B021: f-string used as docstring.

README.rst

@@ -134,6 +134,9 @@ data available in ``ex``.
 
 **B020**: Loop control variable overrides iterable it iterates
 
+**B021**: f-string used as docstring. This will be interpreted by python
+as a joined string rather than a docstring.
+
 
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~

bugbear.py

@@ -350,11 +350,13 @@ def visit_FunctionDef(self, node):
         self.check_for_b902(node)
         self.check_for_b006(node)
         self.check_for_b018(node)
+        self.check_for_b021(node)
         self.generic_visit(node)
 
     def visit_ClassDef(self, node):
         self.check_for_b903(node)
         self.check_for_b018(node)
+        self.check_for_b021(node)
         self.generic_visit(node)
 
     def visit_Try(self, node):
@@ -685,6 +687,20 @@ def check_for_b018(self, node):
             ):
                 self.errors.append(B018(subnode.lineno, subnode.col_offset))
 
+    def check_for_b021(self, node):
+        if (
+            node.body
+            and isinstance(node.body[0], ast.Expr)
+            and isinstance(node.body[0].value, ast.JoinedStr)
+            and ast.get_source_segment(
+                "".join(self.lines),
+                node.body[0].value,
+            ).startswith('f"""')
+        ):
+            self.errors.append(
+                B021(node.body[0].value.lineno, node.body[0].value.col_offset)
+            )
+
 
 @attr.s
 class NameFinder(ast.NodeVisitor):
@@ -892,6 +908,12 @@ def visit(self, node):
         + "with each iterable value."
     )
 )
+B021 = Error(
+    message=(
+        "B021 f-string used as docstring."
+        "This will be interpreted by python as a joined string rather than a docstring."
+    )
+)
 
 # Warnings disabled by default.
 B901 = Error(

tests/b021.py

@@ -0,0 +1,22 @@
+"""
+Should emit:
+B021 - on lines 14, 22
+"""
+
+VARIABLE = "world"
+
+
+def foo1():
+    """hello world!"""
+
+
+def foo2():
+    f"""hello {VARIABLE}!"""
+
+
+class bar1:
+    """hello world!"""
+
+
+class bar2:
+    f"""hello {VARIABLE}!"""

tests/test_bugbear.py

@@ -30,6 +30,7 @@
     B017,
     B018,
     B020,
+    B021,
     B901,
     B902,
     B903,
@@ -262,6 +263,13 @@ def test_b020(self):
             ),
         )
 
+    def test_b021_classes(self):
+        filename = Path(__file__).absolute().parent / "b021.py"
+        bbc = BugBearChecker(filename=str(filename))
+        errors = list(bbc.run())
+        expected = self.errors(B021(14, 4), B021(22, 4))
+        self.assertEqual(errors, expected)
+
     def test_b901(self):
         filename = Path(__file__).absolute().parent / "b901.py"
         bbc = BugBearChecker(filename=str(filename))