global variables in python

Question

class AlphaBetaAgent(MultiAgentSearchAgent):

  def action(self,gamestate):
    self.alpha= -9999
    self.beta = 9999

    def abc(gamestate, depth, alpha, beta):


    def bvc(gamestate, depth, alpha, beta):


   return abc(gamestate, 0, alpha, beta)

I am calling the getAction function which itself calling the abc funct and abc function calling the bvc funct. The functions abc and bvc are working in recursive way. I need to modify the values of alpha and beta as per the situation demands so I made them local. BUt it is not letting me to do that. Error occurs

Error occurs:- global name 'alpha' is not defined

Answer 1

To use a global variable you have to do something like:

global alpha
alpha = -9999

Answer 2

In Python, global variables must be declared outside of the function. Then, any function can read that variable without any problems, but if a function wants to write to it it has to declare it global. Example:

def fun1():
     print a
def fun2():
     a = 3
def fun3():
     global a
     a = 3
a = 0
fun1() # <- Will work
fun2() # <- Will raise exception
fun3() # <- Will work

Answer 3

Use a class. Storing state & behaviour is what classes are for.

Answer 4

If you want functions like abc and bvc to use common variables, you generally want to define an object for them to be methods of, like so:

class ActionState(object):
   def abc(self, gamestate, depth):
       self.alpha = -9999
       self.beta = 9999
   def bvc(self, gamestate, depth):
       self.alpha = -9999
       self.beta = 99999

def action(self, gamestate):
   state = ActionState()
   state.abc(gamestate, 0)

Alternatively, if you really want to, you can enclose a mutable object like a dict around to hold your data:

def action(self, gamestate):
    actionstate = { 'alpha': 0, 'beta': 0 }

    def abc(gamestate, depth):
        actionstate['alpha'] = -9999
        actionstate['beta'] = 9999

    def bvc(gamestate, depth):
        actionstate['alpha'] = -9999
        actionstate['beta'] = 9999

    abc(gamestate, 0)

Note that the actionstate parameter isn't passed here -- it's inherited from the enclosing scope. You can pass it explicity instead if you want, in which case abc and bvc no longer need to be defined inside of action. The reason this works and your example doesn't is that Python binds any primitive identifier lexically to the most-local function where it is assigned. So when you assign to alpha in abc, Python defines an alpha as local to abc, not action. I believe the only way to get a closure in Python is not to assign to the enclosed variable within the inner function.