Serhiy Storchaka added the comment:
The classes can't be *correctly* copied and unpickled because the pickle data
doesn't contain object's state. "Copied" and "unpickled" objects are not equal
to original objects, they are non-initialized instances. You can test
attributes of copied NameAssignment instance to ensure that they are not
restored. May be copied values are not used in Cython building, except rare
exclusive cases when using them fails.
Example in the stdlib is csv.Dialect objects:
>>> import csv, pickle
>>> d = csv.get_dialect('excel-tab')
>>> d2 = pickle.loads(pickle.dumps(d))
>>> d.delimiter, d.doublequote, d.escapechar, d.lineterminator, d.quotechar,
>>> d.quoting, d.skipinitialspace, d.strict
('\t', 1, None, '\r\n', '"', 0, 0, 0)
>>> d2.delimiter, d2.doublequote, d2.escapechar, d2.lineterminator,
>>> d2.quotechar, d2.quoting, d2.skipinitialspace, d2.strict
(',', 1, None, '\r\n', '"', 0, 0, 0)
You just silently get wrong result. Since it by accident matches the instance
of most used 'excel' dialect, this error is left unnoticed.
I think we have to left this restriction in 3.6 (and Cython should fix its bug
by providing either __getnewargs__/ __getnewargs_ex__, __getstate__ or
__reduce__/__reduce_ex__). But in 2.7 and 3.5 we should allow current Cython to
work (even obtaining wrong result) by removing the check at all or by making it
emit only a warning.
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<http://bugs.python.org/issue22995>
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