fun with lambdas

[Juan Pablo Romero]

Thanks to all

I settled with this:

def partial1(f,b):

    return lambda a:f(a,b)


def partial2(f,a):

    return lambda b:f(a,b)



   Juan Pablo

2005/10/20, Mike Meyer <mwm at mired.org>:
> Robert Kern <robert.kern at gmail.com> writes:
> > Juan Pablo Romero wrote:
> >> Hello!
> >>
> >> given the definition
> >>
> >> def f(a,b): return a+b
> >>
> >> With this code:
> >>
> >> fs = [ lambda x: f(x,o) for o in [0,1,2]]
> >>
> >> or this
> >>
> >> fs = []
> >> for o in [0,1,2]:
> >>     fs.append( lambda x: f(x,o) )
> >>
> >> I'd expect that fs contains partial evaluated functions, i.e.
> >>
> >> fs[0](0) == 0
> >> fs[1](0) == 1
> >> fs[2](0) == 2
> >>
> >> But this is not the case :(
> >>
> >> What is happening here?
> >
> > Namespaces. The functions are looking up o at runtime. In both cases, o
> > is bound to the last object in [0,1,2] once the iteration is finished.
> >
> > Try this (although I'm sure there are better ways):
> >
> > In [4]: fs = [lambda x, o=o: f(x, o) for o in [0,1,2]]
> >
> > In [5]: fs[0](0)
> > Out[5]: 0
> >
> > In [6]: fs[0](1)
> > Out[6]: 1
> >
> > In [7]: fs[0](2)
> > Out[7]: 2
>
> Right explanation. Right solution. Wrong examples. He wanted to see:
>
> >>> fs[0](0)
> 0
> >>> fs[1](0)
> 1
> >>> fs[2](0)
> 2
> >>>
>
> List comprehensions were purposely designed to mimic for loops, which
> (unlike other languages) don't create a new variable for each pass
> through the loop. ((They also "leak" the variable to the surrounding
> namespace.) So you need to capture the current value of the loop index
> somewhere. An alternative (and maybe slightly cleaner) method is:
>
> >>> def makef(a):
> ...  return lambda b: a + b
> ...
> >>> fs = [makef(o) for o in [0, 1, 2]]
> >>> fs[0](0)
> 0
> >>> fs[1](0)
> 1
> >>> fs[2](0)
> 2
> >>>
>
> Generator comprehensions have the same issue:
>
> >>> fs = list(lambda x: f(o, x) for o in [0, 1, 2])
> >>> fs[0](0)
> 2
> >>> fs[1](0)
> 2
> >>> fs[2](0)
> 2
> >>>
>
>           <mike
> --
> Mike Meyer <mwm at mired.org>                      http://www.mired.org/home/mwm/
> Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
> --
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>