Python constructors with default values?

Question

So, my assignment is to create a Ship class that has the attributes 'name' and 'fuel.' The name should have a default value of "Enterprise" and the fuel should have a default of 0. And I have to create a object with no parameters, each single parameter, and with both. The only one that's not working is the one with only the fuel and I'm not sure why.

Here's my code:

class Ship(object):

    def __init__(self, name = "Enterprise", fuel = 0):
        self.name = name
        self.fuel = fuel

    def status(self):
        if self.fuel < 0:
            self.fuel = 0
        print self.name, "\nFuel:", self.fuel, "\n"

#main

ship1 = Ship()
ship1.status()
ship2 = Ship("Space Ship1")
ship2.status()
ship3 = Ship(10)
ship3.status()
ship4 = Ship("Space Ship2", 10)
ship4.status()

And here's what it's outputting:

Enterprise 
Fuel: 0 

Space Ship1 
Fuel: 0 

10 
Fuel: 0 

Space Ship2 
Fuel: 10

The third one is printing the fuel as the name and defaulting the fuel to 0 which isn't right. How can I fix it?

As @Hyperboreus noted, you can explicitly assign the value to a parameter in the constructor. If you don't specify, they will be assigned from left to right, then use default values for everything remaining to the right. — Cory Kramer
– Cory Kramer, Commented Apr 4, 2014 at 20:27

Hyperboreus · Accepted Answer · 2014-04-04 20:26:00Z

7

In ship3 = Ship(10) the first parameter 10 is the name of the ship.

You might want to use ship3 = Ship(fuel=10).

Not directly related, but important nevertheless. Remember that if you use mutable objects as default arguments (which you don't in your example), those are evaluated once and not for each call.

answered Apr 4, 2014 at 20:26

Hyperboreus

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