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A way to write properties

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  A way to write properties Arnaud Delobelle <arnodel@gmail.com> - 2012-01-23 11:45 +0000
    Re: A way to write properties HEK <elkarouh@gmail.com> - 2012-01-24 01:51 -0800

#19258 — A way to write properties

Hi all,

It just occurred to me that there's a very simple but slightly
different way to implement properties:

class PropertyType(type):
    def __get__(self, obj, objtype):
        return self if obj is None else self.get(obj)
    def __set__(self, obj, val):
        self.set(obj, val)
    def __delete__(self, obj):
        self.delete(obj)

class Property(metaclass=PropertyType):
    pass

# Here is an example:

class Test:
    class x(Property):
        "My property"
        def get(self):
            return "Test.x"
        def set(self, val):
            print("Setting Test.x to", val)

# This gives:

>>> t = Test()
>>> t.x
'Test.x'
>>> t.x = 42
Setting Test.x to 42
>>> Test.x
<property 'x'>
>>> Test.x.__doc__
'My property'

It also allows defining properties outside class scopes:

class XPlus1(Property):
    "My X Property + 1"
    def get(self):
        return self.x + 1
    def set(self, val):
        self.x = val - 1

class A:
    def __init__(self):
        self.x = 0
    x_plus_one = XPlus1

class B:
    def __init__(self):
        self.x = 2
    x_plus_one = XPlus1

>>> a = A()
>>> b = B()
>>> a.x
0
>>> a.x_plus_one
1
>>> b.x_plus_one
3

I don't know why one would want to do this though :)

-- 
Arnaud

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#19334

On Jan 23, 12:45 pm, Arnaud Delobelle <arno...@gmail.com> wrote:
> Hi all,
>
> It just occurred to me that there's a very simple but slightly
> different way to implement properties:
>
> class PropertyType(type):
>     def __get__(self, obj, objtype):
>         return self if obj is None else self.get(obj)
>     def __set__(self, obj, val):
>         self.set(obj, val)
>     def __delete__(self, obj):
>         self.delete(obj)
>
> class Property(metaclass=PropertyType):
>     pass
>
> # Here is an example:
>
> class Test:
>     class x(Property):
>         "My property"
>         def get(self):
>             return "Test.x"
>         def set(self, val):
>             print("Setting Test.x to", val)
>
> # This gives:
>
> >>> t = Test()
> >>> t.x
> 'Test.x'
> >>> t.x = 42
>
> Setting Test.x to 42>>> Test.x
> <property 'x'>
> >>> Test.x.__doc__
>
> 'My property'
>
> It also allows defining properties outside class scopes:
>
> class XPlus1(Property):
>     "My X Property + 1"
>     def get(self):
>         return self.x + 1
>     def set(self, val):
>         self.x = val - 1
>
> class A:
>     def __init__(self):
>         self.x = 0
>     x_plus_one = XPlus1
>
> class B:
>     def __init__(self):
>         self.x = 2
>     x_plus_one = XPlus1
>
> >>> a = A()
> >>> b = B()
> >>> a.x
> 0
> >>> a.x_plus_one
> 1
> >>> b.x_plus_one
>
> 3
>
> I don't know why one would want to do this though :)
>
> --
> Arnaud

Nice idea.
What would be the python2.7 version (adding __metaclass__=PropertyType
didn't help) ?
Many thanks

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