Unbound Local Error with global variable

Question

I am trying to figure out why I get an UnboundLocalError in my pygame application, Table Wars. Here is a summary of what happens:

The variables, REDGOLD , REDCOMMAND , BLUEGOLD and BLUECOMMAND , are initialised as global variables:

#Red Stat Section
REDGOLD = 50
REDCOMMAND = 100
#Blue Stat Section
BLUEGOLD = 50
BLUECOMMAND = 100

def main():    

    [...]

    global REDGOLD
    global REDCOMMAND
    global BLUEGOLD
    global BLUECOMMAND

This works when spawning units within the main loop, subtracting funds to spawn units.

Right now, I am trying to set up a system so that when a unit dies, the killer refunds the victim's COMMAND and earns GOLD based on what he killed:

class Red_Infantry(pygame.sprite.Sprite):
def __init__(self, screen):
    [...]
    self.reward = 15
    self.cmdback = 5

[...]

def attack(self):
    if self.target is None: return
    if self.target.health <= 0:
        REDGOLD += self.target.reward          #These are the problem lines
        BLUECOMMAND += self.target.cmdback     #They will cause the UnboundLocalError
                                               #when performed
        self.target = None
    if not self.cooldown_ready(): return
    self.target.health -= self.attack_damage
    print "Target's health: %d" % self.target.health

This works right up until the unit dies. Then this happens:

Traceback (most recent call last):
File "C:\Users\Oventoaster\Desktop\Games\Table Wars\Table Wars.py", line 606, in <module>
main()
File "C:\Users\Oventoaster\Desktop\Games\Table Wars\Table Wars.py", line 123, in main
RedTeam.update()
File "C:\Python27\lib\site-packages\pygame\sprite.py", line 399, in update
for s in self.sprites(): s.update(*args)
File "C:\Users\Oventoaster\Desktop\Games\Table Wars\Table Wars.py", line 304, in update
self.attack()
File "C:\Users\Oventoaster\Desktop\Games\Table Wars\Table Wars.py", line 320, in attack
REDGOLD += self.target.reward
UnboundLocalError: local variable 'REDGOLD' referenced before assignment

How do I get the global variables mentioned above to change with the attack block? If it helps, I am using Pygame 2.7.x, so nonlocal won't work :/

Answer 1

global make the global variable visible in the current code block. You only put the global statement in main , not in attack .

ADDENDUM

Here is an illustration of the need to use global more than once. Try this:

RED=1

def main():
    global RED
    RED += 1
    print RED
    f()

def f():
    #global RED
    RED += 1
    print RED

main()

You will get the error UnboundLocalError: local variable 'RED' referenced before assignment .

Now uncomment the global statement in f and it will work.

The global declaration is active in a LEXICAL, not a DYNAMIC scope.

Answer 2

You need to declare the variable as global in each scope where they are being modified

Better yet find a way to not use globals. Does it make sense for those to be class attributes for example?

Answer 3

Found that variables in main act like global "read only" variables in function. If we try to reassign the value, it will generate error.

Try:

#!/usr/bin/env python
RED=1
A=[1,2,3,4,5,6]

def f():
    print A[RED]

f()

It's ok.

But:

#!/usr/bin/env python
RED=1
A=[1,2,3,4,5,6]

def f():
    print A[RED]
    A = [1,1,1,1,1]

f()

Generate

  File "./test.py", line 6, in f
    print A[RED]
UnboundLocalError: local variable **'A'** referenced before assignment

and:

#!/usr/bin/env python
RED=1
A=[1,2,3,4,5,6]

def f():
    print A[RED]
    RED = 2

f()

Generate

  File "./test.py", line 6, in f
    print A[RED]
UnboundLocalError: local variable **'RED'** referenced before assignment