Property setter and lambda question

Anthony Kong anthony.hw.kong at gmail.com
Mon Jul 11 18:53:29 CEST 2011


Thanks again for your input, Thomas.

I normally prefer

not_here = property(lambda self: self.__get_not_here(), lambda self, v:
self.__set_not_here(v))

than

not_here = property(__get_not_here, __set_not_here)

Because it allows me to have a pair getter/setter (when there is a need for
it). Use of lambda there is ensure derived class of A can provide their
custom version of getter/setter.


But decorator! Of course! Thanks for reminding me this.

In your example, where does '@not_here' come from? (Sorry, this syntax is
new to me)

Cheers

On Tue, Jul 12, 2011 at 2:23 AM, Thomas Jollans <t at jollybox.de> wrote:

> On 07/11/2011 05:54 PM, Anthony Kong wrote:
> > Hi, all,
> >
> > This question is in the same context of my two earlier questions. This
> > question was raised by some python beginners, and I would like to check
> > with the list to ensure I provide a correct answer.
> >
> > Here is a code snippet I used to demonstrate the keyword *property*:
> >
> >
> > class A(object):
> >
> >     def __init__(self):
> >         self.__not_here = 1
> >
> >     def __get_not_here(self):
> >         return self.__not_here
> >
> >     def __set_not_here(self, v):
> >         print "I am called"
> >         self.__not_here = v
> >
> >     not_here = property(lambda self: self.__get_not_here(), lambda self,
> > v: self.__set_not_here(v))
> >     # not_here = property(lambda self: self.__not_here, lambda self, v:
> > self.__not_here = v)
> >
> > So the question: is it possible to use lambda expression at all for the
> > setter? (As in the last, commented-out line)
> >
> > Python interpreter will throw an exception right there if I use the last
> > line ('SyntaxError: lambda cannot contain assignment'). I'd use pass a
> > setter method anyway.
> >
> > What is your preferred solution?
>
> No, a lambda can only contain an expression, not a statement. This is
> not C, assignments are not expressions.
>
> As to what I would do:
> There's really no need to use lambdas at all here:
>
> class A(object):
>    def __init__(self):
>        self.not_here = 1
>    def __get_not_here(self):
>        return self.__not_here
>     def __set_not_here(self, val):
>        self.__not_here = val
>    not_here = property(__get_not_here, __set_not_here)
>
> My favourite way to create properties is of course with decorators:
>
> class A(object):
>    def __init__(self):
>        self.not_here = 1
>
>     @property
>    def not_here(self):
>        return self.__not_here
>
>    @not_here.setter
>    def not_here(self, val):
>        self.__not_here = val
> --
> http://mail.python.org/mailman/listinfo/python-list
>



-- 

Tony Kong
*blog:* www.ahwkong.com

Don’t EVER make the mistake that you can design something better than what
> you get from ruthless massively parallel trial-and-error with a feedback
> cycle. That’s giving your intelligence *much* too much credit.


- Linus Torvalds
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