Python: how to get create instance of a class within same class?

Question

I am trying to create a class which has a static method which returns list of its own instances. How can I do this without referring to the class name:

 1: class MyCar(object):
 2:  def __init__(self, number):
 3:    self._num = number
 4: 
 5:  @staticmethod
 6:  def get_cars_from(start=0, end=10):
 7:    """ This method get a list of cars from number 1 to 10.
 8:    """
 9:    return_list = []
10:    for i in range(start, end):
11:      instance = MyCar(i)
12:      return_list.append(instance)
13:    return return_list

This code works perfectly fine. But I have to reuse this code (copy+paste) in various classes, like Bus, Ship, Plane, Truck.

I am looking for a way to reuse above code in all these classes by making a generic way of instantiating instance of current class. Basically replace line #11 from:

  11: instance = MyCar(i)

to a more generic state which can be reused in any class. How can I do this ?

Yes, can't place them in base class, the example is a little different from the real implementation I'm working on. For now assume that I can't put this in base class. — Pranav
– Pranav, Commented Nov 23, 2015 at 14:55
It looks like you want to create a "Vehicle" class then make your classes Car, Bus, Plane etc inherit from it. There are very good tutorials on Google for inheritance in Python. — Dese
– Dese, Commented Nov 23, 2015 at 14:56
You could create a base class with that static method and then cast the result objects to your specific type — Phate01
– Phate01, Commented Nov 23, 2015 at 15:56

chepner · Accepted Answer · 2015-11-23 14:55:50Z

2

Use a class method, not a static method. That way, assuming for example that Bus inherits MyCar, then Bus.get_cars_from() will call the inherited MyCar.get_cars_from, but the cls argument will be set to Bus.

@classmethod
def get_cars_from(cls, start=0, end=10):
    """ This method get a list of cars from number 1 to 10.
    """
    return_list = []
    for i in range(start, end):
        instance = cls(i)
        return_list.append(instance)
    return return_list

Also, a list comprehension makes this a more-efficient one-liner:

@classmethod
def get_cars_from(cls, start=0, end=10):
    return [cls(i) for i in range(start, end)]

(but use xrange instead of range in Python 2).

answered Nov 23, 2015 at 14:55

chepner

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