b042: ignore overloaded init, ignore if str+pickle dunder, improve README

README.rst

@@ -288,7 +288,7 @@ second usage. Save the result to a list if the result is needed multiple times.
 
 **B041**: Repeated key-value pair in dictionary literal. Only emits errors when the key's value is *also* the same, being the opposite of the pyflakes like check.
 
-**B042**: Remember to call super().__init__() in custom exceptions initalizer.
+**B042**: Exception classes with a custom `__init__` should pass all args to `super().__init__()` in order to work correctly with `copy.copy` and `pickle`. Both `BaseException.__reduce__` and `BaseException.__str__` relies 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__` ignores. 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.
 
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~

bugbear.py

@@ -1749,10 +1749,39 @@ def is_exception(s: str):
             else:
                 return
 
+        # if the user defines __str__ + a pickle dunder they're probably in the clear.
+        has_pickle_dunder = False
+        has_str = False
+        for fun in node.body:
+            if isinstance(fun, ast.FunctionDef) and fun.name in (
+                "__getnewargs_ex__",
+                "__getnewargs__",
+                "__getstate__",
+                "__setstate__",
+                "__reduce__",
+                "__reduce_ex__",
+            ):
+                if has_str:
+                    return
+                has_pickle_dunder = True
+            elif isinstance(fun, ast.FunctionDef) and fun.name == "__str__":
+                if has_pickle_dunder:
+                    return
+                has_str = True
+
         # iterate body nodes looking for __init__
         for fun in node.body:
             if not (isinstance(fun, ast.FunctionDef) and fun.name == "__init__"):
                 continue
+            if any(
+                (isinstance(decorator, ast.Name) and decorator.id == "overload")
+                or (
+                    isinstance(decorator, ast.Attribute)
+                    and decorator.attr == "overload"
+                )
+                for decorator in fun.decorator_list
+            ):
+                continue
             if fun.args.kwonlyargs or fun.args.kwarg:
                 # kwargs cannot be passed to super().__init__()
                 self.add_error("B042", fun)
@@ -2411,7 +2440,7 @@ def __call__(self, lineno: int, col: int, vars: tuple[object, ...] = ()) -> erro
     "B042": Error(
         message=(
             "B042 Exception class with `__init__` should pass all args to "
-            "`super().__init__()` in order to work with `copy.copy()`. "
+            "`super().__init__()` to work in edge cases of `pickle` and `copy.copy()`. "
             "It should also not take any kwargs."
         )
     ),

tests/eval_files/b042.py

@@ -1,3 +1,7 @@
+import typing
+from typing import overload
+
+
 class MyError_no_args(Exception):
     def __init__(self):  # safe
         ...
@@ -46,6 +50,25 @@ class MyError_posonlyargs(Exception):
     def __init__(self, x, /, y):
         super().__init__(x, y)
 
+# ignore overloaded __init__
+class MyException(Exception):
+    @overload
+    def __init__(self, x: int): ...
+    @overload
+    def __init__(self, x: float): ...
+
+    def __init__(self, x):
+        super().__init__(x)
+
+class MyException2(Exception):
+    @typing.overload
+    def __init__(self, x: int): ...
+    @typing.overload
+    def __init__(self, x: float): ...
+
+    def __init__(self, x):
+        super().__init__(x)
+
 # triggers if class name ends with, or
 # if it inherits from a class whose name ends with, any of
 # 'Error', 'Exception', 'ExceptionGroup', 'Warning', 'ExceptionGroup'
@@ -70,5 +93,23 @@ def __init__(self, x): ...  # B042: 4
 class ExceptionHandler(Anything):
     def __init__(self, x): ...  # safe
 
-class FooException:
+class FooException: # safe, doesn't inherit from anything
+    def __init__(self, x): ...
+
+### Ignore classes that define __str__ + any pickle dunder
+class HasReduceStr(Exception):
+    def __reduce__(self): ...
+    def __str__(self): ...
+    def __init__(self, x): ...
+
+class HasReduce(Exception):
+    def __reduce__(self): ...
+    def __init__(self, x): ...  # B042: 4
+class HasStr(Exception):
+    def __str__(self): ...
+    def __init__(self, x): ...  # B042: 4
+
+class HasStrReduceEx(Exception):
+    def __reduce_ex__(self, protocol): ...
+    def __str__(self): ...
     def __init__(self, x): ...