Use X | Y union syntax in error messages (#15102)

This should fix #15082:
If the python version is set to 3.10 or later union error reports are
going to be displayed using the `X | Y` syntax.
If None is found in a union it is shown as the last element in order to
improve visibility (`Union[bool, None, str]` becomes `"bool | str |
None"`.
To achieve this an option `use_or_syntax()` is created that is set to
true if the python version is 3.10 or later.

For testing a hidden flag --force-union-syntax is used that sets the
option to false.

mypy/main.py

@@ -738,6 +738,10 @@ 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
+    )
+
     lint_group = parser.add_argument_group(
         title="Configuring warnings",
         description="Detect code that is sound but redundant or problematic.",

mypy/messages.py

@@ -2359,6 +2359,12 @@ def format(typ: Type) -> str:
     def format_list(types: Sequence[Type]) -> str:
         return ", ".join(format(typ) for typ in types)
 
+    def format_union(types: Sequence[Type]) -> str:
+        formatted = [format(typ) for typ in types if format(typ) != "None"]
+        if any(format(typ) == "None" for typ in types):
+            formatted.append("None")
+        return " | ".join(formatted)
+
     def format_literal_value(typ: LiteralType) -> str:
         if typ.is_enum_literal():
             underlying_type = format(typ.fallback)
@@ -2457,9 +2463,17 @@ def format_literal_value(typ: LiteralType) -> str:
             )
 
             if len(union_items) == 1 and isinstance(get_proper_type(union_items[0]), NoneType):
-                return f"Optional[{literal_str}]"
+                return (
+                    f"{literal_str} | None"
+                    if options.use_or_syntax()
+                    else f"Optional[{literal_str}]"
+                )
             elif union_items:
-                return f"Union[{format_list(union_items)}, {literal_str}]"
+                return (
+                    f"{literal_str} | {format_union(union_items)}"
+                    if options.use_or_syntax()
+                    else f"Union[{format_list(union_items)}, {literal_str}]"
+                )
             else:
                 return literal_str
         else:
@@ -2470,10 +2484,17 @@ 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"Optional[{format(rest[0])}]"
+                return (
+                    f"{format(rest[0])} | None"
+                    if options.use_or_syntax()
+                    else f"Optional[{format(rest[0])}]"
+                )
             else:
-                s = f"Union[{format_list(typ.items)}]"
-
+                s = (
+                    format_union(typ.items)
+                    if options.use_or_syntax()
+                    else f"Union[{format_list(typ.items)}]"
+                )
             return s
     elif isinstance(typ, NoneType):
         return "None"

mypy/options.py

@@ -356,12 +356,18 @@ def __init__(self) -> None:
         self.disable_memoryview_promotion = False
 
         self.force_uppercase_builtins = False
+        self.force_union_syntax = False
 
     def use_lowercase_names(self) -> bool:
         if self.python_version >= (3, 9):
             return not self.force_uppercase_builtins
         return False
 
+    def use_or_syntax(self) -> bool:
+        if self.python_version >= (3, 10):
+            return not self.force_union_syntax
+        return False
+
     # To avoid breaking plugin compatibility, keep providing new_semantic_analyzer
     @property
     def new_semantic_analyzer(self) -> bool:

mypy/test/helpers.py

@@ -383,6 +383,7 @@ def parse_options(
         options.error_summary = False
         options.hide_error_codes = True
         options.force_uppercase_builtins = True
+        options.force_union_syntax = True
 
     # Allow custom python version to override testfile_pyversion.
     if all(flag.split("=")[0] not in ["--python-version", "-2", "--py2"] for flag in flag_list):

mypy/test/testcheck.py

@@ -127,6 +127,8 @@ def run_case_once(
             options.allow_empty_bodies = not testcase.name.endswith("_no_empty")
         if "lowercase" not in testcase.file:
             options.force_uppercase_builtins = True
+        if "union-error" not in testcase.file:
+            options.force_union_syntax = True
 
         if incremental_step and options.incremental:
             # Don't overwrite # flags: --no-incremental in incremental test cases

mypy/test/testcmdline.py

@@ -63,6 +63,8 @@ def test_python_cmdline(testcase: DataDrivenTestCase, step: int) -> None:
         args.append("--allow-empty-bodies")
     if "--no-force-uppercase-builtins" not in args:
         args.append("--force-uppercase-builtins")
+    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()

test-data/unit/check-union-error-syntax.test

@@ -0,0 +1,61 @@
+[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
+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]")
+
+[case testOrNoneErrorSyntax]
+# flags: --python-version 3.10 --no-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 "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
+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
+from typing import Union
+from typing_extensions import Literal
+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 Union
+from typing_extensions import Literal
+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
+from typing import Union
+from typing_extensions import Literal
+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 Union
+from typing_extensions import Literal
+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]

test-data/unit/daemon.test

@@ -312,7 +312,7 @@ def bar() -> None:
     foo(arg='xyz')
 
 [case testDaemonGetType_python38]
-$ dmypy start --log-file log.txt -- --follow-imports=error --no-error-summary
+$ dmypy start --log-file log.txt -- --follow-imports=error --no-error-summary --python-version 3.8
 Daemon started
 $ dmypy inspect foo:1:2:3:4
 Command "inspect" is only valid after a "check" command (that produces no parse errors)