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

[Lloyd Kvam]

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.

> 
> Thanks!
> Bruce
> 
> Bruce Labitt
> Autoliv Electronics
> 1011B Pawtucket Blvd, PO Box 1858
> Lowell, MA  01853
> 
> Email: bruce.labitt at autoliv.com. 
> Tel:  (978) 674-6526
> Fax: (978) 674-6581 
> 
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-- 
Lloyd Kvam
Venix Corp
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