全局范围变量在python中保持不变

Question

In this code

 money = .3 Things = ["Nothing"] def main(): print "go to buy things" print "Current Money %s" % (money) print "Things you have:" for item in Things: print item wait = raw_input() buythings(money) def buythings(money): print "Type Buy to buy things" buy = raw_input() if buy == "buy": money = money - .3 Things.append("Things") main() else: print "you bought nothing" print "" main() 

Why after buying the things does the money not go down? This has been a problem to me for a while now and I cant seem to understand how the scope works in this situation.

Answer 1

The global variable money is shadowed by the function parameter money in buythings(money) function. You should remove the parameter for it to work:

def main():
    global money
    global Things
    ...

def buythings():
    global money
    global Things
    ...

However, A better approach, as alfasin pointed out, would be passing money and Things as parameters to both functions and not using global keyword at all:

def main(money, things):
    ...
    for item in things:
        print item
    wait = raw_input()
    buythings(money, things)


def buythings(money, things):
    ...
    if buy == "buy":
        money = money - .3
        Things.append("Things")
        main(money, things)
    else:
        ...
        main(money, things)

---

>>> money = .3
>>> Things = ["Nothing"]
>>> main(money, Things)

Hope this helps.

Answer 2

You can use a global variable in other functions by declaring it as global in each function that assigns to it:

money = 0

def set_money_to_one():
    global money    # Needed to modify global copy of money
    money = 1

def print_money():
    print money     # No need for global declaration to read value of money

set_money_to_one()
print_money()       # Prints 1

In your case :

def buythings():
    global money

Python wants to make sure that you really know what you're playing with by explicitly requiring the global keyword.