error when deep copying module is confusing

[mrjbq7]

BPO	18195
Nosy	@mrjbq7, @bitdancer, @serhiy-storchaka

^{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 = None
closed_at = <Date 2017-02-19.20:55:25.541>
created_at = <Date 2013-06-11.22:45:32.576>
labels = ['type-feature']
title = 'error when deep copying module is confusing'
updated_at = <Date 2017-02-19.20:55:25.540>
user = 'https://github.com/mrjbq7'

bugs.python.org fields:

activity = <Date 2017-02-19.20:55:25.540>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-02-19.20:55:25.541>
closer = 'serhiy.storchaka'
components = []
creation = <Date 2013-06-11.22:45:32.576>
creator = 'mrjbq7'
dependencies = []
files = []
hgrepos = []
issue_num = 18195
keywords = []
message_count = 4.0
messages = ['190996', '191002', '261006', '288164']
nosy_count = 3.0
nosy_names = ['mrjbq7', 'r.david.murray', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue18195'
versions = []

[mrjbq7]

If you have a simple module (say "foo.py"):

$ cat foo.py
bar = 1

You get weird errors when trying to deep copy them (which I did by accident, not intentionally trying to deep copy modules):

Python 2.7.2:

>>> import foo
>>> import copy
>>> copy.deepcopy(foo)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 190, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 334, in _reconstruct
    state = deepcopy(state, memo)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 163, in deepcopy
    y = copier(x, memo)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 257, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 163, in deepcopy
    y = copier(x, memo)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 257, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 190, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 329, in _reconstruct
    y = callable(*args)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy_reg.py", line 93, in __newobj__
    return cls.__new__(cls, *args)
TypeError: object.__new__(NotImplementedType) is not safe, use NotImplementedType.__new__()

Python 3.3.2:

>>> import foo
>>> import copy
>>> copy.deepcopy(foo)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.3/copy.py", line 174, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/lib/python3.3/copy.py", line 301, in _reconstruct
    y.__dict__.update(state)
AttributeError: 'NoneType' object has no attribute 'update'

I'm not expecting to be able to deep copy a module, but it would be really great if it is not possible for the error message to say something like "deepcopy doesn't work for modules" rather than two different funky tracebacks that don't really explain the problem...

Thanks,

[bitdancer]

Well, we don't generally complicate the code to handle edge cases. That said, it might not be too complicated to add copy protocol methods to the module object which just raise a more useful error.

[serhiy-storchaka]

Python 3.6:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/copy.py", line 97, in copy
    rv = reductor(4)
TypeError: can't pickle module objects

[serhiy-storchaka]

This issue looks as enhancement rather than bug fix. Since it is fixed in 3.6 I'm closing it.