Python变量不会改变吗？

Question

I'm making a game in python, and I have some code set up as such:

istouching = False
death = True

def checkdead():
    if istouching:
        print "Is touching"     
        death = True

while death is False:
    print death
    game logic

I know the game logic is working, because "Is touching" prints, but then when I print out the value of death, it remains false, any help?

Answer 1

use global to change global variables inside a function, otherwise death=True inside checkdead() will actually define a new local variable.

def checkdead():
    global death
    if istouching == True:      #use == here for comparison
        print "Is touching"     
        death = True

Answer 2

Make checkdead return a value:

def checkdead():
    if istouching:
        print "Is touching"     
        return True

death = checkdead()

You could also use global , as @AshwiniChaudhar shows, but I think it is preferable to write functions that return values instead of functions that modify globals, since such functions can be unit-tested more easily, and it makes explicit what external variables are changed.

PS. if istouching = True should have resulted in a SyntaxError since you can not make a variable assignment inside a conditional expression.

Instead, use

if istouching:

Answer 3

That's scope-related.

death = False        
def f():
    death = True      # Here python doesn't now death, so it creates a new, different variable
f()
print(death)          # False

death = False       
def f():
    global death
    death = True
f()
print(death)      # True