Question

What is a clean way to declare multiple constructors for one class?

For example, let’s say I have an Item class. One way to create an Item (for example is)

item = Item(product_id, name, description,price)

another way to do the same thing might

item = Item(otherItem)

And then another way to do this.. maybe in some cases I don’t have the price so I want to pass just

item = Item(product_id, name,description)

and yet another case might be

item = Item(product_id,price)

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The other question I have is:
There are some private variables which might be initialized during runtime.
Let’s say I have some random variable itemCount and I want to keep a track of it internally.

How do I declare it that I dont have to put it in initialization mode, but rather, somewhere in the run time..
I can do something like

self._count +=1

Thanks

Answer 1

You could use default arguments:

class Item(object):

  def __init__(self, product_id = None, name = None, description = None, price = None)
      ... implementation ...

You can substitute in any value you want for None, if the default value should be something different.

Example usage:

item1 = Item(product_id = 4, price = 13) # use the field name!
item2 = Item(name = "hammer", description = "used to belong to Thor")

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For the copy constructor, item = Item(otherItem), @Raymond’s suggestions of class methods and factory functions may be the most Pythonic way to go.

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Update: here’s a question about multiple constructors in Python. It also mentions using *args and **kwargs.