CODEWITHSUNDEEP
pythoninheritancepython-3.x
amundi12
amundi12

Reputation: 137

Inheritance has no attribute?

class Airplane:

    def __init__(self, plane_model, serial_num, num_seats, miles_travelled):
        self.model = plane_model
        self.serial = serial_num
        self.seat = num_seats
        self.miles = miles_travelled

class Flight:

    def __init__(self, plane):
        self.passangers = []
        self.airplane = plane

I am trying to complete the following function:

    def add(self, passenger):
        """(Flight, str) -> bool

        If there are still seats avalaible on this flight, add passenger to the passenger list.       
        Return true iff passenger is added to this flight.

        >>> a = Airplane('Cesna 150E', '9378', 1, 824.8)
        >>> f = Flight(a)
        >>> f.add('Myrto') # Decreases the seat by 1
        True
        >>> f.add('Jen') # No more seats avalaible
        False
        """

So far, my attempt:

        if self.seat > 0:
            self.seat -= 1 #num_seats
            return True
        else:
            return False

Of course, naturally, the instance variable num_seats would decrease by one if a passenger is added. The number of seats available is indicated on the third parameter on the Airplane class.

If I use the function that I have designed, attempting the exact same action like the one in the docstring returns the following error:

Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    f.add('a')
  File "/Users/N/Desktop/Untitled.py", line 39, in add
    if self.seat > 0:
AttributeError: 'Flight' object has no attribute 'seat'

I think I may be missing something but I am not sure what it is.

Upvotes: 1

Views: 348

Answers (1)

jonrsharpe
jonrsharpe

Reputation: 122032

Your specific error is that a Flight instance doesn't have a seat attribute, that's an attribute of an Airplane instance and should therefore be accessed via the self.airplane attribute (which provides the link to the Airplane instance that will be used for the Flight instance):

if self.airplane.seat > 0:
      # ^ access via 'airplane' attribute

However, I think your modelling is slightly off generally. The addition of passengers changes the number of free seats on the Flight instance, but does not alter the number of seats on the Airplane instance (as far as I'm aware, they don't start stripping seats out of the plane as people book!) Also, the distance to be travelled is a property of the flight, not the aeroplane. With that in mind, I would implement as:

class Airplane(object):

    def __init__(self, model, seats):
        self.model = model
        self.seats = seats


class Flight(object):

    def __init__(self, plane, code, distance):
        self.plane = plane
        self.code = code
        self.distance = distance
        self.passengers = []

    @property
    def free_seats(self):
        return self.plane.seats - len(self.passengers)

    def add(self, passenger):
        if not self.free_seats:
            raise Exception("Flight full.")
        self.passengers.append(passenger)

Note that Flight.add checks to ensure that the current passengers list will fit into the plane.seats. There is no need to keep a separate attribute - free_seats can be calculated from other attributes, so I've made it a read-only property. In use:

>>> f = Flight(Airplane("747", 5), "BA0123", 123)
>>> for person in ['John Cleese', 'Terry Gilliam', 'Eric Idle', 'Terry Jones', 'Michael Palin']:
    f.add(person)


>>> f.passengers
['John Cleese', 'Terry Gilliam', 'Eric Idle', 'Terry Jones', 'Michael Palin']
>>> f.add('Graham Chapman')

Traceback (most recent call last):
  File "<pyshell#32>", line 1, in <module>
    f.add('Graham Chapman')
  File "<pyshell#25>", line 13, in add
    raise Exception("Flight full.")
Exception: Flight full.
>>> f.free_seats
0

(Don't worry, Graham's past caring at this point.)

This does not quite meet your specification, as Flight.add now raises an error if the flight is full, rather than returning False. This is a more Pythonic way of coding; see "Easier to ask for forgiveness than permission".

Upvotes: 1

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