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

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

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

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.