Elegant pattern for mutually exclusive keyword args?


Question

Sometimes in my code I have a function which can take an argument in one of two ways. Something like:

def func(objname=None, objtype=None):
    if objname is not None and objtype is not None:
        raise ValueError("only 1 of the ways at a time")
    if objname is not None:
        obj = getObjByName(objname)
    elif objtype is not None:
        obj = getObjByType(objtype)
    else:
        raise ValueError("not given any of the ways")

    doStuffWithObj(obj)

Is there any more elegant way to do this? What if the arg could come in one of three ways? If the types are distinct I could do:

def func(objnameOrType):
    if type(objnameOrType) is str:
        getObjByName(objnameOrType)
    elif type(objnameOrType) is type:
        getObjByType(objnameOrType)
    else:
        raise ValueError("unk arg type: %s" % type(objnameOrType))

But what if they are not? This alternative seems silly:

def func(objnameOrType, isName=True):
    if isName:
        getObjByName(objnameOrType)
    else:
        getObjByType(objnameOrType)

cause then you have to call it like func(mytype, isName=False) which is weird.

1
9
4/29/2012 8:09:47 PM

Accepted Answer

How about using something like a command dispatch pattern:

def funct(objnameOrType):
   dispatcher = {str: getObjByName,
                 type1: getObjByType1,
                 type2: getObjByType2}
   t = type(objnameOrType)
   obj = dispatcher[t](objnameOrType)
   doStuffWithObj(obj)

where type1,type2, etc are actual python types (e.g. int, float, etc).

6
3/3/2011 6:07:18 PM

For whatever it's worth, similar kinds of things happen in the Standard Libraries; see, for example, the beginning of GzipFile in gzip.py (shown here with docstrings removed):

class GzipFile:
    myfileobj = None
    max_read_chunk = 10 * 1024 * 1024   # 10Mb
    def __init__(self, filename=None, mode=None,
                 compresslevel=9, fileobj=None):
        if mode and 'b' not in mode:
            mode += 'b'
        if fileobj is None:
            fileobj = self.myfileobj = __builtin__.open(filename, mode or 'rb')
        if filename is None:
            if hasattr(fileobj, 'name'): filename = fileobj.name
            else: filename = ''
        if mode is None:
            if hasattr(fileobj, 'mode'): mode = fileobj.mode
            else: mode = 'rb'

Of course this accepts both filename and fileobj keywords and defines a particular behavior in the case that it receives both; but the general approach seems pretty much identical.


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