[python] How to overload __init__ method based on argument type?

Let's say I have a class that has a member called data which is a list.

I want to be able to initialize the class with, for example, a filename (which contains data to initialize the list) or with an actual list.

What's your technique for doing this?

Do you just check the type by looking at __class__?

Is there some trick I might be missing?

I'm used to C++ where overloading by argument type is easy.

You probably want the isinstance builtin function:

self.data = data if isinstance(data, list) else self.parse(data)

My preferred solution is:

class MyClass:
    _data = []
    __init__(self,data=None):
        # do init stuff
        if not data: return
        self._data = list(data) # list() copies the list, instead of pointing to it.

Then invoke it with either MyClass() or MyClass([1,2,3]).

Hope that helps. Happy Coding!

OK, great. I just tossed together this example with a tuple, not a filename, but that's easy. Thanks all.

class MyData:
    def __init__(self, data):
        self.myList = []
        if isinstance(data, tuple):
            for i in data:
                self.myList.append(i)
        else:
            self.myList = data

    def GetData(self):
        print self.myList

a = [1,2]

b = (2,3)

c = MyData(a)

d = MyData(b)

c.GetData()

d.GetData()

[1, 2]

[2, 3]

Why don't you go even more pythonic?

class AutoList:
def __init__(self, inp):
    try:                        ## Assume an opened-file...
        self.data = inp.read()
    except AttributeError:
        try:                    ## Assume an existent filename...
            with open(inp, 'r') as fd:
                self.data = fd.read()
        except:
            self.data = inp     ## Who cares what that might be?

Excellent question. I've tackled this problem as well, and while I agree that "factories" (class-method constructors) are a good method, I would like to suggest another, which I've also found very useful:

Here's a sample (this is a read method and not a constructor, but the idea is the same):

def read(self, str=None, filename=None, addr=0):
    """ Read binary data and return a store object. The data
        store is also saved in the interal 'data' attribute.

        The data can either be taken from a string (str 
        argument) or a file (provide a filename, which will 
        be read in binary mode). If both are provided, the str 
        will be used. If neither is provided, an ArgumentError 
        is raised.
    """
    if str is None:
        if filename is None:
            raise ArgumentError('Please supply a string or a filename')

        file = open(filename, 'rb')
        str = file.read()
        file.close()
    ...
    ... # rest of code

The key idea is here is using Python's excellent support for named arguments to implement this. Now, if I want to read the data from a file, I say:

obj.read(filename="blob.txt")

And to read it from a string, I say:

obj.read(str="\x34\x55")

This way the user has just a single method to call. Handling it inside, as you saw, is not overly complex

A better way would be to use isinstance and type conversion. If I'm understanding you right, you want this:

def __init__ (self, filename):
    if isinstance (filename, basestring):
        # filename is a string
    else:
        # try to convert to a list
        self.path = list (filename)

You should use isinstance

isinstance(...)
    isinstance(object, class-or-type-or-tuple) -> bool

    Return whether an object is an instance of a class or of a subclass thereof.
    With a type as second argument, return whether that is the object's type.
    The form using a tuple, isinstance(x, (A, B, ...)), is a shortcut for
    isinstance(x, A) or isinstance(x, B) or ... (etc.).

with python3, you can use Implementing Multiple Dispatch with Function Annotations as Python Cookbook wrote:

import time


class Date(metaclass=MultipleMeta):
    def __init__(self, year:int, month:int, day:int):
        self.year = year
        self.month = month
        self.day = day

    def __init__(self):
        t = time.localtime()
        self.__init__(t.tm_year, t.tm_mon, t.tm_mday)

and it works like:

>>> d = Date(2012, 12, 21)
>>> d.year
2012
>>> e = Date()
>>> e.year
2018

Quick and dirty fix

class MyData:
    def __init__(string=None,list=None):
        if string is not None:
            #do stuff
        elif list is not None:
            #do other stuff
        else:
            #make data empty

Then you can call it with

MyData(astring)
MyData(None, alist)
MyData()

Excellent question. I've tackled this problem as well, and while I agree that "factories" (class-method constructors) are a good method, I would like to suggest another, which I've also found very useful:

Here's a sample (this is a read method and not a constructor, but the idea is the same):

def read(self, str=None, filename=None, addr=0):
    """ Read binary data and return a store object. The data
        store is also saved in the interal 'data' attribute.

        The data can either be taken from a string (str 
        argument) or a file (provide a filename, which will 
        be read in binary mode). If both are provided, the str 
        will be used. If neither is provided, an ArgumentError 
        is raised.
    """
    if str is None:
        if filename is None:
            raise ArgumentError('Please supply a string or a filename')

        file = open(filename, 'rb')
        str = file.read()
        file.close()
    ...
    ... # rest of code

The key idea is here is using Python's excellent support for named arguments to implement this. Now, if I want to read the data from a file, I say:

obj.read(filename="blob.txt")

And to read it from a string, I say:

obj.read(str="\x34\x55")

This way the user has just a single method to call. Handling it inside, as you saw, is not overly complex