Classes in Python 2.7 - not sure what's wrong

Question

Just a simple program to deposit and withdraw from an account. I'm trying to learn classes by just testing them out.

class bank:
    def __init__(self):
        self.origBal = 0
    def deposit(self, amount):
        self.origBal += amount
    def withdraw(self, amount):
        self.origBal -= amount
b = bank()
d = bank()
w = bank()

That problem I'm having is probably best seen from the output.

For example, here is the output.

w.withdraw(3423)
b.origBal
-3423
d.deposit(3423)
b.origBal
-3423
d.deposit(322423)
d.origBal
325846
d.deposit(3223)
d.origBal
329069
w.withdraw(324334)
b.origBal
-3423
w.withdraw(234)
b.origBal
-3423

Not exactly sure what's happening.

I'm sure I could fix it by just manually entering (-n) or (+n) and only have one method, but I'd like to avoid that.

Answer 1

When you do b = bank() you create one bank. When you do d = bank() you create a second bank. When you do w = bank() you create a third bank. Each bank has its own origBal . Calling deposit or withdraw on one of the three objects won't affect either of the other two objects. If you do

b = bank()
b.deposit(10)
b.withdraw(100)

. . . then things should work as you seem to expect.

You should read the Python tutorial to learn how classes work.

Answer 2

To do what you want you need to use class variables not object variables. These are defined and used by the class as a whole as seen below.

class Bank(object):
    origBal = 0
    def deposit(self, amount):
        Bank.origBal += amount
    def withdraw(self, amount):
        Bank.origBal -= amount

b = Bank()
d = Bank()
w = Bank()

w.withdraw(3423)
b.origBal
-3423
d.deposit(3423)
b.origBal
0
d.deposit(322423)
b.origBal
322423
d.deposit(3223)
b.origBal
325646
w.withdraw(324334)
b.origBal
1312
w.withdraw(234)
b.origBal
1078