    normalized_output = [line.replace(test_temp_dir + json_os_separator, "") for line in output]
+
+    assert normalized_output == testcase.output

test-data/unit/outputgithub.test

@@ -0,0 +1,45 @@
+-- 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::(`call-arg`) Too many arguments for "foo"
+
+[case testOutputGitHubWithHint]
+# flags: --output=json
+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::(`misc`) Revealed type is "Overload(def (), def (x: builtins.int))"
+::error file=main,line=14,col=0::(`call-overload`) No overload variant of "foo" matches argument type "str"
+::error file=main,line=17,col=0::(`call-arg`) Too many arguments for "bar"
+