Multiple constructor with Python

Question

I have a class A that can be generated from two different ways.

a = A(path_to_xml_file)
a = A(listA, listB)

The first method has file path as an input to parse from XML file to get listA, and listB. The second method is given two lists.

I can think of two ways to implement multiple constructor. What do you think? What method normally Python guys use for this case?

Check the type

class A():
    def __init__(self, arg1, arg2 = None):
        if isinstance(arg1, str): 
            ...
        elif isinstance(arg1, list):
            ...

a = A("abc")
b = A([1,2,3],[4,5,6])

Make different builders

class A2():
    def __init__(self):
        pass
    def genFromPath(self, path):
        ...
    def genFromList(self, list1, list2):
        ...
a = A2()
a.genFromPath("abc")
b = A2()
b.genFromList([1,2,3],[4,5,6])

Ignacio Vazquez-Abrams · Accepted Answer · 2010-10-19 20:15:26Z

7

Make the constructor take the two lists. Write a factory classmethod that parses the XML and returns the object.

answered Oct 19, 2010 at 20:15

Ignacio Vazquez-Abrams

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4 Comments

Peter Gibson Over a year ago

Any reason why a classmethod over a staticmethod?

Ignacio Vazquez-Abrams Over a year ago

@Peter: So that if the class is derived from, the method will pick the proper class to instantiate regardless.

max Over a year ago

Why the asymmetry? That is, why not two factory classmethods, one for XML and one for a pair of lists? Or is it because you assume the "pair of list" is a "natural" way to build this object, so that factory method could rely on the "pair of list" constructor?

Ignacio Vazquez-Abrams Over a year ago

@max: list is a native Python type. XML is not.

petraszd · Accepted Answer · 2010-10-19 20:19:00Z

3

Use classmethod for second one

class A(object):
    @classmethod
    def from_string(cls, string):
        # ...

    @classmethod
    def from_lists(cls, list1, list2):
        # ...

Use module's functions

def from_string(string):
    # ...

def from_lists(list1, list2):
    # ...

class A(object):
    pass

answered Oct 19, 2010 at 20:19

petraszd

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Comments

Ray · Accepted Answer · 2010-10-19 21:28:11Z

3

Since the number of arguments passed to the initializer is different in each case, you can avoid type-checking by using the extended call syntax:

class A(object):
    def __init__(self, *args):
        if len(args) == 1:
            path = args[0]
            ...
        elif len(args) == 2:
            list1 = args[0]
            list2 = args[1]
            ...
        else:
            raise SomeException()

answered Oct 19, 2010 at 21:28

Ray

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1 Comment

Ignacio Vazquez-Abrams Over a year ago

Note that this is not really a good method since it goes against Python's usually self-documenting nature.

Chris B. · Accepted Answer · 2010-10-19 20:17:43Z

2

Looking at the problem more closely, I'd suggest having the class take two lists, and include a helper function in the module:

class A(object):
    def __init__(self, list1, list2):
        # Init from lists here
        pass

def create_A_from_path(path):
    list1, list2 = parse_xml_into_lists(path)
    return A(list1, list2)

answered Oct 19, 2010 at 20:17

Chris B.

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Comments

Dantalion · Accepted Answer · 2010-10-19 21:57:35Z

2

class A(object):
    @staticmethod
    def from_string(str):
        obj =A()
        obj.str = str
        return obj

    @staticmethod
    def from_list(lis):
        obj = A()
        obj.lis = lis
        return obj

>>>
(obj1, obj2) = A.from_string('hello'), A.from_list(['one', 'two'])

answered Oct 19, 2010 at 21:57

Dantalion

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