Cannot deepcopy unittest.TestCase instances

[spiv]

BPO	5660
Nosy	@gpshead, @voidspace

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/gpshead'
closed_at = <Date 2009-04-02.05:52:35.022>
created_at = <Date 2009-04-02.00:27:53.138>
labels = ['library']
title = 'Cannot deepcopy unittest.TestCase instances'
updated_at = <Date 2009-04-02.05:52:35.021>
user = 'https://bugs.python.org/spiv'

bugs.python.org fields:

activity = <Date 2009-04-02.05:52:35.021>
actor = 'michael.foord'
assignee = 'gregory.p.smith'
closed = True
closed_date = <Date 2009-04-02.05:52:35.022>
closer = 'michael.foord'
components = ['Library (Lib)']
creation = <Date 2009-04-02.00:27:53.138>
creator = 'spiv'
dependencies = []
files = []
hgrepos = []
issue_num = 5660
keywords = []
message_count = 4.0
messages = ['85123', '85163', '85166', '85171']
nosy_count = 3.0
nosy_names = ['gregory.p.smith', 'spiv', 'michael.foord']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue5660'
versions = ['Python 2.7']

[spiv]

Here's a demonstration of the bug:

>>> from unittest import TestCase
>>> class MyTest(TestCase):
...     def test_foo(self): pass
... 
>>> tc = MyTest('test_foo')
>>> import copy
>>> copy.deepcopy(tc)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 189, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 338, in
_reconstruct
    state = deepcopy(state, memo)
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 162, in deepcopy
    y = copier(x, memo)
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 255, in
_deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 162, in deepcopy
    y = copier(x, memo)
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 255, in
_deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 189, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/local/stow/py2.7/lib/python2.7/copy.py", line 323, in
_reconstruct
    y = callable(*args)
  File "/usr/local/bin/../stow/py2.7/lib/python2.7/copy_reg.py", line
93, in __newobj__
    return cls.__new__(cls, *args)
TypeError: instancemethod expected at least 2 arguments, got 0

This regression breaks bzr's test suite, which copies test objects to
run the same test against multiple scenarios (e.g. to run all the same
tests against all the implementations of bzrlib.transport.Transport,
such as HTTPTransport and SFTPTransport.)
<https://launchpad.net/testtools\> also implements test parameterisation
in this way, and it is extremely useful.

I suspect the __test_equality_funcs on TestCase is the problem:

>>> tc.__dict__
{'_testMethodDoc': None, '_TestCase__type_equality_funcs': {<type
'dict'>: <bound method MyTest.assertDictEqual of <__main__.MyTest
testMethod=test_foo>>, <type 'tuple'>: <bound method
MyTest.assertTupleEqual of <__main__.MyTest testMethod=test_foo>>, <type
'frozenset'>: <bound method MyTest.assertSetEqual of <__main__.MyTest
testMethod=test_foo>>, <type 'list'>: <bound method
MyTest.assertListEqual of <__main__.MyTest testMethod=test_foo>>, <type
'set'>: <bound method MyTest.assertSetEqual of <__main__.MyTest
testMethod=test_foo>>}, '_testMethodName': 'test_foo'}

copy.deepcopy can't deepcopy bound methods, reasonably enough.

[voidspace]

This is a workaround:

import copy
copy._deepcopy_dispatch[types.MethodType] = copy._deepcopy_atomic

[voidspace]

We can fix this by wrapping the assert functions in our assert register
as deep-copyable objects. (And then unwrapping when we fetch them.)

[voidspace]

Fixed in revision 71043.