Provide a better fallback value for the python_version option (#19162)

Followup to #19157. After dropping
support for an old Python version, mypy should assume the next oldest
one instead of the current interpreter version.

Author: cdce8p

mypy/config_parser.py

@@ -28,6 +28,14 @@
 _INI_PARSER_CALLABLE: _TypeAlias = Callable[[Any], _CONFIG_VALUE_TYPES]
 
 
+class VersionTypeError(argparse.ArgumentTypeError):
+    """Provide a fallback value if the Python version is unsupported."""
+
+    def __init__(self, *args: Any, fallback: tuple[int, int]) -> None:
+        self.fallback = fallback
+        super().__init__(*args)
+
+
 def parse_version(v: str | float) -> tuple[int, int]:
     m = re.match(r"\A(\d)\.(\d+)\Z", str(v))
     if not m:
@@ -44,7 +52,7 @@ def parse_version(v: str | float) -> tuple[int, int]:
             if isinstance(v, float):
                 msg += ". You may need to put quotes around your Python version"
 
-            raise argparse.ArgumentTypeError(msg)
+            raise VersionTypeError(msg, fallback=defaults.PYTHON3_VERSION_MIN)
     else:
         raise argparse.ArgumentTypeError(
             f"Python major version '{major}' out of range (must be 3)"
@@ -548,6 +556,9 @@ def parse_section(
                     continue
                 try:
                     v = ct(section.get(key))
+                except VersionTypeError as err_version:
+                    print(f"{prefix}{key}: {err_version}", file=stderr)
+                    v = err_version.fallback
                 except argparse.ArgumentTypeError as err:
                     print(f"{prefix}{key}: {err}", file=stderr)
                     continue

test-data/unit/cmdline.test

@@ -440,6 +440,20 @@ python_version = 3.9
 python_version = 3.14
 [out]
 
+[case testPythonVersionFallback]
+# cmd: mypy main.py
+[file main.py]
+import sys
+if sys.version_info == (3, 9):  # Update here when bumping the min Python version!
+    reveal_type("good")
+[file mypy.ini]
+\[mypy]
+python_version = 3.8
+[out]
+mypy.ini: [mypy]: python_version: Python 3.8 is not supported (must be 3.9 or higher)
+main.py:3: note: Revealed type is "Literal['good']?"
+== Return code: 0
+
 -- This should be a dumping ground for tests of plugins that are sensitive to
 -- typeshed changes.
 [case testTypeshedSensitivePlugins]