Implement logic on object.attribute and object.attribute()

Marc Aymerich glicerinu at gmail.com
Sun Nov 24 15:16:38 CET 2013


On Sun, Nov 24, 2013 at 2:45 PM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> On Sun, 24 Nov 2013 05:04:16 -0800, Devin Jeanpierre wrote:
>
>> On Sun, Nov 24, 2013 at 4:52 AM, Marc Aymerich <glicerinu at gmail.com>
>> wrote:
>>> Hi,
>>> I'm playing with python internals to make objects behave like this:
>>>
>>> if I access to "object.attribute" I want to return the result of an
>>> HTTP GET request. However if i call "object.attribute()" I want an HTTP
>>> POST request to be executed.
>>
>> Uh oh. What you want is impossible. You cannot call an attribute without
>> first accessing it. :(
>
> Not quite impossible. All you need is an object that behaves like a
> string, except it has a __call__ method. Here's a sketch of a solution,
> completely untested.
>
>
> class CallableString(str):
>     # Like a string, but callable.
>     def function(self):
>         raise NotImplementedError(
>                 "this must be overridden on the instance"
>                 )
>     def __call__(self):
>         return self.function()
>
>
> class Magic_HTTP_Thing:
>     @property
>     def attribute(self):
>         result = CallableStr(self.do_get())
>         result.function = lambda: self.do_put()
>     def do_get(self):
>         # Do a HTTP GET request.
>         return "Get stuff"
>     def do_put(self):
>         # Do a HTTP PUT request.
>         return "Put stuff"


OMG steven, it actually works :)

>>> class CallableString(str):
...     # Like a string, but callable.
...     def function(self):
...         raise NotImplementedError(
...                 "this must be overridden on the instance"
...                 )
...     def __call__(self):
...         return self.function()
...
>>>
>>> class Magic_HTTP_Thing:
...     @property
...     def attribute(self):
...         result = CallableString(self.do_get())
...         result.function = lambda: self.do_put()
...         return result
...     def do_get(self):
...         # Do a HTTP GET request.
...         return "Get stuff"
...     def do_put(self):
...         # Do a HTTP PUT request.
...         return "Put stuff"
...
>>>
>>> Magic_HTTP_Thing().attribute
'Get stuff'
>>> Magic_HTTP_Thing().attribute()
'Put stuff'

>
> Possible or not, it doesn't seem like a reasonable API to me.

yeah, this is a "corner case" of our REST API, I have some badly
design endpoints that mostly behave like functions, but some of them
also contain state information that you can GET, I was trying to map
this behavior to python objects and this interface is the best that
occurred to me :)


-- 
Marc



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