Keep a running total(budget)

[mcmxl22]

I am trying to write a budget program. I need to figure out how to keep a running total. How do I update the variable bal via a user input? I have tried while and for loops. This is as close as I can get. 

bal = 0.00

def budget():
    menu = ('1 Set Ballance', '2 Add Transaction', '3 Check Ballance')
    for m in menu:
        print '%s' % m

    what = raw_input('What do you want to do?\n> ').isdigit()

    print bal + what 
    
budget()

Output:
1 Set Ballance
2 Add Transaction
3 Check Ballance
What do you want to do?
> 1
1.0