Remove --force-union-syntax flag (python#20405)

This PR removed the `--force-union-syntax` flag. It also removes the
temporary `--overwrite-union-syntax` flag which was added to be able to
update the mypy test output.

Author: cdce8p

CHANGELOG.md

@@ -11,6 +11,12 @@ Support for this will be dropped in the first half of 2026!
 
 Contributed by Marc Mueller (PR [20156](https://github.com/python/mypy/pull/20156)).
 
+### Removed Flag: `--force-union-syntax`
+
+Mypy only supports Python 3.10+. Removed the `--force-union-syntax` flag as it's no longer necessary.
+
+Contributed by Marc Mueller (PR [20405](https://github.com/python/mypy/pull/20405))
+
 ## Mypy 1.19
 
 We’ve just uploaded mypy 1.19.0 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)).

docs/source/command_line.rst

@@ -958,12 +958,6 @@ in error messages.
     useful or they may be overly noisy. If ``N`` is negative, there is
     no limit. The default limit is -1.
 
-.. option:: --force-union-syntax
-
-    Always use ``Union[]`` and ``Optional[]`` for union types
-    in error messages (instead of the ``|`` operator),
-    even on Python 3.10+.
-
 
 .. _incremental:
 

docs/source/config_file.rst

@@ -930,15 +930,6 @@ These options may only be set in the global section (``[mypy]``).
 
     Show absolute paths to files.
 
-.. confval:: force_union_syntax
-
-    :type: boolean
-    :default: False
-
-    Always use ``Union[]`` and ``Optional[]`` for union types
-    in error messages (instead of the ``|`` operator),
-    even on Python 3.10+.
-
 Incremental mode
 ****************
 

mypy/main.py

@@ -820,14 +820,6 @@ def add_invertible_flag(
         "--force-uppercase-builtins", default=False, help=argparse.SUPPRESS, group=none_group
     )
 
-    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",
         description="Detect code that is sound but redundant or problematic.",

mypy/messages.py

@@ -1813,10 +1813,7 @@ def need_annotation_for_var(
             type_dec = "<type>"
             if not node.type.type:
                 # partial None
-                if options.use_or_syntax():
-                    recommended_type = f"{type_dec} | None"
-                else:
-                    recommended_type = f"Optional[{type_dec}]"
+                recommended_type = f"{type_dec} | None"
             elif node.type.type.fullname in reverse_builtin_aliases:
                 # partial types other than partial None
                 name = node.type.type.fullname.partition(".")[2]
@@ -2713,17 +2710,9 @@ def format_literal_value(typ: LiteralType) -> str:
             )
 
             if len(union_items) == 1 and isinstance(get_proper_type(union_items[0]), NoneType):
-                return (
-                    f"{literal_str} | None"
-                    if options.use_or_syntax()
-                    else f"Optional[{literal_str}]"
-                )
+                return f"{literal_str} | None"
             elif union_items:
-                return (
-                    f"{literal_str} | {format_union(union_items)}"
-                    if options.use_or_syntax()
-                    else f"Union[{', '.join(format_union_items(union_items))}, {literal_str}]"
-                )
+                return f"{literal_str} | {format_union(union_items)}"
             else:
                 return literal_str
         else:
@@ -2734,17 +2723,9 @@ def format_literal_value(typ: LiteralType) -> str:
             )
             if print_as_optional:
                 rest = [t for t in typ.items if not isinstance(get_proper_type(t), NoneType)]
-                return (
-                    f"{format(rest[0])} | None"
-                    if options.use_or_syntax()
-                    else f"Optional[{format(rest[0])}]"
-                )
+                return f"{format(rest[0])} | None"
             else:
-                s = (
-                    format_union(typ.items)
-                    if options.use_or_syntax()
-                    else f"Union[{', '.join(format_union_items(typ.items))}]"
-                )
+                s = format_union(typ.items)
             return s
     elif isinstance(typ, NoneType):
         return "None"

mypy/options.py

@@ -412,9 +412,6 @@ def __init__(self) -> None:
         self.disable_memoryview_promotion = False
         # 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
@@ -433,11 +430,6 @@ def use_lowercase_names(self) -> bool:
         )
         return True
 
-    def use_or_syntax(self) -> bool:
-        if self.python_version >= (3, 10):
-            return not self.force_union_syntax
-        return self.overwrite_union_syntax
-
     def use_star_unpack(self) -> bool:
         return self.python_version >= (3, 11)
 

mypy/suggestions.py

@@ -891,7 +891,7 @@ def visit_typeddict_type(self, t: TypedDictType) -> str:
     def visit_union_type(self, t: UnionType) -> str:
         if len(t.items) == 2 and is_overlapping_none(t):
             s = remove_optional(t).accept(self)
-            return f"{s} | None" if self.options.use_or_syntax() else f"Optional[{s}]"
+            return f"{s} | None"
         else:
             return super().visit_union_type(t)
 

mypy/test/helpers.py

@@ -357,7 +357,6 @@ def parse_options(
         options = Options()
         options.error_summary = False
         options.hide_error_codes = True
-        options.overwrite_union_syntax = True
 
     # Allow custom python version to override testfile_pyversion.
     if all(flag.split("=")[0] != "--python-version" for flag in flag_list):

mypy/test/testcheck.py

@@ -136,7 +136,6 @@ def run_case_once(
         options = parse_options(original_program_text, testcase, incremental_step)
         options.use_builtins_fixtures = True
         options.show_traceback = True
-        options.overwrite_union_syntax = True
 
         if options.num_workers:
             options.fixed_format_cache = True

mypy/test/testcmdline.py

@@ -60,7 +60,6 @@ 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: