Better way to overload constructor in python?

Question

So I am aware that class methods can be used to "overload" the init method in python, but I'm running into a bit of a problem. Say, for example, I have a constructor like this:

def __init__(self, serviceName, endpoint, exportedInterface, exportedObject, remoteObjectInterface, remoteObjectProxy,
                interruptionHandler, invalidationHandler):

    self._serviceName = serviceName;
    #etc.

@classmethod
def createConnection(cls, serviceName):
    return cls(serviceName, None, None, None, None, None, None, None)

Do I really have to list out a value for every single data member in the class method, just to create an instance of the class with a class method? Also, I plan to add more data members to the class later on, but I'm not to happy about having this unwieldy init function with like 15 arguments. Is there a better way to go about this?

Marcy · Accepted Answer · 2016-06-13 19:08:44Z

6

You can use default values for the parameters. Then you can specify which one you want to replace.

def __init__(self, serviceName, endpoint = None, exportedInterface = None, 
             exportedObject = None, remoteObjectInterface = None, remoteObjectProxy = None,
             interruptionHandler = None, invalidationHandler = None):

    self._serviceName = serviceName;
    #etc.

@classmethod
def createConnection(cls, serviceName):
    return cls(serviceName)

Now you can set only some arguments like this (the other will use their default values):

@classmethod
def createConnectionWithInterruptionHandler(cls, serviceName, interruptionHandler):
    return cls(serviceName, interruptionHandler=interruptionHandler)

answered Jun 13, 2016 at 19:08

Marcy

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1 Comment

Rohan Shankar Over a year ago

Thanks! This helps :)

score 0 · Accepted Answer · 2016-06-13 19:38:18Z

0

Two options are:

Use keyword arguments. e.g.

def __init__(self, **kwargs):
    for names in ("serviceName", ...):
        setattr(self, "_" + name, kwargs.get(names, None))

Make __init__ do the bare minimum and use factory functions to do the appropriate initialization

def __init__(self): pass

@classmethod
def createConnection(cls, serviceName):
    foo = cls()
    foo._serviceName = serviceName
    return foo

edited Jun 13, 2016 at 19:38

answered Jun 13, 2016 at 19:10

user1084944

2 Comments

Tadhg McDonald-Jensen Over a year ago

why are you using __setitem__ directly instead of self["_"+name] = kwargs.get(names)? Or do you maybe mean to use setattr instead?

user1084944 Over a year ago

@Tadhg: Yes, I meant setattr

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