Improve B018 to include useless calls (#547)

* Improve B018 to include useless calls

Closes #546

* Fix

* Add changelog

Author: kasium

README.rst

@@ -173,6 +173,7 @@ using ``pytest.raises``), or use the context manager form with a target
 .. _B018:
 
 **B018**: Found useless expression. Either assign it to a variable or remove it.
+The check also considers function calls without side-effects such as ``isinstance``.
 Note that dangling commas will cause things to be interpreted as useless tuples.
 For example, in the statement ``print(".."),`` is the same as ``(print(".."),)``
 which is an unassigned tuple. Simply remove the comma to clear the error.
@@ -292,16 +293,16 @@ second usage. Save the result to a list if the result is needed multiple times.
 
 .. _B042:
 
-**B042**: Exception classes with a custom `__init__` should pass all args to `super().__init__()` to work correctly with `copy.copy` and `pickle`. 
-Both `BaseException.__reduce__` and `BaseException.__str__` rely on the `args` attribute being set correctly, which is set in `BaseException.__new__` and `BaseException.__init__`. 
-If you define `__init__` yourself without passing all arguments to `super().__init__` it is very easy to break pickling, especially if they pass keyword arguments which both 
-`BaseException.__new__` and `BaseException.__init__` ignore. It's also important that `__init__` not accept any keyword-only parameters. 
-Alternately you can define both `__str__` and `__reduce__` to bypass the need for correct handling of `args`. 
+**B042**: Exception classes with a custom `__init__` should pass all args to `super().__init__()` to work correctly with `copy.copy` and `pickle`.
+Both `BaseException.__reduce__` and `BaseException.__str__` rely on the `args` attribute being set correctly, which is set in `BaseException.__new__` and `BaseException.__init__`.
+If you define `__init__` yourself without passing all arguments to `super().__init__` it is very easy to break pickling, especially if they pass keyword arguments which both
+`BaseException.__new__` and `BaseException.__init__` ignore. It's also important that `__init__` not accept any keyword-only parameters.
+Alternately you can define both `__str__` and `__reduce__` to bypass the need for correct handling of `args`.
 If you define `__str__/__reduce__` in super classes this check is unable to detect it, and we advise disabling it.
 
 .. _B043:
 
-**B043**: Do not call ``delattr(x, 'attr')``, instead use ``del x.attr``. 
+**B043**: Do not call ``delattr(x, 'attr')``, instead use ``del x.attr``.
 There is no additional safety in using ``delattr`` if you know the attribute name ahead of time.
 
 
@@ -494,6 +495,11 @@ MIT
 Change Log
 ----------
 
+UNRELEASED
+~~~~~~~~~~
+
+* B018: handle also useless calls such as `isinstance(x, int)` without assigning or using the result
+
 25.11.29
 ~~~~~~~~
 

bugbear.py

@@ -42,6 +42,22 @@
     ast.GeneratorExp,
 )
 FUNCTION_NODES = (ast.AsyncFunctionDef, ast.FunctionDef, ast.Lambda)
+FUNCTIONS_WITHOUT_SIDE_EFFECTS = (
+    "all",
+    "any",
+    "dict",
+    "frozenset",
+    "isinstance",
+    "issubclass",
+    "len",
+    "max",
+    "min",
+    "repr",
+    "set",
+    "sorted",
+    "str",
+    "tuple",
+)
 B908_pytest_functions = {"raises", "warns"}
 B908_unittest_methods = {
     "assertRaises",
@@ -1466,19 +1482,27 @@ def check_for_b903(self, node: ast.ClassDef) -> None:
     def check_for_b018(self, node: ast.AST) -> None:
         if not isinstance(node, ast.Expr):
             return
-        if isinstance(
-            node.value,
-            (
-                ast.List,
-                ast.Set,
-                ast.Dict,
-                ast.Tuple,
-            ),
-        ) or (
-            isinstance(node.value, ast.Constant)
-            and (
-                isinstance(node.value.value, (int, float, complex, bytes, bool))
-                or node.value.value is None
+        if (
+            isinstance(
+                node.value,
+                (
+                    ast.List,
+                    ast.Set,
+                    ast.Dict,
+                    ast.Tuple,
+                ),
+            )
+            or (
+                isinstance(node.value, ast.Constant)
+                and (
+                    isinstance(node.value.value, (int, float, complex, bytes, bool))
+                    or node.value.value is None
+                )
+            )
+            or (
+                isinstance(node.value, ast.Call)
+                and isinstance(node.value.func, ast.Name)
+                and node.value.func.id in FUNCTIONS_WITHOUT_SIDE_EFFECTS
             )
         ):
             self.add_error("B018", node, node.value.__class__.__name__)

tests/eval_files/b018_calls.py

@@ -0,0 +1,24 @@
+"""
+Should emit:
+B018 - on lines 11-24
+"""
+
+something()
+assert issubclass(object, int)
+x = sorted(foo)
+
+# calls to be found
+all()  # B018: 0, "Call"
+any()  # B018: 0, "Call"
+dict()  # B018: 0, "Call"
+frozenset()  # B018: 0, "Call"
+isinstance()  # B018: 0, "Call"
+issubclass()  # B018: 0, "Call"
+len()  # B018: 0, "Call"
+max()  # B018: 0, "Call"
+min()  # B018: 0, "Call"
+repr()  # B018: 0, "Call"
+set()  # B018: 0, "Call"
+sorted()  # B018: 0, "Call"
+str()  # B018: 0, "Call"
+tuple()  # B018: 0, "Call"

tests/eval_files/b023.py

@@ -111,50 +111,50 @@ def myfunc(x):
 # argument or in a consumed `filter()` (even if a comprehension is better style)
 for x in range(2):
     # It's not a complete get-out-of-linting-free construct - these should fail:
-    min([None, lambda: x], key=repr)  # B023: 23, "x"
-    sorted([None, lambda: x], key=repr)  # B023: 26, "x"
-    any(filter(bool, [None, lambda: x]))  # B023: 36, "x"
-    list(filter(bool, [None, lambda: x]))  # B023: 37, "x"
-    all(reduce(bool, [None, lambda: x]))  # B023: 36, "x"
+    _ = min([None, lambda: x], key=repr)  # B023: 27, "x"
+    _ = sorted([None, lambda: x], key=repr)  # B023: 30, "x"
+    _ = any(filter(bool, [None, lambda: x]))  # B023: 40, "x"
+    _ = list(filter(bool, [None, lambda: x]))  # B023: 41, "x"
+    _ = all(reduce(bool, [None, lambda: x]))  # B023: 40, "x"
 
     # But all these ones should be OK:
-    min(range(3), key=lambda y: x * y)
-    max(range(3), key=lambda y: x * y)
-    sorted(range(3), key=lambda y: x * y)
-
-    any(map(lambda y: x < y, range(3)))
-    all(map(lambda y: x < y, range(3)))
-    set(map(lambda y: x < y, range(3)))
-    list(map(lambda y: x < y, range(3)))
-    tuple(map(lambda y: x < y, range(3)))
-    sorted(map(lambda y: x < y, range(3)))
-    frozenset(map(lambda y: x < y, range(3)))
-
-    any(filter(lambda y: x < y, range(3)))
-    all(filter(lambda y: x < y, range(3)))
-    set(filter(lambda y: x < y, range(3)))
-    list(filter(lambda y: x < y, range(3)))
-    tuple(filter(lambda y: x < y, range(3)))
-    sorted(filter(lambda y: x < y, range(3)))
-    frozenset(filter(lambda y: x < y, range(3)))
-
-    any(reduce(lambda y: x | y, range(3)))
-    all(reduce(lambda y: x | y, range(3)))
-    set(reduce(lambda y: x | y, range(3)))
-    list(reduce(lambda y: x | y, range(3)))
-    tuple(reduce(lambda y: x | y, range(3)))
-    sorted(reduce(lambda y: x | y, range(3)))
-    frozenset(reduce(lambda y: x | y, range(3)))
+    _ = min(range(3), key=lambda y: x * y)
+    _ = max(range(3), key=lambda y: x * y)
+    _ = sorted(range(3), key=lambda y: x * y)
+
+    _ = any(map(lambda y: x < y, range(3)))
+    _ = all(map(lambda y: x < y, range(3)))
+    _ = set(map(lambda y: x < y, range(3)))
+    _ = list(map(lambda y: x < y, range(3)))
+    _ = tuple(map(lambda y: x < y, range(3)))
+    _ = sorted(map(lambda y: x < y, range(3)))
+    _ = frozenset(map(lambda y: x < y, range(3)))
+
+    _ = any(filter(lambda y: x < y, range(3)))
+    _ = all(filter(lambda y: x < y, range(3)))
+    _ = set(filter(lambda y: x < y, range(3)))
+    _ = list(filter(lambda y: x < y, range(3)))
+    _ = tuple(filter(lambda y: x < y, range(3)))
+    _ = sorted(filter(lambda y: x < y, range(3)))
+    _ = frozenset(filter(lambda y: x < y, range(3)))
+
+    _ = any(reduce(lambda y: x | y, range(3)))
+    _ = all(reduce(lambda y: x | y, range(3)))
+    _ = set(reduce(lambda y: x | y, range(3)))
+    _ = list(reduce(lambda y: x | y, range(3)))
+    _ = tuple(reduce(lambda y: x | y, range(3)))
+    _ = sorted(reduce(lambda y: x | y, range(3)))
+    _ = frozenset(reduce(lambda y: x | y, range(3)))
 
     import functools
 
-    any(functools.reduce(lambda y: x | y, range(3)))
-    all(functools.reduce(lambda y: x | y, range(3)))
-    set(functools.reduce(lambda y: x | y, range(3)))
-    list(functools.reduce(lambda y: x | y, range(3)))
-    tuple(functools.reduce(lambda y: x | y, range(3)))
-    sorted(functools.reduce(lambda y: x | y, range(3)))
-    frozenset(functools.reduce(lambda y: x | y, range(3)))
+    _ = any(functools.reduce(lambda y: x | y, range(3)))
+    _ = all(functools.reduce(lambda y: x | y, range(3)))
+    _ = set(functools.reduce(lambda y: x | y, range(3)))
+    _ = list(functools.reduce(lambda y: x | y, range(3)))
+    _ = tuple(functools.reduce(lambda y: x | y, range(3)))
+    _ = sorted(functools.reduce(lambda y: x | y, range(3)))
+    _ = frozenset(functools.reduce(lambda y: x | y, range(3)))
 
 
 # OK because the lambda which references a loop variable is defined in a `return`