Instances of subclasses of BaseException created with keyword argument fail to copy properly as demonstrated by:

    import copy

    class E(BaseException):
        def __init__(self, x):
            self.x=x

    # works fine
    e = E(None)
    copy.copy(e)

    # raises
    e = E(x=None)
    copy.copy(e)

This seems to affect all Python versions I've tested (3.6 <= Python <= 3.9).

I've currently partially worked around the issue with a custom pickler that just restores __dict__, but:

 * "args" is not part of __dict__, and setting "args" key in __dict__ does not create a "working object" (i.e. the key is set, but is ignored for all intents and purposes except direct lookup in __dict__)

 * pickle is friendly: you can provide a custom pickler that chooses the reduce function for each single class.
   copy module is much less friendly: copyreg.pickle() only allow registering custom functions for specific classes. That means there is no way (that I know) to make copy.copy() select a custom reduce for a whole subclass tree.


One the root of the issue:

 * exception from the standard library prevent keyword arguments (maybe because of that issue ?), but there is no such restriction on user-defined classes.
 * the culprit is BaseException_reduce() (in Objects/exceptions.c) [1]

It seems that the current behavior is a consequence of the __dict__ being created lazily, I assume for speed and memory efficiency

There seems to be a few approaches that would solve the issue:

 * keyword arguments passed to the constructor could be fused with the positional arguments in BaseException_new (using the signature, but signature might be not be available for extension types I suppose)

 * keyword arguments could just be stored like "args" in a "kwargs" attribute in PyException_HEAD, so they are preserved and passed again to __new__ when the instance is restored upon copying/pickling.

 * the fact that keyword arguments were used could be saved as a bool in PyException_HEAD. When set, this flag would make BaseException_reduce() only use __dict__ and not "args". This would technically probably be a breaking change, but the only cases I can think of where this would be observable are a bit far fetched (if __new__ or __init__ have side effects beyond storing attributes in __dict__).

[1] https://github.com/python/cpython/blob/master/Objects/exceptions.c#L134

The solution based on the signature is something along those lines:

    class E(BaseException):
        def __new__(cls, *args, **kwargs):
            """
            Fix exception copying.

            Turn all the keyword arguments into positional arguments, so that the
            :exc:`BaseException` machinery has all the parameters for a valid call
            to ``__new__``, instead of missing all the keyword arguments.
            """
            sig = inspect.signature(cls.__init__)
            bound_args = sig.bind_partial(*args, **kwargs)
            bound_args.apply_defaults()
            args = tuple(bound_args.arguments.values())
            return super().__new__(cls, *args)

        def __init__(self, x):
            self.x=x

But there are a many shortcomings to that approach:

 * What if super().__new__() consumes arguments before passing the rest to __init__() ? This fix is blind to that since it only cares about __init__ signature

 * What if inspect.signature() does not provide a signature (extension modules) ?

 * Defaults are "hardcoded" in the args, so the object will always be restored with the defaults of the time it was created. This is a breaking change, as currently the defaults used when restoring the instance are the current ones.

 * Also uses more memory for args (and for pickle files), since it contains all the defaults

See issue32696, issue30005, issue29466