[Python-talk] Calling functions with variable number of arguments

[bruce.labitt at autoliv.com]

Lloyd Kvam <python at venix.com> wrote on 09/01/2009 05:23:41 PM:

> On Tue, 2009-09-01 at 13:05 -0400, bruce.labitt at autoliv.com wrote:
> > Say one has three functions, function 1 has one input argument, 
function 2 
> > has 2 input arguments and function 3 has no input arguments.
> > 
> > I have made a function list, and an argument list
> > 
> > def func1(n):
> >         print 'Function '+str(n)+' ran'
> > 
> > def func2( n, a ):
> >         print 'Function '+str(n)+' ran '+str(a)+'times'
> > 
> > def func3:
> >         print 'Function 3 ran'
> > 
> > funclist = [func1, func2, func3]
> > arglist  = [ a, (b,c), None ]
> > 
> > I'd like to execute something like this (which does not work)
> > 
> > for item, arg in zip( funclist, arglist ):
> >         item( arg )
> > 
> > If I run this, it dies on the call to func2, giving a TypeError 
Exception. 
> >  So I use *arg.  This only works for func2, but fails for func1 with 
> > TypeError because "argument after * must be a sequence"
> 
> To get past this hurdle, define arglist as:
> arglist = [ [a], (b,c), [] ]
> 
> Now the *arg notation will work for each call.
> > 
> > I can get func1 & func3 to work if I modify the function func3 from 
above 
> > to
> > 
> > def func3(a): 
> >         print 'Function 3 ran'
> > 
> > 
> > So is there a way to get all three functions to work using a similar 
(or 
> > any) construction?
> 
> There is still likely to be a hurdle.  You need to create a new arglist
> every time a, b, or c changes.  In other words, the a in arglist (unless
> a is mutable) is determined when arglist is created.  Changing a
> variable named a elsewhere in your program has no impact on arglist[0].
> These are not memory references as in C.
> 
> Possibly you have a case like:
> 
> def do_stuff( a, b, c, d, e):
>         # now you could create arglist here
>         arglist = [ [a], (b,c), [] ]
>         # but you are probably better off doing this
>         def use_func1():
>            return func1( a)
>         def use_func2():
>            return func2( b, c)
>         def use_func3():
>            return func3()
>         funclist = [use_func1, use_func2, use_func3]
>         # but this is all pretty wordy and you probably do not really
>         care
>         # about these function names so
>         funclist = [
>                 lambda : func1(a),
>                 lambda : func2( b,c),
>                 lambda : func3(),
>                 ]
>         # and you no longer have explicit args for the funclist
>         functions
>         # instead the args are picked up from the local scope.
>         for f in funclist:
>             f()
> 
> One last comment.  If you are pushed into an older Python version or if
> the lambdas are being created within a loop where a, b, and c are all
> varying, and then executed outside the loop, you might need to code the
> funclist functions like this:
>         lambda a=a: func1(a)   # parameter a defaults to the local scope
>         variable a
>         # the duplication of names is odd looking and not required
>         lambda b=b,c=c: func2( b, c)
> 
> You still do not provide arguments in your func calls because the
> defaults are built into the definitions.
> 
> > 


Lloyd, thank you so much.  I have learned an awful lot from the list.

-Bruce

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