How best to pass arbitrary parameters from one function to another

[John O'Hagan]

On Tue Sep 30 11:32:41 CEST 2008, Steven D'Aprano

>On Tue, 30 Sep 2008 08:58:15 +0000, John O'Hagan wrote:
>
>> Hi Pythonistas,
>> 
>> I'm looking for the best way to pass an arbitrary number and type of
>> variables created by one function to another. They can't be global
>> because they may have different values each time they are used in the
>> second function.
>> 
>> So far I'm trying to do something like this:
>> 
>> 
>> def process_args( [list, of, command-line, arguments] ):
>
>
>If you are trying to process commandline args, I suggest you don't re-
>invent the wheel. Have a look at the two standard modules, optparse and 
>getopt. Of the two, getopt is probably simpler to get started with, but 
>optparse is more powerful.
>

Thanks, both to you and Bruno for pointing this out, I'll certainly be using 
it in future.

>To answer your more general question, see below.
>
>
>>       do stuff
>>       return {dictionary : of, local : variables }
>> 
>> def main_function( **kwargs ):
>> 
>>       do stuff
>>       return result
>> 
>> kw1 = process_args( [some, list] )
>> kw2 = process_args( [a, different, list] )
>> 
>> for i in main_function( **kw1 ):
>> 
>>       kw2[ var1 ] = i
>>       kw2[ var2 ] = len( i )
>> 
>>       for j in main_function(**kw2):
>> 
>>               print j
>> 
>> This only seems to work if I specify a default value for every possible
>> parameter of main_function and also for any others which may be passed
>> to it, which is a bit tedious because there are very many of them but
>> only a few are used in any given execution of the program.
>>
>Er, yes. Presumably main_function actually does something. So it expects 
>some arguments, and if the arguments aren't given, then it will fail. To 
>prevent it failing when arguments aren't given, they must have default 
>values. So define them, once, and be done with it:

[...snip code example...] 

>Default values are a feature, not a problem to be solved.

I take your point, but in this case all necessary parameters are present in 
the keyword dictionary, so I guess I was surprised that default values still 
need to specified. 

Also, the function body tests for the presence of each parameter and only acts 
on those that are there, so it does not fail because of a missing argument. 
However, I'm sure there's a good reason that it works this way.

>As for your nested for-loops (see above), I'm guessing that you're trying 
>to copy items from one dictionary kw1 to another kw2, before processing 
>kw2. You should check out the update method on dictionaries. 
[...]

That's also of great use, thank you.

What I'm actually trying to do, though, is add some of the output of one call 
of main_function to the arguments of another call of the same function (with 
different arguments). It's a long story, but I have a number-crunching 
function which produces lists of numbers representing musical notes, either 
rhythmic or melodic, to be printed out as a score using Lilypond and/or 
played by samples or synthesizers using sox.

The melodic and rhythmic parts work by themselves, but I'm trying to get the 
function to feed itself rhythms for its melodies, and vice-versa.

I guess that's already too much information....

Thanks for your help.

John