How to use a class property to store function variables?

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From: alex23 on 27 Apr 2010 22:20

---

GZ <zyzhu2...(a)gmail.com> wrote:
> I do not think it will help me. I am not trying to define a function
> fn() in the class, but rather I want to make it a "function reference"
> so that I can initialize it any way I like later.

It always helps to try an idea out before dismissing it out of hand.
Experimentation in the interpreter is cheap and easy.

>>> class A(object):
.... fn = staticmethod(lambda x: x*x)
....
>>> A.fn(10)
100
>>> A.fn = staticmethod(lambda x: x**x)
>>> A.fn(3)
27
>>> def third(x): return x/3
....
>>> A.fn = staticmethod(third)
>>> A.fn(9)
3

However, I'm assuming you're wanting to do something like this:

>>> class B(object):
.... def act(self):
.... print self.fn()

That is, providing a hook in .act() that you can redefine on demand.
If so, note that you only need to decorate functions as staticmethods
if you're assigning them to the class. If you intend on overriding on
_instances_, you don't:

>>> B.fn = staticmethod(lambda: 'one') # assign on class
>>> b = B() # instantiate
>>> b.act() # act on instance
one
>>> B.fn = staticmethod(lambda: 'two') # assign on class
>>> b.act() # existing instance calls new version on class
two
>>> b.fn = staticmethod(lambda: 'three') # assign on instance
>>> b.act()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in act
TypeError: 'staticmethod' object is not callable
>>> b.fn = lambda: 'three' # look Ma, no staticmethod!
>>> b.act()
three

Incidentally, this is known as the Strategy pattern, and you can see a
simple example of it in Python here: http://en.wikipedia.org/wiki/Strategy_pattern#Python

Hope this helps.

From: GZ on 28 Apr 2010 01:55

---

On Apr 27, 9:20 pm, alex23 <wuwe...(a)gmail.com> wrote:
> GZ <zyzhu2...(a)gmail.com> wrote:
> > I do not think it will help me. I am not trying to define a function
> > fn() in the class, but rather I want to make it a "function reference"
> > so that I can initialize it any way I like later.
>
> It always helps to try an idea out before dismissing it out of hand.
> Experimentation in the interpreter is cheap and easy.
>
> >>> class A(object):
>
> ...   fn = staticmethod(lambda x: x*x)
> ...>>> A.fn(10)
> 100
> >>> A.fn = staticmethod(lambda x: x**x)
> >>> A.fn(3)
> 27
> >>> def third(x): return x/3
> ...
> >>> A.fn = staticmethod(third)
> >>> A.fn(9)
>
> 3
>
> However, I'm assuming you're wanting to do something like this:
>
> >>> class B(object):
>
> ...   def act(self):
> ...     print self.fn()
>
> That is, providing a hook in .act() that you can redefine on demand.
> If so, note that you only need to decorate functions as staticmethods
> if you're assigning them to the class. If you intend on overriding on
> _instances_, you don't:
>
> >>> B.fn = staticmethod(lambda: 'one') # assign on class
> >>> b = B() # instantiate
> >>> b.act() # act on instance
> one
> >>> B.fn = staticmethod(lambda: 'two') # assign on class
> >>> b.act() # existing instance calls new version on class
> two
> >>> b.fn = staticmethod(lambda: 'three') # assign on instance
> >>> b.act()
>
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 3, in act
> TypeError: 'staticmethod' object is not callable>>> b.fn = lambda: 'three' # look Ma, no staticmethod!
> >>> b.act()
>
> three
>
> Incidentally, this is known as the Strategy pattern, and you can see a
> simple example of it in Python here:http://en.wikipedia.org/wiki/Strategy_pattern#Python
>
> Hope this helps.

Ah, this totally works. The key is to use the staticmethod function.
Thanks a lot.

From: GZ on 28 Apr 2010 02:02

---

On Apr 27, 9:20 pm, alex23 <wuwe...(a)gmail.com> wrote:
> GZ <zyzhu2...(a)gmail.com> wrote:
> > I do not think it will help me. I am not trying to define a function
> > fn() in the class, but rather I want to make it a "function reference"
> > so that I can initialize it any way I like later.
>
> It always helps to try an idea out before dismissing it out of hand.
> Experimentation in the interpreter is cheap and easy.
>
> >>> class A(object):
>
> ...   fn = staticmethod(lambda x: x*x)
> ...>>> A.fn(10)
> 100
> >>> A.fn = staticmethod(lambda x: x**x)
> >>> A.fn(3)
> 27
> >>> def third(x): return x/3
> ...
> >>> A.fn = staticmethod(third)
> >>> A.fn(9)
>
> 3
>
> However, I'm assuming you're wanting to do something like this:
>
> >>> class B(object):
>
> ...   def act(self):
> ...     print self.fn()
>
> That is, providing a hook in .act() that you can redefine on demand.
> If so, note that you only need to decorate functions as staticmethods
> if you're assigning them to the class. If you intend on overriding on
> _instances_, you don't:
>
> >>> B.fn = staticmethod(lambda: 'one') # assign on class
> >>> b = B() # instantiate
> >>> b.act() # act on instance
> one
> >>> B.fn = staticmethod(lambda: 'two') # assign on class
> >>> b.act() # existing instance calls new version on class
> two
> >>> b.fn = staticmethod(lambda: 'three') # assign on instance
> >>> b.act()
>
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 3, in act
> TypeError: 'staticmethod' object is not callable>>> b.fn = lambda: 'three' # look Ma, no staticmethod!
> >>> b.act()
>
> three
>
> Incidentally, this is known as the Strategy pattern, and you can see a
> simple example of it in Python here:http://en.wikipedia.org/wiki/Strategy_pattern#Python
>
> Hope this helps.

Another question: I am not sure how staticmethod works internally. And
the python doc does not seem to say. What does it do?

From: Chris Rebert on 28 Apr 2010 02:20

---

On Tue, Apr 27, 2010 at 11:02 PM, GZ <zyzhu2000(a)gmail.com> wrote:
> On Apr 27, 9:20 pm, alex23 <wuwe...(a)gmail.com> wrote:
>> GZ <zyzhu2...(a)gmail.com> wrote:
>> > I do not think it will help me. I am not trying to define a function
>> > fn() in the class, but rather I want to make it a "function reference"
>> > so that I can initialize it any way I like later.
>>
>> It always helps to try an idea out before dismissing it out of hand.
>> Experimentation in the interpreter is cheap and easy.
>>
>> >>> class A(object):
>>
>> ...   fn = staticmethod(lambda x: x*x)
>> ...>>> A.fn(10)
>> 100
>> >>> A.fn = staticmethod(lambda x: x**x)
>> >>> A.fn(3)
>> 27
>> >>> def third(x): return x/3
>> ...
>> >>> A.fn = staticmethod(third)
>> >>> A.fn(9)
>>
>> 3
>>
>> However, I'm assuming you're wanting to do something like this:
>>
>> >>> class B(object):
>>
>> ...   def act(self):
>> ...     print self.fn()
>>
>> That is, providing a hook in .act() that you can redefine on demand.
>> If so, note that you only need to decorate functions as staticmethods
>> if you're assigning them to the class. If you intend on overriding on
>> _instances_, you don't:
>>
>> >>> B.fn = staticmethod(lambda: 'one') # assign on class
>> >>> b = B() # instantiate
>> >>> b.act() # act on instance
>> one
>> >>> B.fn = staticmethod(lambda: 'two') # assign on class
>> >>> b.act() # existing instance calls new version on class
>> two
>> >>> b.fn = staticmethod(lambda: 'three') # assign on instance
>> >>> b.act()
>>
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>>   File "<stdin>", line 3, in act
>> TypeError: 'staticmethod' object is not callable>>> b.fn = lambda: 'three' # look Ma, no staticmethod!
>> >>> b.act()
>>
>> three
>>
>> Incidentally, this is known as the Strategy pattern, and you can see a
>> simple example of it in Python here:http://en.wikipedia.org/wiki/Strategy_pattern#Python
>>
>> Hope this helps.
>
> Another question: I am not sure how staticmethod works internally. And
> the python doc does not seem to say. What does it do?

It involves the relatively arcane magic of "descriptors".
See http://docs.python.org/reference/datamodel.html#implementing-descriptors
or for a more complete but advanced explanation, the "Static methods
and class methods" section of
http://www.python.org/download/releases/2.2.3/descrintro/

Understanding exactly how staticmethod() and friends work is not too
essential in practice though.

Cheers,
Chris
--
http://blog.rebertia.com

From: GZ on 28 Apr 2010 04:04

---

On Apr 28, 1:20 am, Chris Rebert <c...(a)rebertia.com> wrote:
> On Tue, Apr 27, 2010 at 11:02 PM, GZ <zyzhu2...(a)gmail.com> wrote:
> > On Apr 27, 9:20 pm, alex23 <wuwe...(a)gmail.com> wrote:
> >> GZ <zyzhu2...(a)gmail.com> wrote:
> >> > I do not think it will help me. I am not trying to define a function
> >> > fn() in the class, but rather I want to make it a "function reference"
> >> > so that I can initialize it any way I like later.
>
> >> It always helps to try an idea out before dismissing it out of hand.
> >> Experimentation in the interpreter is cheap and easy.
>
> >> >>> class A(object):
>
> >> ...   fn = staticmethod(lambda x: x*x)
> >> ...>>> A.fn(10)
> >> 100
> >> >>> A.fn = staticmethod(lambda x: x**x)
> >> >>> A.fn(3)
> >> 27
> >> >>> def third(x): return x/3
> >> ...
> >> >>> A.fn = staticmethod(third)
> >> >>> A.fn(9)
>
> >> 3
>
> >> However, I'm assuming you're wanting to do something like this:
>
> >> >>> class B(object):
>
> >> ...   def act(self):
> >> ...     print self.fn()
>
> >> That is, providing a hook in .act() that you can redefine on demand.
> >> If so, note that you only need to decorate functions as staticmethods
> >> if you're assigning them to the class. If you intend on overriding on
> >> _instances_, you don't:
>
> >> >>> B.fn = staticmethod(lambda: 'one') # assign on class
> >> >>> b = B() # instantiate
> >> >>> b.act() # act on instance
> >> one
> >> >>> B.fn = staticmethod(lambda: 'two') # assign on class
> >> >>> b.act() # existing instance calls new version on class
> >> two
> >> >>> b.fn = staticmethod(lambda: 'three') # assign on instance
> >> >>> b.act()
>
> >> Traceback (most recent call last):
> >>   File "<stdin>", line 1, in <module>
> >>   File "<stdin>", line 3, in act
> >> TypeError: 'staticmethod' object is not callable>>> b.fn = lambda: 'three' # look Ma, no staticmethod!
> >> >>> b.act()
>
> >> three
>
> >> Incidentally, this is known as the Strategy pattern, and you can see a
> >> simple example of it in Python here:http://en.wikipedia.org/wiki/Strategy_pattern#Python
>
> >> Hope this helps.
>
> > Another question: I am not sure how staticmethod works internally. And
> > the python doc does not seem to say. What does it do?
>
> It involves the relatively arcane magic of "descriptors".
> Seehttp://docs.python.org/reference/datamodel.html#implementing-descriptors
> or for a more complete but advanced explanation, the "Static methods
> and class methods" section ofhttp://www.python.org/download/releases/2.2.3/descrintro/
>
> Understanding exactly how staticmethod() and friends work is not too
> essential in practice though.
>
> Cheers,
> Chris
> --http://blog.rebertia.com- Hide quoted text -
>
> - Show quoted text -

Got it. I appreciate your help.

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