Add hidden --overwrite-union-syntax option

Author: cdce8p

mypy/main.py

@@ -815,6 +815,10 @@ def add_invertible_flag(
     add_invertible_flag(
         "--force-union-syntax", default=False, help=argparse.SUPPRESS, group=none_group
     )
+    # For internal use only! Will be removed once Mypy drops support for Python 3.9.
+    add_invertible_flag(
+        "--overwrite-union-syntax", default=False, help=argparse.SUPPRESS, group=none_group
+    )
 
     lint_group = parser.add_argument_group(
         title="Configuring warnings",

mypy/options.py

@@ -412,6 +412,8 @@ def __init__(self) -> None:
         # Deprecated, Mypy only supports Python 3.9+
         self.force_uppercase_builtins = False
         self.force_union_syntax = False
+        # Mypy internal use only! Set during test run.
+        self.overwrite_union_syntax = False
 
         # Sets custom output format
         self.output: str | None = None
@@ -433,7 +435,7 @@ def use_lowercase_names(self) -> bool:
     def use_or_syntax(self) -> bool:
         if self.python_version >= (3, 10):
             return not self.force_union_syntax
-        return False
+        return False or self.overwrite_union_syntax
 
     def use_star_unpack(self) -> bool:
         return self.python_version >= (3, 11)

mypy/test/testcmdline.py

@@ -55,14 +55,13 @@ def test_python_cmdline(testcase: DataDrivenTestCase, step: int) -> None:
     args = parse_args(testcase.input[0])
     custom_cwd = parse_cwd(testcase.input[1]) if len(testcase.input) > 1 else None
     args.append("--show-traceback")
+    args.append("--overwrite-union-syntax")
     if "--error-summary" not in args:
         args.append("--no-error-summary")
     if "--show-error-codes" not in args:
         args.append("--hide-error-codes")
     if "--disallow-empty-bodies" not in args:
         args.append("--allow-empty-bodies")
-    if "--no-force-union-syntax" not in args:
-        args.append("--force-union-syntax")
     # Type check the program.
     fixed = [python3_path, "-m", "mypy"]
     env = os.environ.copy()

mypy/test/testpythoneval.py

@@ -52,6 +52,7 @@ def test_python_evaluation(testcase: DataDrivenTestCase, cache_dir: str) -> None
         "--no-error-summary",
         "--hide-error-codes",
         "--allow-empty-bodies",
+        "--overwrite-union-syntax",
         "--test-env",  # Speeds up some checks
     ]
     interpreter = python3_path
@@ -71,9 +72,6 @@ def test_python_evaluation(testcase: DataDrivenTestCase, cache_dir: str) -> None
                     return
         mypy_cmdline.extend(additional_flags)
 
-    if "--no-force-union-syntax" not in mypy_cmdline:
-        mypy_cmdline.append("--force-union-syntax")
-
     # Write the program to a file.
     program = "_" + testcase.name + ".py"
     program_path = os.path.join(test_temp_dir, program)

mypy/types.py

@@ -3736,6 +3736,7 @@ class TypeStrVisitor(SyntheticTypeVisitor[str]):
     Notes:
      - Represent unbound types as Foo? or Foo?[...].
      - Represent the NoneType type as None.
+     - Represent Union[x, y] as x | y
     """
 
     def __init__(self, id_mapper: IdMapper | None = None, *, options: Options) -> None: