Add error-code for truthy-function (#13686)

A separate error code for `truthy-function` so it can be enabled by default.

Closes #12621

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Author: cdce8p

docs/source/error_code_list2.rst

@@ -258,6 +258,19 @@ except that attempting to invoke an undefined method (e.g. ``__len__``) results
 while attempting to evaluate an object in boolean context without a concrete implementation results in a truthy value.
 
 
+Check that function isn't used in boolean context [truthy-function]
+-------------------------------------------------------------------
+
+Functions will always evaluate to true in boolean contexts.
+
+.. code-block:: python
+
+    def f():
+        ...
+
+    if f:  # Error: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
+        pass
+
 .. _ignore-without-code:
 
 Check that ``# type: ignore`` include an error code [ignore-without-code]

mypy/errorcodes.py

@@ -155,6 +155,11 @@ def __str__(self) -> str:
     "General",
     default_enabled=False,
 )
+TRUTHY_FUNCTION: Final[ErrorCode] = ErrorCode(
+    "truthy-function",
+    "Warn about function that always evaluate to true in boolean contexts",
+    "General",
+)
 NAME_MATCH: Final = ErrorCode(
     "name-match", "Check that type definition has consistent naming", "General"
 )

mypy/message_registry.py

@@ -148,7 +148,7 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
     code=codes.TRUTHY_BOOL,
 )
 FUNCTION_ALWAYS_TRUE: Final = ErrorMessage(
-    "Function {} could always be true in boolean context", code=codes.TRUTHY_BOOL
+    "Function {} could always be true in boolean context", code=codes.TRUTHY_FUNCTION
 )
 NOT_CALLABLE: Final = "{} not callable"
 TYPE_MUST_BE_USED: Final = "Value of type {} must be used"

test-data/unit/check-errorcodes.test

@@ -842,19 +842,21 @@ if bad_union:  # E: "__main__.bad_union" has type "Union[Foo, object]" of which
 if not bad_union:  # E: "__main__.bad_union" has type "object" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
     pass
 
-def f():
-    pass
-if f:  # E: Function "Callable[[], Any]" could always be true in boolean context  [truthy-bool]
-    pass
-if not f:  # E: Function "Callable[[], Any]" could always be true in boolean context  [truthy-bool]
-    pass
-conditional_result = 'foo' if f else 'bar'  # E: Function "Callable[[], Any]" could always be true in boolean context  [truthy-bool]
-
 lst: List[int] = []
 if lst:
     pass
 [builtins fixtures/list.pyi]
 
+[case testTruthyFunctions]
+# flags: --strict-optional
+def f():
+    pass
+if f:  # E: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
+    pass
+if not f:  # E: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
+    pass
+conditional_result = 'foo' if f else 'bar'  # E: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
+
 [case testNoOverloadImplementation]
 from typing import overload
 

test-data/unit/check-incremental.test

@@ -6017,12 +6017,15 @@ tmp/m.py:2: error: Argument "age" to "foo" has incompatible type "str"; expected
 [case testDisableEnableErrorCodesIncremental]
 # flags: --disable-error-code truthy-bool
 # flags2: --enable-error-code truthy-bool
-def foo() -> int: ...
+class Foo:
+    pass
+
+foo = Foo()
 if foo:
     ...
 [out]
 [out2]
-main:4: error: Function "Callable[[], int]" could always be true in boolean context
+main:7: error: "__main__.foo" has type "Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context
 
 [case testModuleAsProtocolImplementationSerialize]
 import m

test-data/unit/check-inline-config.test

@@ -166,9 +166,11 @@ main:1: error: Setting "strict" not supported in inline configuration: specify i
 [case testInlineErrorCodes]
 # flags: --strict-optional
 # mypy: enable-error-code="ignore-without-code,truthy-bool"
+class Foo:
+    pass
 
-def foo() -> int: ...
-if foo: ...  # E: Function "Callable[[], int]" could always be true in boolean context
+foo = Foo()
+if foo: ...  # E: "__main__.foo" has type "Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context
 42 + "no"  # type: ignore  # E: "type: ignore" comment without error code (consider "type: ignore[operator]" instead)
 
 [case testInlineErrorCodesOverrideConfig]
@@ -178,8 +180,10 @@ import tests.bar
 import tests.baz
 [file foo.py]
 # mypy: disable-error-code="truthy-bool"
+class Foo:
+    pass
 
-def foo() -> int: ...
+foo = Foo()
 if foo: ...
 42 + "no"  # type: ignore  # E: "type: ignore" comment without error code (consider "type: ignore[operator]" instead)
 
@@ -193,8 +197,10 @@ if foo: ...  # E: Function "Callable[[], int]" could always be true in boolean c
 
 [file tests/baz.py]
 # mypy: disable-error-code="truthy-bool"
+class Foo:
+    pass
 
-def foo() -> int: ...
+foo = Foo()
 if foo: ...
 42 + "no"  # type: ignore
 

test-data/unit/check-python38.test

@@ -310,7 +310,7 @@ def f(x: int = (c := 4)) -> int:
         z2: NT  # E: Variable "NT" is not valid as a type \
                 # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
 
-    if Alias := int:
+    if Alias := int:  # E: Function "Type[int]" could always be true in boolean context
         z3: Alias  # E: Variable "Alias" is not valid as a type \
                    # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
 

test-data/unit/check-unreachable-code.test

@@ -936,7 +936,8 @@ class Case1:
         return False and self.missing()  # E: Right operand of "and" is never evaluated
 
     def test2(self) -> bool:
-        return not self.property_decorator_missing and self.missing()  # E: Right operand of "and" is never evaluated
+        return not self.property_decorator_missing and self.missing()  # E: Function "Callable[[], bool]" could always be true in boolean context \
+            # E: Right operand of "and" is never evaluated
 
     def property_decorator_missing(self) -> bool:
         return True