CODEWITHSUNDEEP
pythonpython-3.x
user3605780
user3605780

Reputation: 7072

Why are both class instances changed?

I was reading this article about functional programming in Python (3).

However I don't understand this example in the text:

class Bus(object):
     passengers = set()
     def add_passenger(self, person):
        self.passengers.add(person)

bus1 = Bus()
bus2 = Bus()
bus1.add_passenger('abe')
bus2.add_passenger('bertha')
bus1.passengers  # returns ['abe', 'bertha']
bus2.passengers  # also ['abe', 'bertha']

Why would calling add_passenger() on bus1 instance of the class change the passenger set of bus2?

And what would be the correct way to do it when you don't want this behaviour?

Upvotes: 0

Views: 59

Answers (1)

ForceBru
ForceBru

Reputation: 44838

Why would calling add_passenger() on bus1 instance of the class change the passenger set of bus2?

Because there is no "passenger set of bus2" (and no passenger set of bus1). In this code:

class Bus(object):
     passengers = set()

...passengers is a class variable which is shared among all instances of this class, yet belongs not to these instances, but to the class itself, so when you change self.passengers, you actually change Bus.passengers, and since bus1.passengers and bus2.passengers refer to Bus.passengers, bus1.passengers == bus2.passengers is always true.

If you do not want this behaviour, implement an __init__ method:

class Bus:
    def __init__(self):
        self.passengers = set()

Upvotes: 5

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