Add error-code for truthy-iterable (#13762)

`Iterable` does not implement `__len__`, so passing a `Generator` would
always evaluate to `true`. Suggest `Collection` as alternative.

In most cases this isn't an issue as the function is often called with
`list` anyway. However, the dedicated check can pinpoint subtle errors
that might get unnoticed otherwise.

Author: cdce8p

docs/source/error_code_list2.rst

@@ -231,31 +231,29 @@ since unless implemented by a sub-type, the expression will always evaluate to t
     if foo:
        ...
 
+The check is similar in concept to ensuring that an expression's type implements an expected interface (e.g. ``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.
 
-This check might falsely imply an error. For example, ``Iterable`` does not implement
-``__len__`` and so this code will be flagged:
 
-.. code-block:: python
+Check that iterable is not implicitly true in boolean context [truthy-iterable]
+-------------------------------------------------------------------------------
 
-    # Use "mypy -enable-error-code truthy-bool ..."
+``Iterable`` does not implement ``__len__`` and so this code will be flagged:
+
+.. code-block:: python
 
     from typing import Iterable
 
-    def transform(items: Iterable[int]) -> Iterable[int]:
-        # Error: "items" has type "Iterable[int]" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+    def transform(items: Iterable[int]) -> list[int]:
+        # Error: "items" has type "Iterable[int]" which can always be true in boolean context. Consider using "Collection[int]" instead.  [truthy-iterable]
         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: Sequence[int]``.
-
-This is similar in concept to ensuring that an expression's type implements an expected interface (e.g. ``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.
+If called with a ``Generator`` like ``int(x) for x 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 is recommended to annotate ``items: Collection[int]``.
 
 
 Check that function isn't used in boolean context [truthy-function]

mypy/checker.py

@@ -5106,6 +5106,14 @@ def format_expr_type() -> str:
             self.fail(message_registry.FUNCTION_ALWAYS_TRUE.format(format_type(t)), expr)
         elif isinstance(t, UnionType):
             self.fail(message_registry.TYPE_ALWAYS_TRUE_UNIONTYPE.format(format_expr_type()), expr)
+        elif isinstance(t, Instance) and t.type.fullname == "typing.Iterable":
+            _, info = self.make_fake_typeinfo("typing", "Collection", "Collection", [])
+            self.fail(
+                message_registry.ITERABLE_ALWAYS_TRUE.format(
+                    format_expr_type(), format_type(Instance(info, t.args))
+                ),
+                expr,
+            )
         else:
             self.fail(message_registry.TYPE_ALWAYS_TRUE.format(format_expr_type()), expr)
 

mypy/errorcodes.py

@@ -160,6 +160,12 @@ def __str__(self) -> str:
     "Warn about function that always evaluate to true in boolean contexts",
     "General",
 )
+TRUTHY_ITERABLE: Final[ErrorCode] = ErrorCode(
+    "truthy-iterable",
+    "Warn about Iterable 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"
 )

mypy/message_registry.py

@@ -153,6 +153,10 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 FUNCTION_ALWAYS_TRUE: Final = ErrorMessage(
     "Function {} could always be true in boolean context", code=codes.TRUTHY_FUNCTION
 )
+ITERABLE_ALWAYS_TRUE: Final = ErrorMessage(
+    "{} which can always be true in boolean context. Consider using {} instead.",
+    code=codes.TRUTHY_ITERABLE,
+)
 NOT_CALLABLE: Final = "{} not callable"
 TYPE_MUST_BE_USED: Final = "Value of type {} must be used"
 

mypy/semanal.py

@@ -51,7 +51,7 @@
 from __future__ import annotations
 
 from contextlib import contextmanager
-from typing import Any, Callable, Iterable, Iterator, List, TypeVar, cast
+from typing import Any, Callable, Collection, Iterable, Iterator, List, TypeVar, cast
 from typing_extensions import Final, TypeAlias as _TypeAlias
 
 from mypy import errorcodes as codes, message_registry
@@ -6202,7 +6202,9 @@ def add_plugin_dependency(self, trigger: str, target: str | None = None) -> None
             target = self.scope.current_target()
         self.cur_mod_node.plugin_deps.setdefault(trigger, set()).add(target)
 
-    def add_type_alias_deps(self, aliases_used: Iterable[str], target: str | None = None) -> None:
+    def add_type_alias_deps(
+        self, aliases_used: Collection[str], target: str | None = None
+    ) -> None:
         """Add full names of type aliases on which the current node depends.
 
         This is used by fine-grained incremental mode to re-check the corresponding nodes.

mypyc/test-data/irbuild-statements.test

@@ -1006,9 +1006,9 @@ L5:
     return 1
 
 [case testForZip]
-from typing import List, Iterable
+from typing import List, Iterable, Sequence
 
-def f(a: List[int], b: Iterable[bool]) -> None:
+def f(a: List[int], b: Sequence[bool]) -> None:
     for x, y in zip(a, b):
         if b:
             x = 1

test-data/unit/check-errorcodes.test

@@ -857,6 +857,13 @@ if not f:  # E: Function "Callable[[], Any]" could always be true in boolean con
     pass
 conditional_result = 'foo' if f else 'bar'  # E: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
 
+[case testTruthyIterable]
+# flags: --strict-optional --enable-error-code truthy-iterable
+from typing import Iterable
+def func(var: Iterable[str]) -> None:
+    if var:  # E: "var" has type "Iterable[str]" which can always be true in boolean context. Consider using "Collection[str]" instead.  [truthy-iterable]
+        ...
+
 [case testNoOverloadImplementation]
 from typing import overload