[Python-bugs-list] [ python-Bugs-451547 ] pickle / cPickle can't load lambdas

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Bugs item #451547, was opened at 2001-08-16 06:33
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Category: Python Library
Group: None
Status: Open
Resolution: None
Priority: 3
Submitted By: Gordon B. McMillan (gmcm)
Assigned to: Guido van Rossum (gvanrossum)
Summary: pickle / cPickle can't load lambdas

Initial Comment:
pickle and cPickle will happily dump a lambda, but on 
load, both report:
SystemError: Failed to import class <lambda> from 
module __main__

Seen on Py 2.1 & 1.5.2

>>> f = lambda x: x in (1,2,3)
>>> o = cPickle.dumps(f)
>>> f2 = cPickle.loads(o)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
SystemError: Failed to import class <lambda> from 
module __main__
>>> o = pickle.dumps(f)
>>> f2 = pickle.loads(o)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\python21\lib\pickle.py", line 951, in loads
    return Unpickler(file).load()
  File "c:\python21\lib\pickle.py", line 567, in load
    dispatch[key](self)
  File "c:\python21\lib\pickle.py", line 780, in 
load_global
    klass = self.find_class(module, name)
  File "c:\python21\lib\pickle.py", line 790, in 
find_class
    raise SystemError, \
SystemError: Failed to import class <lambda> from 
module __main__



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>Comment By: Guido van Rossum (gvanrossum)
Date: 2001-08-16 09:31

Message:
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user_id=6380

I guess what you're missing is that pickling a function
doesn't pickle the bytecode! It pickles the name instead. So
a reference to "foo.bar" is pickled as "foo.bar", and the
unpickler imports bar from foo.

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Comment By: Gordon B. McMillan (gmcm)
Date: 2001-08-16 09:03

Message:
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Hmm. In the (simplistic) case I was trying, I can't see any 
significant difference between the lambda and the 
equivalent function (names differ, and the func has 2 
appended & apparently unreachable bytecodes, but otherwise 
the func_* and func_code.co_* attributes match). So what am 
I missing?

Lowering priority - I can live without it easily enough.

But if lambda's won't load, they probably shouldn't dump.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2001-08-16 08:11

Message:
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user_id=6380

I think I'll have to close this with a won't fix, or "then
don't do that" resolution.

The problem is that whenever a function or class is pickled,
pickle must accept on blind faith that it can also be
unpickled. How would you check that this is indeed the case?


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