Question

Only one argument is passed. Still I’m getting error that 2 arguments have been passed. head and tail are not being initialized to -1.

class Queue_demo:
    head=-1
    tail=-1
    a=[]

    def enqueue(data=10):
        if(head==-1 and tail==-1):
            head=head+1
            tail=tail+1
            a.append(data)
        else:
            tail=tail+1
            a.append(data)

    def dequeue():
        y=a[head]
        if(head==tail):
            head,tail=-1,-1
        else:
            head=head+1
        return y

q1=Queue_demo()
q2=Queue_demo()
q1.enqueue(12)

while(q1.tail==-1):
    print(q1.dequeue())

Answer 1

There are a few issues with your code.

The one that is directly causing your error is that you have not given your methods a self argument. When you call q1.enqueue(12) Python translates this to Queue_demo.enqueue(q1, 12). The object the method is being called on is passed to the method as the first argument. By convention, it is usually named self.

This leads me to the second issue you’re going to encounter once you get past the wrong number of arguments error. Your instances are going to all be sharing the same set of data members, since they’re currently accessing class variables head, tail and a, rather than instance variables. This will be very confusing, as adding an item to one queue will make it appear in all other queues too.

To fix this, what you will want to do is create those variables in a constructor (which is simply a method named __init__), rather than defining them in the class definition. Here’s what your __init__ method will probably look like:

def __init__(self):
    self.head = -1
    self.tail = -1
    self.a = []

Remember that Python is often different from other programming languages! You don’t need to declare your member variables, just start assigning things as values on self and you’ll be all set.