Use --no-implicit-optional by default

mypy/fastparse.py

@@ -997,7 +997,7 @@ def do_func_def(
         return retval
 
     def set_type_optional(self, type: Optional[Type], initializer: Optional[Expression]) -> None:
-        if self.options.no_implicit_optional:
+        if not self.options.implicit_optional:
             return
         # Indicate that type should be wrapped in an Optional if arg is initialized to None.
         optional = isinstance(initializer, NameExpr) and initializer.name == "None"

mypy/main.py

@@ -719,10 +719,9 @@ def add_invertible_flag(
         "https://mypy.readthedocs.io/en/stable/kinds_of_types.html#no-strict-optional",
     )
     add_invertible_flag(
-        "--no-implicit-optional",
+        "--implicit-optional",
         default=False,
-        strict_flag=True,
-        help="Don't assume arguments with default values of None are Optional",
+        help="Assume arguments with default values of None are Optional",
         group=none_group,
     )
     none_group.add_argument("--strict-optional", action="store_true", help=argparse.SUPPRESS)

mypy/options.py

@@ -35,19 +35,19 @@ class BuildType:
     "disallow_untyped_calls",
     "disallow_untyped_decorators",
     "disallow_untyped_defs",
-    "follow_imports",
     "follow_imports_for_stubs",
+    "follow_imports",
     "ignore_errors",
     "ignore_missing_imports",
+    "implicit_optional",
     "implicit_reexport",
     "local_partial_types",
     "mypyc",
-    "no_implicit_optional",
     "show_none_errors",
     "strict_concatenate",
     "strict_equality",
-    "strict_optional",
     "strict_optional_whitelist",
+    "strict_optional",
     "warn_no_return",
     "warn_return_any",
     "warn_unreachable",
@@ -165,8 +165,8 @@ def __init__(self) -> None:
         # Alternate way to show/hide strict-None-checking related errors
         self.show_none_errors = True
 
-        # Don't assume arguments with default values of None are Optional
-        self.no_implicit_optional = False
+        # Assume arguments with default values of None are Optional
+        self.implicit_optional = False
 
         # Don't re-export names unless they are imported with `from ... as ...`
         self.implicit_reexport = True

test-data/unit/check-flags.test

@@ -827,6 +827,7 @@ standard.f(None)
 [file mypy.ini]
 \[mypy]
 strict_optional = False
+implicit_optional = true
 \[mypy-optional]
 strict_optional = True
 
@@ -846,6 +847,7 @@ standard.f(None)
 [file pyproject.toml]
 \[tool.mypy]
 strict_optional = false
+implicit_optional = true
 \[[tool.mypy.overrides]]
 module = 'optional'
 strict_optional = true

test-data/unit/check-kwargs.test

@@ -24,6 +24,7 @@ class A: pass
 class B: pass
 
 [case testOneOfSeveralOptionalKeywordArguments]
+# flags: --implicit-optional
 import typing
 def f(a: 'A' = None, b: 'B' = None, c: 'C' = None) -> None: pass
 f(a=A())

test-data/unit/check-optional.test

@@ -127,6 +127,7 @@ def f(x: None) -> None: pass
 f(None)
 
 [case testInferOptionalFromDefaultNone]
+# flags: --implicit-optional
 def f(x: int = None) -> None:
   x + 1  # E: Unsupported left operand type for + ("None") \
          # N: Left operand is of type "Optional[int]"
@@ -140,6 +141,7 @@ def f(x: int = None) -> None:  # E: Incompatible default for argument "x" (defau
 [out]
 
 [case testInferOptionalFromDefaultNoneComment]
+# flags: --implicit-optional
 def f(x=None):
   # type: (int) -> None
   x + 1  # E: Unsupported left operand type for + ("None") \

test-data/unit/fine-grained.test

@@ -7852,7 +7852,7 @@ class Foo(a.I):
 ==
 
 [case testImplicitOptionalRefresh1]
-# flags: --strict-optional
+# flags: --strict-optional --implicit-optional
 from x import f
 def foo(x: int = None) -> None:
     f()

test-data/unit/fixtures/tuple.pyi

@@ -1,6 +1,6 @@
 # Builtins stub used in tuple-related test cases.
 
-from typing import Iterable, Iterator, TypeVar, Generic, Sequence, Any, overload, Tuple, Type
+from typing import Iterable, Iterator, TypeVar, Generic, Sequence, Optional, overload, Tuple, Type
 
 T = TypeVar("T")
 Tco = TypeVar('Tco', covariant=True)
@@ -46,6 +46,6 @@ class list(Sequence[T], Generic[T]):
 
 def isinstance(x: object, t: type) -> bool: pass
 
-def sum(iterable: Iterable[T], start: T = None) -> T: pass
+def sum(iterable: Iterable[T], start: Optional[T] = None) -> T: pass
 
 class BaseException: pass