Do not trigger B901 with explicit Generator return type (#481)

bugbear.py

@@ -1187,10 +1187,27 @@ def _loop(parent, node):
         for child in node.body:
             yield from _loop(node, child)
 
-    def check_for_b901(self, node):
+    def check_for_b901(self, node: ast.FunctionDef) -> None:
         if node.name == "__await__":
             return
 
+        # If the user explicitly wrote the 3-argument version of Generator as the
+        # return annotation, they probably know what they were doing.
+        if (
+            node.returns is not None
+            and isinstance(node.returns, ast.Subscript)
+            and (
+                is_name(node.returns.value, "Generator")
+                or is_name(node.returns.value, "typing.Generator")
+                or is_name(node.returns.value, "collections.abc.Generator")
+            )
+        ):
+            slice = node.returns.slice
+            if sys.version_info < (3, 9) and isinstance(slice, ast.Index):
+                slice = slice.value
+            if isinstance(slice, ast.Tuple) and len(slice.elts) == 3:
+                return
+
         has_yield = False
         return_node = None
 
@@ -1204,9 +1221,8 @@ def check_for_b901(self, node):
             if isinstance(x, ast.Return) and x.value is not None:
                 return_node = x
 
-            if has_yield and return_node is not None:
-                self.errors.append(B901(return_node.lineno, return_node.col_offset))
-                break
+        if has_yield and return_node is not None:
+            self.errors.append(B901(return_node.lineno, return_node.col_offset))
 
     # taken from pep8-naming
     @classmethod
@@ -1703,6 +1719,16 @@ def compose_call_path(node):
         yield node.id
 
 
+def is_name(node: ast.expr, name: str) -> bool:
+    if "." not in name:
+        return isinstance(node, ast.Name) and node.id == name
+    else:
+        if not isinstance(node, ast.Attribute):
+            return False
+        rest, attr = name.rsplit(".", maxsplit=1)
+        return node.attr == attr and is_name(node.value, rest)
+
+
 def _transform_slice_to_py39(slice: ast.expr | ast.Slice) -> ast.Slice | ast.expr:
     """Transform a py38 style slice to a py39 style slice.
 

tests/b901.py

@@ -1,11 +1,11 @@
 """
 Should emit:
-B901 - on lines 9, 36
+B901
 """
 
 def broken():
     if True:
-        return [1, 2, 3]
+        return [1, 2, 3]  # B901
 
     yield 3
     yield 2
@@ -32,7 +32,7 @@ def not_broken3():
 
 
 def broken2():
-    return [3, 2, 1]
+    return [3, 2, 1]  # B901
 
     yield from not_broken()
 
@@ -75,3 +75,35 @@ class NotBroken9(object):
     def __await__(self):
         yield from function()
         return 42
+
+
+def broken3():
+    if True:
+        return [1, 2, 3]  # B901
+    else:
+        yield 3
+
+
+def broken4() -> Iterable[str]:
+    yield "x"
+    return ["x"]  # B901
+
+
+def broken5() -> Generator[str]:
+    yield "x"
+    return ["x"]  # B901
+
+
+def not_broken10() -> Generator[str, int, float]:
+    yield "x"
+    return 1.0
+
+
+def not_broken11() -> typing.Generator[str, int, float]:
+    yield "x"
+    return 1.0
+
+
+def not_broken12() -> collections.abc.Generator[str, int, float]:
+    yield "x"
+    return 1.0

tests/test_bugbear.py

@@ -792,7 +792,10 @@ def test_b901(self):
         filename = Path(__file__).absolute().parent / "b901.py"
         bbc = BugBearChecker(filename=str(filename))
         errors = list(bbc.run())
-        self.assertEqual(errors, self.errors(B901(8, 8), B901(35, 4)))
+        self.assertEqual(
+            errors,
+            self.errors(B901(8, 8), B901(35, 4), B901(82, 8), B901(89, 4), B901(94, 4)),
+        )
 
     def test_b902(self):
         filename = Path(__file__).absolute().parent / "b902.py"