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What use is '__reduce__'?

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  What use is '__reduce__'? Robert <rxjwg98@gmail.com> - 2016-01-08 08:42 -0800
    Re: What use is '__reduce__'? Ian Kelly <ian.g.kelly@gmail.com> - 2016-01-08 10:03 -0700
      Re: What use is '__reduce__'? Robert <rxjwg98@gmail.com> - 2016-01-08 09:45 -0800
    Re: What use is '__reduce__'? Peter Otten <__peter__@web.de> - 2016-01-08 21:12 +0100

#101386 — What use is '__reduce__'?

Hi,

When I try to run the following code:



/////
from collections import Counter, OrderedDict

class OrderedCounter(Counter, OrderedDict):
     'Counter that remembers the order elements are first seen'
     def __repr__(self):
         return '%s(%r)' % (self.__class__.__name__,
                            OrderedDict(self))
     def __reduce__(self):
         return self.__class__, (OrderedDict(self),)

oc = OrderedCounter('abracadabra') 
-----

I don't know the use of '__reduce__', even I look it up on Python website.
On that website, it explains 'pickle' module:
https://docs.python.org/2/library/pickle.html

But the above example without import that module. Is it from built-in?
Anyhow, I don't find a built-in explanation about '__reduce__'.

What use of the above two new self methods are in class OrderedCounter?

Thanks,

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#101388

As you found by searching, __reduce__ is used to determine how
instances of the class are pickled. If the example you're using
doesn't do any pickling, then it's not really relevant to the example.
It's probably included so that it won't be missed when the code is
copied.

On Fri, Jan 8, 2016 at 9:42 AM, Robert <rxjwg98@gmail.com> wrote:
> Hi,
>
> When I try to run the following code:
>
>
>
> /////
> from collections import Counter, OrderedDict
>
> class OrderedCounter(Counter, OrderedDict):
>      'Counter that remembers the order elements are first seen'
>      def __repr__(self):
>          return '%s(%r)' % (self.__class__.__name__,
>                             OrderedDict(self))
>      def __reduce__(self):
>          return self.__class__, (OrderedDict(self),)
>
> oc = OrderedCounter('abracadabra')
> -----
>
> I don't know the use of '__reduce__', even I look it up on Python website.
> On that website, it explains 'pickle' module:
> https://docs.python.org/2/library/pickle.html
>
> But the above example without import that module. Is it from built-in?
> Anyhow, I don't find a built-in explanation about '__reduce__'.
>
> What use of the above two new self methods are in class OrderedCounter?
>
> Thanks,
>
>
> --
> https://mail.python.org/mailman/listinfo/python-list

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#101390

On Friday, January 8, 2016 at 12:04:21 PM UTC-5, Ian wrote:
> As you found by searching, __reduce__ is used to determine how
> instances of the class are pickled. If the example you're using
> doesn't do any pickling, then it's not really relevant to the example.
> It's probably included so that it won't be missed when the code is
> copied.
> 
> On Fri, Jan 8, 2016 at 9:42 AM, Robert <ail.com> wrote:
> > Hi,
> >
> > When I try to run the following code:
> >
> >
> >
> > /////
> > from collections import Counter, OrderedDict
> >
> > class OrderedCounter(Counter, OrderedDict):
> >      'Counter that remembers the order elements are first seen'
> >      def __repr__(self):
> >          return '%s(%r)' % (self.__class__.__name__,
> >                             OrderedDict(self))
> >      def __reduce__(self):
> >          return self.__class__, (OrderedDict(self),)
> >
> > oc = OrderedCounter('abracadabra')
> > -----
> >
> > I don't know the use of '__reduce__', even I look it up on Python website.
> > On that website, it explains 'pickle' module:
> > https://docs.python.org/2/library/pickle.html
> >
> > But the above example without import that module. Is it from built-in?
> > Anyhow, I don't find a built-in explanation about '__reduce__'.
> >
> > What use of the above two new self methods are in class OrderedCounter?
> >
> > Thanks,
> >
> >
> > --
> > https://mail.python.org/mailman/listinfo/python-list

Thanks for your reply.

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#101396

Ian Kelly wrote:

> As you found by searching, __reduce__ is used to determine how
> instances of the class are pickled. If the example you're using
> doesn't do any pickling, then it's not really relevant to the example.
> It's probably included so that it won't be missed when the code is
> copied.

__reduce__() also makes copying work as expected:

>>> class OrderedCounter(Counter, OrderedDict):
...      'Counter that remembers the order elements are first seen'
...      def __repr__(self):
...          return '%s(%r)' % (self.__class__.__name__,
...                             OrderedDict(self))
...      def __reduce__(self):
...          return self.__class__, (OrderedDict(self),)
... 
>>> oc = OrderedCounter('abracadabra') 
>>> import copy
>>> copy.copy(oc)
OrderedCounter(OrderedDict([('a', 5), ('b', 2), ('r', 2), ('c', 1), ('d', 
1)]))

Now take away the __reduce__ method:

>>> del OrderedCounter.__reduce__
>>> copy.copy(oc)
OrderedCounter(OrderedDict([('b', 2), ('a', 5), ('c', 1), ('r', 2), ('d', 
1)]))

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