如何在python中使用可选参数正确调用函数

Question

I'm a beginner with python and I'm facing a problem with a function that requires optional parameters. This function gets as parameters a variable number of file paths, that can be from 2 to n parameters. After that, a certain number of optional parameters can be passed to this function. I tried to do something like that:

def compareNfilesParameters(*args):
    start_time = time.time()

    listFiles = []
    listParameters = []

    for argument in args:
        if str(argument).endswith(".vcf"):
            listFiles.append(str(argument))
        else:
            listParameters.append(argument)

So if the parameters has the file extension it is considered as one of the file path parameters, the others are seen as the optional parameters.

What I want to do is letting the user call the function like:

function('a.vcf', 'b.vcf', 'c.vcf') 

or

function('a.vcf', 'b.vcf', 'c.vcf', 0, 1)

or

function('a.vcf', 'b.vcf', 'c.vcf', 0, 1, 4,...,3)

I tried different approaches but none of them satisfies me.

The first approach is declaring the function as:

def compareNfilesParameters(*args)  

but this way, if I get for example 3 parameters, 2 will certainly be the files path, and the last one I don't know on which variable it refers. So I need to specify every value and pass '-1' for the parameters that I want to use default value.

The 2nd approach is the following:

def compareNfilesParameters(*args, par1 = 10, par2 = 15 ..)

But this way I need to call the function like:

compareNfilesParameters(path1, path2, path3, par1 = 10)

and not like

compareNfilesParameters(path1, path2, path3, 10)

or the 10 will be considered in the args input, right? I wouldn't like to use this approach because it becomes very verbose to call the function.

How would you do this?

Answer 1

Make the user pass in the filenames as a sequence; don't try to cram everything into separate arguments:

def compareNfilesParameters(files, *params):

and call this as:

compareNfilesParameters(('a.vcf', 'b.vcf', 'c.vcf'), 0, 1, 4)

This makes the files explicit and removes the need to separate files from other parameters.

If your remaining parameters are distinct options (and not a homogenous series of integers), I'd use keyword arguments:

def compareNfilesParameters(files, op1=default_value, op2=default_value, op3=default_value):

You don't have to use keyword arguments with keywords when calling; you can still treat them as positional:

compareNfilesParameters(('a.vcf', 'b.vcf', 'c.vcf'), 0, 1, 4)

would give op1 the value 0 , op2 the value 1 , and op3 the value 4 . Only if you want to specify values out of order or for a specific option do you have to use keyword arguments in the call:

compareNfilesParameters(('a.vcf', 'b.vcf', 'c.vcf'), op3=4)

Answer 2

Ok, I solved like using the keyword parameters as suggested.

def compareNfilesParameters(listFiles, **kwargs):
    start_time = time.time()

    if len(listFiles) < MINUMUM_FILES_NUMBER :
        print "You need to specify at least "+ str(MINUMUM_FILES_NUMBER) +" files."
        return

    try:
        operationType = int(kwargs.get("op", DEFAULT_OPERATION_TYPE))
    except ValueError:
        print "Operation type filter has to be an integer."
        return    
    if operationType not in [0,1]:
        print "Operation type must be 0 (intersection), 1 (union)"
        return

and so on for all the parameters. Like this I need to put all the files paths in a list and pass it as a single required parameter, and searching kwargs dictionary for optionals parameters setting the default values if not expressed.