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Is there a function that applies list of functions to a value?

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  Is there a function that applies list of functions to a value? AdamKal <adamkalinski@gmail.com> - 2013-08-28 05:52 -0700
    Re: Is there a function that applies list of functions to a value? Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-08-28 16:10 +0300
      Re: Is there a function that applies list of functions to a value? fp2161@gmail.com - 2013-08-29 15:48 -0700
    Re: Is there a function that applies list of functions to a value? Tim Chase <python.list@tim.thechases.com> - 2013-08-28 08:11 -0500
      Re: Is there a function that applies list of functions to a value? Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-08-28 16:19 +0300
      Re: Is there a function that applies list of functions to a value? AdamKal <adamkalinski@gmail.com> - 2013-08-28 06:23 -0700
        Re: Is there a function that applies list of functions to a value? Chris Angelico <rosuav@gmail.com> - 2013-08-28 23:26 +1000
        Re: Is there a function that applies list of functions to a value? Tim Chase <python.list@tim.thechases.com> - 2013-08-28 08:43 -0500
          Re: Is there a function that applies list of functions to a value? AdamKal <adamkalinski@gmail.com> - 2013-08-28 06:50 -0700
    Re: Is there a function that applies list of functions to a value? ishish <ishish@domhain.de> - 2013-08-28 14:17 +0100
    Re: Is there a function that applies list of functions to a value? Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2013-08-28 15:09 +0200
    Re: Is there a function that applies list of functions to a value? Josh English <Joshua.R.English@gmail.com> - 2013-08-28 11:50 -0700
      Re: Is there a function that applies list of functions to a value? fp2161@gmail.com - 2013-08-29 23:17 -0700
        Re: Is there a function that applies list of functions to a value? alex23 <wuwei23@gmail.com> - 2013-08-30 16:36 +1000
          Re: Is there a function that applies list of functions to a value? Fabrice Pombet <fp2161@gmail.com> - 2013-08-30 01:11 -0700
    Re: Is there a function that applies list of functions to a value? fp2161@gmail.com - 2013-08-29 13:50 -0700
      Re: Is there a function that applies list of functions to a value? Chris Angelico <rosuav@gmail.com> - 2013-08-30 07:05 +1000
        Re: Is there a function that applies list of functions to a value? fp2161@gmail.com - 2013-08-29 14:27 -0700
          Re: Is there a function that applies list of functions to a value? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-30 02:18 +0000
          Re: Is there a function that applies list of functions to a value? Chris Angelico <rosuav@gmail.com> - 2013-08-30 07:35 +1000
            Re: Is there a function that applies list of functions to a value? fp2161@gmail.com - 2013-08-29 23:14 -0700
              Re: Is there a function that applies list of functions to a value? alex23 <wuwei23@gmail.com> - 2013-08-30 16:23 +1000
                Re: Is there a function that applies list of functions to a value? Fabrice Pombet <fp2161@gmail.com> - 2013-08-30 01:06 -0700
        Re: Is there a function that applies list of functions to a value? fp2161@gmail.com - 2013-08-29 14:48 -0700
          Re: Is there a function that applies list of functions to a value? Terry Reedy <tjreedy@udel.edu> - 2013-08-30 00:38 -0400
      Re: Is there a function that applies list of functions to a value? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-30 02:09 +0000
        Re: Is there a function that applies list of functions to a value? fp2161@gmail.com - 2013-08-29 23:06 -0700

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#53278

On Thursday, August 29, 2013 11:35:39 PM UTC+2, Chris Angelico wrote:
> On Fri, Aug 30, 2013 at 7:27 AM,  <fp2161@gmail.com> wrote:
> 
> > Chris, call me a snob, but I resent using lambdas (aren't they usually considered odd/bad practice in python?)
> 
> 
> 
> They're not bad practice; all they are is a function without a name,
> 
> that's restricted to returning a single expression. So they're
> 
> perfectly suited to this task, and ill-suited to some others.
> 
> 
> 
> Like everything, lambda's a tool that can be used or misused.
> 
> 
> 
> ChrisA

For this purpose however, I suspect that a named function with a proper docstring that can be imported and reused over and over again is probably more appropriate than a lambda (the first post is telling us about something happening from time to time...)

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#53280

On 30/08/2013 4:14 PM, fp2161@gmail.com wrote:
> For this purpose however, I suspect that a named function with a proper docstring that can be imported and reused over and over again is probably more appropriate than a lambda

Given that in Chris' example the lambda was returned from a factory, 
importing the inner function by name is never going to be a concern.

It's also possible to assign to both __name__ and __doc__ for a lambda, 
which can be useful at times.

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#53285

On Friday, August 30, 2013 8:23:44 AM UTC+2, alex23 wrote:
> On 30/08/2013 4:14 PM, fp2161@gmail.com wrote:
> 
> > For this purpose however, I suspect that a named function with a proper docstring that can be imported and reused over and over again is probably more appropriate than a lambda
> 
> 
> 
> Given that in Chris' example the lambda was returned from a factory, 
> 
> importing the inner function by name is never going to be a concern.
> 
> 
> 
> It's also possible to assign to both __name__ and __doc__ for a lambda, 
> 
> which can be useful at times.
I am sorry, I can't see any of Chris' code in this thread, where is it???

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#53260

On Thursday, August 29, 2013 11:05:38 PM UTC+2, Chris Angelico wrote:
> On Fri, Aug 30, 2013 at 6:50 AM,  <fp2161@gmail.com> wrote:
> 
> > My way is so obvious that it may not be that interesting...
> 
> >
> 
> > def func4(f1,f2,f3,f4):
> 
> >     def anon(x):
> 
> >         f1(f2(f3(f4(x))))
> 
> >     return anon
> 
> 
> 
> Or have it return the result of f1. And then, since it's an anonymous
> 
> function that simply returns an expression, I'd write it as:
> 
> 
> 
> def func4(f1,f2,f3,f4):
> 
>     return lambda x: f1(f2(f3(f4(x))))
> 
> 
> 
> Of course, that's still restricted to precisely four args. Extending
> 
> this concept to a variable number of arguments is, uhh, left as an
> 
> exercise to the reader. Which will probably end up going back to
> 
> reduce(). :)
> 
> 
> 
> ChrisA

Here is the generalisable version:


def comp(*func):
    def anon(x):
        res=x
        for f in func:
            res=f(res)
        return res
    return anon

you can then compose your function in __main__:
f4=comp(f5,f6,f7)
and call it:
print(x) ...

(I hope that Chris is happy now...)

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#53273

On 8/29/2013 5:48 PM, fp2161@gmail.com wrote:

> Here is the generalisable version:
>
> def comp(*func):
>      def anon(x):
>          res=x
>          for f in func:
>              res=f(res)
>          return res
>      return anon

With a bit more work, one can set the __name__ and __qualname__ attributes.

import math as m

def comp(*func, name=''):
     def anon(x):
         res=x
         for f in func:
             res=f(res)
         return res
     if name:
         anon.__name__ = name
         q = anon.__qualname__.rsplit('.', maxsplit=1)
         q[1] = name
         anon.__qualname__ = '.'.join(q)
     return anon

esincos = comp(m.exp, m.sin, m.cos, name='esincos')
print(esincos)
#
<function comp.<locals>.esincos at 0x00000000033107B8>

-- 
Terry Jan Reedy

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#53268

On Thu, 29 Aug 2013 13:50:39 -0700, fp2161 wrote:

> My way is so obvious that it may not be that interesting...
> 
> def func4(f1,f2,f3,f4):
>     def anon(x):
>         f1(f2(f3(f4(x))))
>     return anon

I don't think "obvious" is quite the right description. Well, perhaps 
"obviously wrong" :-)

You also need to define func1 (trivial), func2, func3, func5, func6, 
func7, func8, ..., func2147483647, plus another master function to choose 
between them, depending on the number of functions provided as argument.

I assume that the maximum number of arguments given is 2**31-1. Python 
may not actually have that limitation, in which case you would need to 
define additional functions.

Or... you would have to come up with an implementation which doesn't hard-
code the number of functions used.


-- 
Steven

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#53277

On Friday, August 30, 2013 4:09:45 AM UTC+2, Steven D'Aprano wrote:
> On Thu, 29 Aug 2013 13:50:39 -0700, fp2161 wrote:
> 
> 
> 
> > My way is so obvious that it may not be that interesting...
> 
> > 
> 
> > def func4(f1,f2,f3,f4):
> 
> >     def anon(x):
> 
> >         f1(f2(f3(f4(x))))
> 
> >     return anon

> I don't think "obvious" is quite the right description. Well, perhaps 
> 
> "obviously wrong" :-)
 
> You also need to define func1 (trivial), func2, func3, func5, func6, 
> 
> func7, func8, ..., func2147483647, plus another master function to choose 
> 
> between them, depending on the number of functions provided as argument.
> I assume that the maximum number of arguments given is 2**31-1. Python 
> 
> may not actually have that limitation, in which case you would need to 
> 
> define additional functions.
> Or... you would have to come up with an implementation which doesn't hard-
> 
> code the number of functions used.
> 
> Steven

I got the generalisation criticism before yours, and generalised it accordingly. Unfortunately it was wrong essentially because it was so obvious that Josh Englsih posted essentially the same one before me...

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