cPickle - module object has no attribute

[ntroutman]

BPO	5509
Nosy	@benjaminp, @basak
Files	Foo.py: Creates a list of Foo objects and pickles them LoadFoo.py: Loads the pickled list

^{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 2009-03-18.19:08:30.441>
created_at = <Date 2009-03-18.17:08:11.311>
labels = []
title = 'cPickle - module object has no attribute'
updated_at = <Date 2009-12-05.18:51:50.355>
user = 'https://bugs.python.org/ntroutman'

bugs.python.org fields:

activity = <Date 2009-12-05.18:51:50.355>
actor = 'rb'
assignee = 'none'
closed = True
closed_date = <Date 2009-03-18.19:08:30.441>
closer = 'benjamin.peterson'
components = ['None']
creation = <Date 2009-03-18.17:08:11.311>
creator = 'ntroutman'
dependencies = []
files = ['13364', '13365']
hgrepos = []
issue_num = 5509
keywords = []
message_count = 5.0
messages = ['83759', '83760', '83766', '83770', '95998']
nosy_count = 3.0
nosy_names = ['benjamin.peterson', 'rb', 'ntroutman']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue5509'
versions = ['Python 2.5']

[ntroutman]

If I define a class Foo in module A and in module A pickle out a list of
Foo objects to 'foo.pkl', then in module B attempt to unpickle 'foo.pkl'
I recieve the error "AttributeError: 'module' object has no attribute 'Foo'"

Attached are:
Foo.py which defines the class Foo and pickles out a list of objects

LoadFoo.py attempts to load the list of objects pickled by Foo

I'm running Vista with "Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08)
[MSC v.1310 32 bit (Intel)] on win32"

[ntroutman]

Why can I only attach one file at a time?

[ntroutman]

I believe I've tracked down the problem. When you run a python module
directly (ie "python Foo.py") any classes defined in the module have
their '__module__' attribute set to '__main__'. Which means the pickle
says the class is in '__main__' of whatever module is trying to load the
file.

I think it would make more sense to actually include the module name,
this means that an external module need simply ensure that the pickled
class's module be imported with the correct name.

[benjaminp]

Implementing this may be difficult (the module may not be on the search
path when its run and there's no way to determine that). Also, it would
break compatibility. Anyway, it's usually better to define classes that
will be pickled in their own permanent module.

[basak]

I've just been stung by this. I've noticed that this seems to apply to
both cPickle and pickle. Even worse, it causes different behaviour when
a program is run under pdb because __main__ is suddenly pdb rather than
the program itself.

So, in summary, neither pickle nor cPickle can pickle a class if it is
not defined in its own module? Surely this is an obvious deficiency of
pickle and if it is not going to be fixed it should at least be
documented as such?