python · hasier · May 23, 2025 · May 23, 2025 · May 23, 2025 · May 27, 2025
diff --git a/mypy/config_parser.py b/mypy/config_parser.py
@@ -292,7 +292,7 @@ def parse_config_file(
    stdout: TextIO | None = None,
    stderr: TextIO | None = None,
 ) -> None:
-    """Parse a config file into an Options object.
+    """Parse a config file into an Options object, following config extend arguments.

    Errors are written to stderr but are not fatal.

@@ -301,30 +301,104 @@ def parse_config_file(
    stdout = stdout or sys.stdout
    stderr = stderr or sys.stderr

+    ret = _parse_and_extend_config_file(
+        options=options,
+        set_strict_flags=set_strict_flags,
+        filename=filename,
+        stdout=stdout,
+        stderr=stderr,
+        visited=set(),
+    )
+
+    if ret is None:
+        return
+
+    file_read, mypy_updates, mypy_report_dirs, module_updates = ret
+
+    options.config_file = file_read
+    os.environ["MYPY_CONFIG_FILE_DIR"] = os.path.dirname(os.path.abspath(file_read))
+
+    for k, v in mypy_updates.items():
+        setattr(options, k, v)
+
+    options.report_dirs.update(mypy_report_dirs)
+
+    for glob, updates in module_updates.items():
+        options.per_module_options[glob] = updates
+
+
+def _merge_updates(existing: dict[str, object], new: dict[str, object]) -> None:
+    existing["disable_error_code"] = list(
+        set(existing.get("disable_error_code", []) + new.pop("disable_error_code"))
+    )
+    existing["enable_error_code"] = list(
+        set(existing.get("enable_error_code", []) + new.pop("enable_error_code"))
+    )
+    existing.update(new)
+
+
+def _parse_and_extend_config_file(
+    options: Options,
+    set_strict_flags: Callable[[], None],
+    filename: str | None,
+    stdout: TextIO,
+    stderr: TextIO,
+    visited: set[str],
+) -> tuple[str, dict[str, object], dict[str, str], dict[str, dict[str, object]]] | None:
    ret = (
        _parse_individual_file(filename, stderr)
        if filename is not None
        else _find_config_file(stderr)
    )
    if ret is None:
-        return
+        return None
    parser, config_types, file_read = ret

-    options.config_file = file_read
-    os.environ["MYPY_CONFIG_FILE_DIR"] = os.path.dirname(os.path.abspath(file_read))
+    abs_file_read = os.path.abspath(file_read)
+    if abs_file_read in visited:
+        print(f"Circular extend detected: {abs_file_read}", file=stderr)
+        return None
+    visited.add(abs_file_read)
+
+    mypy_updates: dict[str, object] = {}
+    mypy_report_dirs: dict[str, str] = {}
+    module_updates: dict[str, dict[str, object]] = {}

    if "mypy" not in parser:
        if filename or os.path.basename(file_read) not in defaults.SHARED_CONFIG_NAMES:
            print(f"{file_read}: No [mypy] section in config file", file=stderr)
    else:
        section = parser["mypy"]
+
+        extend = parser["mypy"].pop("extend", None)
+        if extend:
+            cwd = os.getcwd()
+            try:
+                # process extend relative to the directory where we found current config
+                os.chdir(os.path.dirname(abs_file_read))
+                parse_ret = _parse_and_extend_config_file(
+                    options=options,
+                    set_strict_flags=set_strict_flags,
+                    filename=os.path.abspath(expand_path(extend)),
+                    stdout=stdout,
+                    stderr=stderr,
+                    visited=visited,
+                )
+            finally:
+                os.chdir(cwd)
+
+            if parse_ret is None:
+                print(f"{extend} is not a valid path to extend from {abs_file_read}", file=stderr)
+            else:
+                _, mypy_updates, mypy_report_dirs, module_updates = parse_ret
+
        prefix = f"{file_read}: [mypy]: "
        updates, report_dirs = parse_section(
            prefix, options, set_strict_flags, section, config_types, stderr
        )
-        for k, v in updates.items():
-            setattr(options, k, v)
-        options.report_dirs.update(report_dirs)
+        # extend and overwrite existing values with new ones
+        _merge_updates(mypy_updates, updates)
+        mypy_report_dirs.update(report_dirs)

    for name, section in parser.items():
        if name.startswith("mypy-"):
@@ -367,7 +441,10 @@ def parse_config_file(
                        file=stderr,
                    )
                else:
-                    options.per_module_options[glob] = updates
+                    # extend and overwrite existing values with new ones
+                    _merge_updates(module_updates.setdefault(glob, {}), updates)
+
+    return file_read, mypy_updates, mypy_report_dirs, module_updates


 def get_prefix(file_read: str, name: str) -> str:

diff --git a/mypy/test/test_config_parser.py b/mypy/test/test_config_parser.py
@@ -1,14 +1,16 @@
 from __future__ import annotations

 import contextlib
+import io
 import os
 import tempfile
 import unittest
 from collections.abc import Iterator
 from pathlib import Path

-from mypy.config_parser import _find_config_file
+from mypy.config_parser import _find_config_file, parse_config_file
 from mypy.defaults import CONFIG_NAMES, SHARED_CONFIG_NAMES
+from mypy.options import Options


 @contextlib.contextmanager
@@ -128,3 +130,82 @@ def test_precedence_missing_section(self) -> None:
                result = _find_config_file()
                assert result is not None
                assert Path(result[2]).resolve() == parent_mypy.resolve()
+
+
+class ExtendConfigFileSuite(unittest.TestCase):
+
+    def test_extend_success(self) -> None:
+        with tempfile.TemporaryDirectory() as _tmpdir:
+            tmpdir = Path(_tmpdir)
+            with chdir(tmpdir):
+                pyproject = tmpdir / "pyproject.toml"
+                write_config(
+                    pyproject,
+                    "[tool.mypy]\n"
+                    'extend = "./folder/mypy.ini"\n'
+                    "strict = false\n"
+                    "[[tool.mypy.overrides]]\n"
+                    'module = "c"\n'
+                    'enable_error_code = ["explicit-override"]\n'
+                    "disallow_untyped_defs = true",
+                )
+                folder = tmpdir / "folder"
+                folder.mkdir()
+                write_config(
+                    folder / "mypy.ini",
+                    "[mypy]\n"
+                    "strict = True\n"
+                    "ignore_missing_imports_per_module = True\n"
+                    "[mypy-c]\n"
+                    "disallow_incomplete_defs = True",
+                )
+
+                options = Options()
+                strict_option_set = False
+
+                def set_strict_flags() -> None:
+                    nonlocal strict_option_set
+                    strict_option_set = True
+
+                stdout = io.StringIO()
+                stderr = io.StringIO()
+                parse_config_file(options, set_strict_flags, None, stdout, stderr)
+
+                assert strict_option_set is True
+                assert options.ignore_missing_imports_per_module is True
+                assert options.config_file == str(pyproject.name)
+                os.environ["MYPY_CONFIG_FILE_DIR"] = str(pyproject.parent)
+
+                assert options.per_module_options["c"] == {
+                    "disable_error_code": [],
+                    "enable_error_code": ["explicit-override"],
+                    "disallow_untyped_defs": True,
+                    "disallow_incomplete_defs": True,
+                }
+
+                assert stdout.getvalue() == ""
+                assert stderr.getvalue() == ""
+
+    def test_extend_cyclic(self) -> None:
+        with tempfile.TemporaryDirectory() as _tmpdir:
+            tmpdir = Path(_tmpdir)
+            with chdir(tmpdir):
+                pyproject = tmpdir / "pyproject.toml"
+                write_config(pyproject, '[tool.mypy]\nextend = "./folder/mypy.ini"\n')
+
+                folder = tmpdir / "folder"
+                folder.mkdir()
+                ini = folder / "mypy.ini"
+                write_config(ini, "[mypy]\nextend = ../pyproject.toml\n")
+
+                options = Options()
+
+                stdout = io.StringIO()
+                stderr = io.StringIO()
+                parse_config_file(options, lambda: None, None, stdout, stderr)
+
+                assert stdout.getvalue() == ""
+                assert stderr.getvalue() == (
+                    f"Circular extend detected: /private{pyproject}\n"
+                    f"../pyproject.toml is not a valid path to extend from /private{ini}\n"
+                )