Roundup Robot added the comment:
New changeset d103ba56710e by Ethan Furman in branch 'default':
Issue #19272: slight clarification of pickle docs with regard to lambda.
http://hg.python.org/cpython/rev/d103ba56710e
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Ethan Furman added the comment:
Added some clarification to the docs to make it clearer that lambda functions
cannot be pickled.
Facundo [1], if you want to pursue being able to pickle lambda functions please
open an enhancement issue. Some of the questions that come to mind:
1) for a
Changes by Ethan Furman et...@stoneleaf.us:
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Added file: http://bugs.python.org/file32155/issue19272.stoneleaf.01.patch
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New submission from Facundo Batista:
This is ok:
Python 3.4.0a3+ (default:86af5991c809, Oct 13 2013, 16:42:52)
...
import pickle
def f():
... pass
...
pickle.dumps(f)
b'\x80\x03c__main__\nf\nq\x00.'
However, when trying to pickle a lambda, it fails:
pickle.dumps(lambda:
Amaury Forgeot d'Arc added the comment:
Functions are pickled by name, not by code.
Unpickling will only work if a function with the same name is present in in the
same module (__main__ in your example)
This is why pickling a lambda won't work: they have no individual names.
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Changes by Jesús Cea Avión j...@jcea.es:
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Jesús Cea Avión added the comment:
Would be interesting to be able to pickle function.__code__.
Although, thinking about this, it would be not portable between Python
releases, something that pickle guarantee.
Thoughs?
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Ethan Furman added the comment:
According to the docs[1]:
12.1.4. What can be pickled and unpickled?
The following types can be pickled:
- None, True, and False
- integers, floating point numbers, complex numbers
- strings, bytes, bytearrays
- tuples, lists, sets, and
Facundo Batista added the comment:
Ethan, lambda functions are included in functions defined at the top level of
a module.
Probably we should note there something like except lambdas, because function
pickling is by name, not by code.
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Facundo Batista added the comment:
Jesús, Amaury:
What if pickle would assign a random unique name to the lambda (like, an UUID)
so it can be pickled and unpickled?
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Ethan Furman added the comment:
Yeah, that one line should say, named functions defined at the top level of a
module.
The following three paragraphs do, however, mention named several times.
Sounds like a doc issue.
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Ethan Furman added the comment:
The problem with a randam unique name is making sure you get the same random
unique name across different runs, different pythons, and different orders of
execution.
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R. David Murray added the comment:
So don't make it random, use a hash of the code object :)
(I'm not sure I'm serious, the thought just popped into my head...)
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Jesús Cea Avión added the comment:
Lambdas are anonymous functions, by definition. And, usually, they are not
top level functions, but defined inside others.
If at your top level (module) you do:
a = lambda x: 2*x
You don't have an anonymous function, but a function called a. You can argue
Ethan Furman added the comment:
From the pickle docs:
=
Note that functions (built-in and user-defined) are pickled by “fully
qualified” name reference, not by value. This means that only the
function name is pickled, along
Ethan Furman added the comment:
Jesús Cea Avión added the comment:
If at your top level (module) you do:
a = lambda x: 2*x
You don't have an anonymous function, but a function called a.
Actually, you do have an anonymous function, which happens to be bound to the
name a.
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