Python Newbie Questions - Not printing correct values

Question

I am newbie to python and I doing doing some OOPS concept exploring in Python.

Following is my Account class:

class Account:
    def __init__(self,balance):
        self.__balance=int(balance)

    def deposit(self,deposit_amt):
        self.__balance=self.__balance + int(deposit_amt)

    def withdraw(self,withdraw_amt):
        withdraw_amt=int(withdraw_amt)
        self.__balance=self.__balance -- int(withdraw_amt)
        print(self.__balance)
        print("Subtracting" + str(withdraw_amt))

    def get___balance(self):
        return(self.__balance)

    def __str__(self):
        return("The Balance in the Account is " + str(self.get___balance()))

The account_test program:

import account
def main():
    balance_amt = input("Enter the balance amount \t")
    new_account=account.Account(int(balance_amt))


    deposit_amt=input("Enter the Deposit Amount \t")
    new_account.deposit(deposit_amt)
    print(new_account)

    withdraw_amt=input("Enter the Withdraw Amount \t")
    new_account.withdraw(withdraw_amt)
    print(new_account)


main()

But I am getting the wrong output:

Enter the balance amount    3000
Enter the Deposit Amount    400
The Balance in the Account is 3400
Enter the Withdraw Amount   300
3700
Subtracting 300
The Balance in the Account is 3700

When I do withdraw I am getting the amount added instead of subtracted. What am I doing wrong here?

Since I am newbie, I need some suggestion in my programming practice. Is my coding style appropriate?

On coding style, don't use double-underscores for the balance - just call it `self.balance`. And there's no reason to define a `get_balance` method. Python practice is to access the attribute directly. — Daniel Roseman, Oct 01 '11 at 18:52
Thanks Daniel for your response. But what I was reading is if u want to make u r variable as private you need to add __ in front of variable so that python consider it as private variable for that class.Please correct me if I am wrong. If not how to declare a variable private in python. I know by default python wont let u , but turn around was to add __ . — techrawther, Oct 02 '11 at 00:21
There are no private variables in Python. `__` merely does name mangling, making them hard to find. — S.Lott, Oct 07 '11 at 15:28

chown · Answer 1 · 2011-10-02T03:05:58.700

2

With the double -- (negative) you are subtracting a negative value (i.e. adding a positive value).
A more clear explanation would look like:

self.__balance = self.__balance - (0 - int(withdraw_amt))

Therefore, change this:

self.__balance=self.__balance -- int(withdraw_amt)

To this:

self.__balance=self.__balance - int(withdraw_amt)

Or better yet, to this:

self.__balance -= int(withdraw_amt)

edited Oct 02 '11 at 03:05

answered Oct 01 '11 at 18:33

chown

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score 1 · Answer 2 · answered Oct 01 '11 at 18:36

1

self.__balance=self.__balance -- int(withdraw_amt)

is actually parsed as

self.__balance=self.__balance - (- int(withdraw_amt))

which is to say, it is adding the withdraw amount. try with a single -

answered Oct 01 '11 at 18:36

Foo Bah

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Python Newbie Questions - Not printing correct values

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