Whats the pythonic way to handle empty *args when creating a set?

Question

Defining a function,

MyFunction(argument, *args): [do something to argument[arg] for arg in *args]

if *args is empty, the function doesn't do anything, but I want to make the default behavior 'use the entire set if length of *args == 0'

def Export(source, target, *args, sep=','):
    for item in source:
        SubsetOutput(WriteFlatFile(target), args).send(item[0])

I don't want to check the length of args on every iteration, and I can't access the keys of item in source until the iteration begins...

so i could

if len(args) != 0:
   for item in source:

else
   for item in source:

which will probably work but doesn't seem 'pythonic' enough?

is this (is there) a standard way to approach *args or **kwargs and default behavior when either is empty?

More Code:

def __coroutine(func):
    """
    a decorator for coroutines to automatically prime the routine
    code and method from 'curous course on coroutines and concurrency'
    by david beazley www.dabeaz.com
    """

    def __start(*args, **kwargs):
        cr = func(*args, **kwargs)
        next(cr)
        return cr
    return __start


def Export(source, target, *args, sep=','):
    if args:
        for item in source:
            SubsetOutput(WriteFlatFile(target, sep), args).send(item)
    else:
        for item in source:
            WriteFlatFile(target, sep).send(item)

@__coroutine
def SubsetOutput(target, *args):
    """
    take *args from the results and pass to target

    TODO
    ----
    raise exception when arg is not in result[0]
    """
    while True:
        result = (yield)
        print([result.arg for arg in result.dict() if arg in args])
        target.send([result.arg for arg in result.dict if arg in args])


@__coroutine
def WriteFlatFile(target, sep):
    """
    take set of results to a flat file

    TODO
    ----
    """
    filehandler = open(target, 'a')
    while True:
        result = (yield)
        line = (sep.join([str(result[var]) for
                        var in result.keys()])).format(result)+'\n'
        filehandler.write(line)

Answer 1

Is there a way to pass an "entire set" argument to SubsetOutput, so you can bury the conditional inside its call rather than have an explicit if? This could be None or [], for example.

# Pass None to use full subset.
def Export(source, target, *args, sep=','):
    for item in source:
        SubsetOutput(WriteFlatFile(target), args or None).send(item[0])

# Pass an empty list [] to use full subset. Even simpler.
def Export(source, target, *args, sep=','):
    for item in source:
        SubsetOutput(WriteFlatFile(target), args).send(item[0])

If not, I would go with the two loop solution, assuming the loop really is a single line. It reads well and is a reasonable use case for a little bit of code duplication.

def Export(source, target, *args, sep=','):
    if args:
        for item in source:
            SubsetOutput(WriteFlatFile(target), args).send(item[0])
    else:
        for item in source:
            FullOutput(WriteFlatFile(target)).send(item[0])

Answer 2

Just check its not none, you don't have to create a separate argument

def test(*args):
    if not args:
        return #break out
    return True #or whatever you want

Answer 3

How about this:

def MyFunc(argument, *args):
    ( DoSomething for i in (filter(args.__contains__ ,argument) if args else argument) )