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Extending the 'function' built-in class

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  Extending the 'function' built-in class "G." <grumsk@grumsk.tz> - 2013-12-01 19:18 +0000
    Re: Extending the 'function' built-in class Roy Smith <roy@panix.com> - 2013-12-01 14:30 -0500
      Re: Extending the 'function' built-in class "G." <grumsk@grumsk.tz> - 2013-12-01 19:37 +0000
    Re: Extending the 'function' built-in class Tim Chase <python.list@tim.thechases.com> - 2013-12-01 13:43 -0600
    Re: Extending the 'function' built-in class Gary Herron <gary.herron@islandtraining.com> - 2013-12-01 11:38 -0800
      Re: Extending the 'function' built-in class "G." <grumsk@grumsk.tz> - 2013-12-01 20:13 +0000
    Re: Extending the 'function' built-in class Robert Kern <robert.kern@gmail.com> - 2013-12-01 20:18 +0000
    Re: Extending the 'function' built-in class Mark Janssen <dreamingforward@gmail.com> - 2013-12-01 17:26 -0800
    Re: Extending the 'function' built-in class alex23 <wuwei23@gmail.com> - 2013-12-02 12:24 +1000
    Re: Extending the 'function' built-in class Steven D'Aprano <steve@pearwood.info> - 2013-12-02 07:01 +0000
      Re: Extending the 'function' built-in class "G." <grumsk@grumsk.tz> - 2013-12-02 09:32 +0000

#60832 — Extending the 'function' built-in class

Hi, I can't figure out how I can extend the 'function' built-in class. I tried:
  class test(function):
    def test(self):
      print("test")
but I get an error. Is it possible ?

Regards, G.

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#60833

In article <529b8ba2$0$2270$426a74cc@news.free.fr>,
 "G." <grumsk@grumsk.tz> wrote:

> Hi, I can't figure out how I can extend the 'function' built-in class. I 
> tried:
>   class test(function):
>     def test(self):
>       print("test")
> but I get an error. Is it possible ?
> 
> Regards, G.

It really helps to give us some basic information when asking questions.  
To start, what version of Python are you using, and what error message 
do you get?

At least in Python 2 (but, I'm guessing, maybe, you're using Python 3, 
since you put parens in your print() statement?), there is no built-in 
class called "function".  There are built-in functions, and they are of 
type builtin_function_or_method.  When I try to subclass that by doing:

class foo(type(open)):
    pass

I get:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    type 'builtin_function_or_method' is not an acceptable base type

So, we're back to asking what version you're using and what error 
message you got.

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#60834

Le 01-12-2013, Roy Smith <roy@panix.com> a écrit :
>
> class foo(type(open)):
>     pass
>
> I get:
>
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: Error when calling the metaclass bases
>     type 'builtin_function_or_method' is not an acceptable base type
>
> So, we're back to asking what version you're using and what error 
> message you got.

Hi, I don't care that much for the version, since I wanted rather to perform
some tests. I tried your code with various versions and got the same message
than yours. Thus I guess the type can't be extended. Regards, G.

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#60835

On 2013-12-01 19:18, G. wrote:
> Hi, I can't figure out how I can extend the 'function' built-in
> class. I tried: class test(function):
>     def test(self):
>       print("test")
> but I get an error. Is it possible ?

While I don't have an answer, I did find this interesting.  First,
"function" doesn't seem to be in the default __buitin__ namespace:

  >>> function
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'function' is not defined

I presume you're doing it with the following:

  >>> from types import FunctionType
  >>> class MyFunc(FunctionType):
  ...     pass
  ... 
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: Error when calling the metaclass bases
      type 'function' is not an acceptable base type

but, as you mention, the inability to subclass it is somewhat
peculiar.  It appears to be metaclass-related.

I'm not quite sure *why* one might want to subclass FunctionType, but
I'm also not sure why you should be *prevented* from subclassing it.

-tkc

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#60836

On 12/01/2013 11:18 AM, G. wrote:
> Hi, I can't figure out how I can extend the 'function' built-in class. I tried:
>    class test(function):
>      def test(self):
>        print("test")
> but I get an error. Is it possible ?
>
> Regards, G.

What error do you get?
What version of Python?
What OS?

And in particular: What 'function' built-in class?  I know of no such 
thing, and the error message I get with your code says exactly that:
   NameError: name 'function' is not defined
Did you not get that same error?

All of which begs the questions: What do you think the function class 
is, and why are you trying to extend it?

Gary Herron

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#60837

Le 01-12-2013, Gary Herron <gary.herron@islandtraining.com> a écrit :
> And in particular: What 'function' built-in class?  I know of no such 
> thing, and the error message I get with your code says exactly that:
>    NameError: name 'function' is not defined
> Did you not get that same error?

Yes, indeed. The 'function' built-in class was the following one:
>>> type(lambda x: 2*x)
<type 'function'>

but I am interested by answers concerning other similar types also.

Regards, G.

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#60839

On 2013-12-01 19:43, Tim Chase wrote:

> I'm not quite sure *why* one might want to subclass FunctionType, but
> I'm also not sure why you should be *prevented* from subclassing it.

Previously:

http://grokbase.com/t/python/python-list/033r5nks47/type-function-does-not-subtype#20030324rcnwbkfedhzbaf3vmiuer3z4xq

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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#60852

> Hi, I can't figure out how I can extend the 'function' built-in class. I tried:
>   class test(function):
>     def test(self):
>       print("test")
> but I get an error. Is it possible ?

It has to do with differing models of computation, and python isn't
designed for this.  Perhaps you're searching for the ultimate lambda?.
-- 
MarkJ
Tacoma, Washington

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#60853

On 2/12/2013 5:18 AM, G. wrote:
> Hi, I can't figure out how I can extend the 'function' built-in class. I tried:
>    class test(function):
>      def test(self):
>        print("test")
> but I get an error. Is it possible ?

Others have pointed out that you cannot subclass the function type. 
Could you explain what you're trying to achieve? It's possible you could 
use a decorator instead:

     def test(fn):
         def _test():
             print('test')
         fn.test = _test
         return fn

     @test
     def foo():
         pass

     >>> foo.test()
     test

(Note that I've only included _test inside the decorator to show that 
you can create a closure to include the wrapped function, as a way of 
replicating 'self' in your class definition.)

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#60856

On Sun, 01 Dec 2013 19:18:58 +0000, G. wrote:

> Hi, I can't figure out how I can extend the 'function' built-in class. I
> tried:
>   class test(function):
>     def test(self):
>       print("test")
> but I get an error. Is it possible ?


You cannot subclass the function type directly, but you can extend 
functions.

Firstly, rather than subclassing, you can use delegation and composition. 
Google for more info on delegation and composition as an alternative to 
subclassing:

https://duckduckgo.com/html/?q=delegation%20as%20alternative%20to%
20subclassing


You can also add attributes to functions:

def spam():
    pass

spam.eggs = 23


Want to add a method to a function? You can do that too:

from types import MethodType
spam.method = MethodType(
    lambda self, n: "%s got %d as arg" % (self.__name__, n),
    spam)


It's even simpler if it doesn't need to be a method:

spam.function = lambda n: "spam got %d as arg" % n)

Want more complex behaviour? Write a callable class:

class MyCallable(object):
    def __call__(self, arg):
        pass

func = MyCallable()



There are plenty of ways to extend functions. Subclassing isn't one of 
them.



-- 
Steven

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#60857

Le 02-12-2013, Steven D'Aprano <steve@pearwood.info> a écrit :
> There are plenty of ways to extend functions. Subclassing isn't one of 
> them.

Thank you very mych for your complete answer; I falled back to your last
proposal by myself in my attempts; I am happyt to learn about the other ways
also.

Regards, G.

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