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Title: Exception copy error
Type: behavior Stage:
Components: Library (Lib) Versions: Python 3.11
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: douglas-raillard-arm, iritkatriel
Priority: normal Keywords:

Created on 2021-03-10 11:30 by douglas-raillard-arm, last changed 2022-04-11 14:59 by admin.

Messages (3)
msg388427 - (view) Author: Douglas Raillard (douglas-raillard-arm) Date: 2021-03-10 11:30
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):

    # works fine
    e = E(None)

    # raises
    e = E(x=None)

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__).

msg388428 - (view) Author: Douglas Raillard (douglas-raillard-arm) Date: 2021-03-10 11:47
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)
            args = tuple(bound_args.arguments.values())
            return super().__new__(cls, *args)

        def __init__(self, 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
msg396600 - (view) Author: Irit Katriel (iritkatriel) * (Python committer) Date: 2021-06-27 21:31
See issue32696, issue30005, issue29466
Date User Action Args
2022-04-11 14:59:42adminsetgithub: 87626
2021-06-27 21:31:34iritkatrielsetversions: + Python 3.11
nosy: + iritkatriel

messages: + msg396600

components: + Library (Lib)
2021-03-10 11:47:51douglas-raillard-armsetmessages: + msg388428
2021-03-10 11:30:36douglas-raillard-armcreate