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

cdce8p · web-flow · commit e7e5a0993941 · 2025-12-13T22:15:11.000-08:00
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.
diff --git a/CHANGELOG.md b/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/)).
diff --git a/docs/source/command_line.rst b/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:
 
diff --git a/docs/source/config_file.rst b/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
 ****************
 
diff --git a/mypy/main.py b/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.",
diff --git a/mypy/messages.py b/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"
diff --git a/mypy/options.py b/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)
 
diff --git a/mypy/suggestions.py b/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)
 
diff --git a/mypy/test/helpers.py b/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):
diff --git a/mypy/test/testcheck.py b/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
diff --git a/mypy/test/testcmdline.py b/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:
diff --git a/mypy/test/testfinegrained.py b/mypy/test/testfinegrained.py
@@ -152,7 +152,6 @@ def get_options(self, source: str, testcase: DataDrivenTestCase, build_cache: bo
         options.use_fine_grained_cache = self.use_cache and not build_cache
         options.cache_fine_grained = self.use_cache
         options.local_partial_types = True
-        options.overwrite_union_syntax = True
         options.export_types = "inspect" in testcase.file
         # Treat empty bodies safely for these test cases.
         options.allow_empty_bodies = not testcase.name.endswith("_no_empty")
diff --git a/mypy/test/testmerge.py b/mypy/test/testmerge.py
@@ -102,7 +102,6 @@ def build(self, source: str, testcase: DataDrivenTestCase) -> BuildResult | None
         options.export_types = True
         options.show_traceback = True
         options.allow_empty_bodies = True
-        options.overwrite_union_syntax = True
         main_path = os.path.join(test_temp_dir, "main")
 
         self.str_conv.options = options
diff --git a/mypy/test/testparse.py b/mypy/test/testparse.py
@@ -38,7 +38,6 @@ def test_parser(testcase: DataDrivenTestCase) -> None:
     The argument contains the description of the test case.
     """
     options = Options()
-    options.overwrite_union_syntax = True
     options.hide_error_codes = True
 
     if testcase.file.endswith("python310.test"):
diff --git a/mypy/test/testpythoneval.py b/mypy/test/testpythoneval.py
@@ -52,7 +52,6 @@ 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
diff --git a/mypy/test/testsemanal.py b/mypy/test/testsemanal.py
@@ -38,7 +38,6 @@ def get_semanal_options(program_text: str, testcase: DataDrivenTestCase) -> Opti
     options.semantic_analysis_only = True
     options.show_traceback = True
     options.python_version = PYTHON3_VERSION
-    options.overwrite_union_syntax = True
     return options
 
 
diff --git a/mypy/test/testtransform.py b/mypy/test/testtransform.py
@@ -38,7 +38,6 @@ def test_transform(testcase: DataDrivenTestCase) -> None:
         options.use_builtins_fixtures = True
         options.semantic_analysis_only = True
         options.show_traceback = True
-        options.overwrite_union_syntax = True
         result = build.build(
             sources=[BuildSource("main", None, src)], options=options, alt_lib_path=test_temp_dir
         )
diff --git a/mypy/test/testtypegen.py b/mypy/test/testtypegen.py
@@ -35,7 +35,6 @@ def run_case(self, testcase: DataDrivenTestCase) -> None:
             options.export_types = True
             options.preserve_asts = True
             options.allow_empty_bodies = True
-            options.overwrite_union_syntax = True
             result = build.build(
                 sources=[BuildSource("main", None, src)],
                 options=options,
diff --git a/mypy/types.py b/mypy/types.py
@@ -3967,9 +3967,7 @@ def visit_literal_type(self, t: LiteralType, /) -> str:
         return f"Literal[{t.value_repr()}]"
 
     def visit_union_type(self, t: UnionType, /) -> str:
-        use_or_syntax = self.options.use_or_syntax()
-        s = self.list_str(t.items, use_or_syntax=use_or_syntax)
-        return s if use_or_syntax else f"Union[{s}]"
+        return self.list_str(t.items, use_or_syntax=True)
 
     def visit_partial_type(self, t: PartialType, /) -> str:
         if t.type is None:
diff --git a/test-data/unit/check-inference.test b/test-data/unit/check-inference.test
@@ -3550,7 +3550,7 @@ if x:
 [builtins fixtures/dict.pyi]
 
 [case testSuggestPep604AnnotationForPartialNone]
-# flags: --local-partial-types --python-version 3.10 --no-force-union-syntax
+# flags: --local-partial-types --python-version 3.10
 x = None # E: Need type annotation for "x" (hint: "x: <type> | None = ...")
 
 [case testTupleContextFromIterable]
diff --git a/test-data/unit/check-union-error-syntax.test b/test-data/unit/check-union-error-syntax.test
@@ -1,74 +1,37 @@
-[case testUnionErrorSyntax]
-# flags: --python-version 3.10 --no-force-union-syntax
-from typing import Union
-x : Union[bool, str]
-x = 3 # E: Incompatible types in assignment (expression has type "int", variable has type "bool | str")
-
 [case testOrErrorSyntax]
-# flags: --python-version 3.10 --force-union-syntax
+# flags: --python-version 3.10
 from typing import Union
 x : Union[bool, str]
-x = 3 # E: Incompatible types in assignment (expression has type "int", variable has type "Union[bool, str]")
+x = 3 # E: Incompatible types in assignment (expression has type "int", variable has type "bool | str")
 
 [case testOrNoneErrorSyntax]
-# flags: --python-version 3.10 --no-force-union-syntax
+# flags: --python-version 3.10
 from typing import Union
 x : Union[bool, None]
 x = 3 # E: Incompatible types in assignment (expression has type "int", variable has type "bool | None")
 
-[case testOptionalErrorSyntax]
-# flags: --python-version 3.10 --force-union-syntax
-from typing import Union
-x : Union[bool, None]
-x = 3 # E: Incompatible types in assignment (expression has type "int", variable has type "Optional[bool]")
-
 [case testNoneAsFinalItem]
-# flags: --python-version 3.10 --no-force-union-syntax
+# flags: --python-version 3.10
 from typing import Union
 x : Union[bool, None, str]
 x = 3 # E: Incompatible types in assignment (expression has type "int", variable has type "bool | str | None")
 
 [case testLiteralOrErrorSyntax]
-# flags: --python-version 3.10 --no-force-union-syntax
+# flags: --python-version 3.10
 from typing import Literal, Union
 x : Union[Literal[1], Literal[2], str]
 x = 3 # E: Incompatible types in assignment (expression has type "Literal[3]", variable has type "Literal[1, 2] | str")
 [builtins fixtures/tuple.pyi]
 
-[case testLiteralUnionErrorSyntax]
-# flags: --python-version 3.10 --force-union-syntax
-from typing import Literal, Union
-x : Union[Literal[1], Literal[2], str]
-x = 3 # E: Incompatible types in assignment (expression has type "Literal[3]", variable has type "Union[str, Literal[1, 2]]")
-[builtins fixtures/tuple.pyi]
-
 [case testLiteralOrNoneErrorSyntax]
-# flags: --python-version 3.10 --no-force-union-syntax
+# flags: --python-version 3.10
 from typing import Literal, Union
 x : Union[Literal[1], None]
 x = 3 # E: Incompatible types in assignment (expression has type "Literal[3]", variable has type "Literal[1] | None")
 [builtins fixtures/tuple.pyi]
 
-[case testLiteralOptionalErrorSyntax]
-# flags: --python-version 3.10 --force-union-syntax
-from typing import Literal, Union
-x : Union[Literal[1], None]
-x = 3 # E: Incompatible types in assignment (expression has type "Literal[3]", variable has type "Optional[Literal[1]]")
-[builtins fixtures/tuple.pyi]
-
-[case testUnionSyntaxRecombined]
-# flags: --python-version 3.10 --force-union-syntax --allow-redefinition-new --local-partial-types
-# The following revealed type is recombined because the finally body is visited twice.
-try:
-    x = 1
-    x = ""
-    x = {1: ""}
-finally:
-    reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.str, builtins.dict[builtins.int, builtins.str]]"
-[builtins fixtures/isinstancelist.pyi]
-
 [case testOrSyntaxRecombined]
-# flags: --python-version 3.10 --no-force-union-syntax --allow-redefinition-new --local-partial-types
+# flags: --python-version 3.10 --allow-redefinition-new --local-partial-types
 # The following revealed type is recombined because the finally body is visited twice.
 try:
     x = 1
diff --git a/test-data/unit/daemon.test b/test-data/unit/daemon.test
@@ -456,7 +456,7 @@ $ dmypy inspect foo.py:3:10:3:17 -vv
 $ dmypy inspect foo.py:9:9:9:11
 "int"
 $ dmypy inspect foo.py:11:1:11:3
-"Callable[[Optional[int]], None]"
+"Callable[[int | None], None]"
 $ dmypy inspect foo.py:11:1:13:1
 "None"
 $ dmypy inspect foo.py:1:2:3:4
