python · hauntsaninja · Sep 3, 2021 · May 29, 2021 · Jun 18, 2021 · Jun 25, 2021
diff --git a/docs/source/error_code_list2.rst b/docs/source/error_code_list2.rst
@@ -214,3 +214,45 @@ mypy generates an error if it thinks that an expression is redundant.
 
         # Error: If condition in comprehension is always true  [redundant-expr]
         [i for i in range(x) if isinstance(i, int)]
+
+
+Check that expression is not implicitly true in boolean context [truthy-bool]
+-----------------------------------------------------------------------------
+
+Warn when an expression whose type does not implement ``__bool__`` or ``__len__`` is used in boolean context,
+since unless implemented by a sub-type, the expression will always evaluate to true.
+
+.. code-block:: python
+
+    # mypy: enable-error-code truthy-bool
+
+    class Foo:
+      pass
+    foo = Foo()
+    # Error: "foo" has type "Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context
+    if foo:
+       ...
+
+
+This check might falsely imply an error. For example, ``typing.Iterable`` does not implement
+``__len__`` and so this code will be flagged:
+
+.. code-block:: python
+
+    # mypy: enable-error-code truthy-bool
+
+    def transform(items: Iterable[int]) -> List[int]:
+        # Error: "items" has type "typing.Iterable[int]" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+        if not items:
+            return [42]
+        return [x + 1 for x in items]
+
+
+
+If called as ``transform((int(s) for s in []))``, this function would not return ``[42]]`` unlike what the author
+might have intended. Of course it's possible that ``transform`` is only passed ``list`` objects, and so there is
+no error in practice. In such case, it might be prudent to annotate ``items: typing.Sequence[int]``.
+
+This is similar in concept to ensuring that an expression's type implements an expected interface (e.g. ``typing.Sized``),
+except that attempting to invoke an undefined method (e.g. ``__len__``) results in an error,
+while attempting to evaluate an object in boolean context without a concrete implementation results in a truthy value.
diff --git a/mypy/checker.py b/mypy/checker.py
@@ -3987,6 +3987,63 @@ def conditional_callable_type_map(self, expr: Expression,
 
         return None, {}
 
+    def _is_truthy_type(self, t: ProperType) -> bool:
+        return (
+            (
+                isinstance(t, Instance) and
+                bool(t.type) and
+                not t.type.has_readable_member('__bool__') and
+                not t.type.has_readable_member('__len__')
+            )
+            or isinstance(t, FunctionLike)
+            or (
+                isinstance(t, UnionType) and
+                all(self._is_truthy_type(t) for t in get_proper_types(t.items))
+            )
+        )
+
+    def _check_for_truthy_type(self, t: Type, expr: Expression) -> None:
+        if not state.strict_optional:
+            return  # if everything can be None, all bets are off
+
+        t = get_proper_type(t)
+        if not self._is_truthy_type(t):
+            return
+
+        def format_expr_type() -> str:
+            if isinstance(expr, MemberExpr):
+                return f'Member "{expr.name}" has type "{t}"'
+            elif isinstance(expr, RefExpr) and expr.fullname:
+                return f'"{expr.fullname}" has type "{t}"'
+            elif isinstance(expr, CallExpr):
+                if isinstance(expr.callee, MemberExpr):
+                    return f'"{expr.callee.name}" returns "{t}"'
+                elif isinstance(expr.callee, RefExpr) and expr.callee.fullname:
+                    return f'"{expr.callee.fullname}" returns "{t}"'
+                return f'Call returns "{t}"'
+            else:
+                return f'Expression has type "{t}"'
+
+        if isinstance(t, FunctionLike):
+            self.msg.fail(
+                f'Function "{t}" could always be true in boolean context', expr,
+                code=codes.TRUTHY_BOOL,
+            )
+        elif isinstance(t, UnionType):
+            self.msg.fail(
+                f"{format_expr_type()} of which no members implement __bool__ or __len__ "
+                "so it could always be true in boolean context",
+                expr,
+                code=codes.TRUTHY_BOOL,
+            )
+        else:
+            self.msg.fail(
+                f'{format_expr_type()} which does not implement __bool__ or __len__ '
+                'so it could always be true in boolean context',
+                expr,
+                code=codes.TRUTHY_BOOL,
+            )
+
     def find_type_equals_check(self, node: ComparisonExpr, expr_indices: List[int]
                                ) -> Tuple[TypeMap, TypeMap]:
         """Narrow types based on any checks of the type ``type(x) == T``
@@ -4089,6 +4146,7 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
         elif is_false_literal(node):
             return None, {}
         elif isinstance(node, CallExpr):
+            self._check_for_truthy_type(type_map[node], node)
             if len(node.args) == 0:
                 return {}, {}
             expr = collapse_walrus(node.args[0])
@@ -4271,6 +4329,7 @@ def has_no_custom_eq_checks(t: Type) -> bool:
             # Restrict the type of the variable to True-ish/False-ish in the if and else branches
             # respectively
             vartype = type_map[node]
+            self._check_for_truthy_type(vartype, node)
             if_type: Type = true_only(vartype)
             else_type: Type = false_only(vartype)
             ref: Expression = node

diff --git a/mypy/errorcodes.py b/mypy/errorcodes.py
@@ -122,6 +122,12 @@ def __str__(self) -> str:
 REDUNDANT_EXPR: Final = ErrorCode(
     "redundant-expr", "Warn about redundant expressions", "General", default_enabled=False
 )
+TRUTHY_BOOL: Final = ErrorCode(
+    'truthy-bool',
+    "Warn about expressions that could always evaluate to true in boolean contexts",
+    'General',
+    default_enabled=False
+)
 NAME_MATCH: Final = ErrorCode(
     "name-match", "Check that type definition has consistent naming", "General"
 )

diff --git a/mypy/test/typefixture.py b/mypy/test/typefixture.py
@@ -5,13 +5,14 @@
 
 from typing import List, Optional, Tuple
 
+from mypy.semanal_shared import set_callable_name
 from mypy.types import (
     Type, TypeVarType, AnyType, NoneType, Instance, CallableType, TypeVarDef, TypeType,
     UninhabitedType, TypeOfAny, TypeAliasType, UnionType, LiteralType
 )
 from mypy.nodes import (
-    TypeInfo, ClassDef, Block, ARG_POS, ARG_OPT, ARG_STAR, SymbolTable,
-    COVARIANT, TypeAlias
+    TypeInfo, ClassDef, FuncDef, Block, ARG_POS, ARG_OPT, ARG_STAR, SymbolTable,
+    COVARIANT, TypeAlias, SymbolTableNode, MDEF,
 )
 
 
@@ -62,6 +63,7 @@ def make_type_var(name: str, id: int, values: List[Type], upper_bound: Type,
                                               typevars=['T'],
                                               variances=[COVARIANT])   # class tuple
         self.type_typei = self.make_type_info('builtins.type')         # class type
+        self.bool_type_info = self.make_type_info('builtins.bool')
         self.functioni = self.make_type_info('builtins.function')  # function TODO
         self.ai = self.make_type_info('A', mro=[self.oi])              # class A
         self.bi = self.make_type_info('B', mro=[self.ai, self.oi])     # class B(A)
@@ -165,6 +167,15 @@ def make_type_var(name: str, id: int, values: List[Type], upper_bound: Type,
         self.type_t = TypeType.make_normalized(self.t)
         self.type_any = TypeType.make_normalized(self.anyt)
 
+        self._add_bool_dunder(self.bool_type_info)
+        self._add_bool_dunder(self.ai)
+
+    def _add_bool_dunder(self, type_info: TypeInfo) -> None:
+        signature = CallableType([], [], [], Instance(self.bool_type_info, []), self.function)
+        bool_func = FuncDef('__bool__', [], Block([]))
+        bool_func.type = set_callable_name(signature, bool_func)
+        type_info.names[bool_func.name] = SymbolTableNode(MDEF, bool_func)
+
     # Helper methods
 
     def callable(self, *a: Type) -> CallableType:

diff --git a/test-data/unit/check-errorcodes.test b/test-data/unit/check-errorcodes.test
@@ -807,3 +807,54 @@ from typing_extensions import TypedDict
 
 Foo = TypedDict("Bar", {})  # E: First argument "Bar" to TypedDict() does not match variable name "Foo"  [name-match]
 [builtins fixtures/dict.pyi]
+[case testTruthyBool]
+# flags: --enable-error-code truthy-bool
+from typing import List, Union
+
+class Foo:
+    pass
+
+foo = Foo()
+if foo:  # E: "__main__.foo" has type "__main__.Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+    pass
+
+zero = 0
+if zero:
+    pass
+
+false = False
+if false:
+    pass
+
+null = None
+if null:
+    pass
+
+s = ''
+if s:
+    pass
+
+good_union: Union[str, int] = 5
+if good_union:
+    pass
+if not good_union:
+    pass
+
+bad_union: Union[Foo, object] = Foo()
+if bad_union:  # E: "__main__.bad_union" has type "Union[__main__.Foo, builtins.object]" of which no members implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+    pass
+if not bad_union:  # E: "__main__.bad_union" has type "builtins.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 "def () -> Any" could always be true in boolean context  [truthy-bool]
+    pass
+if not f:  # E: Function "def () -> Any" could always be true in boolean context  [truthy-bool]
+    pass
+conditional_result = 'foo' if f else 'bar'  # E: Function "def () -> Any" could always be true in boolean context  [truthy-bool]
+
+lst: List[int] = []
+if lst:
+    pass
+[builtins fixtures/list.pyi]
diff --git a/test-data/unit/fixtures/list.pyi b/test-data/unit/fixtures/list.pyi
@@ -16,6 +16,7 @@ class list(Sequence[T]):
     @overload
     def __init__(self, x: Iterable[T]) -> None: pass
     def __iter__(self) -> Iterator[T]: pass
+    def __len__(self) -> int: pass
     def __contains__(self, item: object) -> bool: pass
     def __add__(self, x: list[T]) -> list[T]: pass
     def __mul__(self, x: int) -> list[T]: pass
@@ -26,9 +27,12 @@ class list(Sequence[T]):
 
 class tuple(Generic[T]): pass
 class function: pass
-class int: pass
-class float: pass
-class str: pass
+class int:
+    def __bool__(self) -> bool: pass
+class float:
+    def __bool__(self) -> bool: pass
+class str:
+    def __len__(self) -> bool: pass
 class bool(int): pass
 
 property = object() # Dummy definition.