Lambda alternative?

Aaron Brady castironpi at gmail.com
Fri Apr 17 20:53:42 CEST 2009


On Apr 17, 1:43 pm, "J. Cliff Dyer" <j... at sdf.lonestar.org> wrote:
> On Thu, 2009-04-16 at 13:33 +0200, Hrvoje Niksic wrote:
> > mousemeat <mousem... at gmail.com> writes:
>
> > > Correct me if i am wrong, but i can pickle an object that contains a
> > > bound method (it's own bound method).
>
> > No, you can't:
>
> > >>> import cPickle as p
> > >>> p.dumps([])
> > '(l.'
> > >>> p.dumps([].append)
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> > TypeError: expected string or Unicode object, NoneType found
>
> Yes he can.  mousemeat stated that he could pickle an object that
> *contains* a bound method, not that he could pickle the method itself.
>
> That said, you can make an instance method out of a lambda, just as well
> as any named function, and you can pickle that object, too:
snip

'Contains' here is ambiguous.  If the object contains a bound method,
that is, if a bound method is in its dictionary, you can't.

>>> import pickle as p
>>> class A: pass
...
>>> a= A()
>>> class A:
...     def f( self ): print( 'f' )
...
>>> a= A()
>>> class B: pass
...
>>> b= B()
>>> b.f= a.f
>>> b.f()
f
>>> p.dumps( b )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Programs\Python30\lib\pickle.py", line 1329, in dumps
    Pickler(f, protocol).dump(obj)
_pickle.PicklingError: Can't pickle <class 'method'>: attribute
lookup
builtins.method failed

In this example, 'b' contains a bound method, 'a.f'.  However, for
other definitions of 'contains', such as if 'b' is an instance of a
class that contains methods, you can.  But in that case, the method is
not in 'b.__dict__'.

>>> b.__dict__
{'f': <bound method A.f of <__main__.A object at 0x00B54B90>>}




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