Modifying a global variable inside a function

Question

I have defined the following function:

def GMM(s1, s2, s3, s4, s5, a):
    """The GMM objective function.

    Arguments
    ---------
        si: float
            standard deviations of preference distribution

        a: float
            marginal utility of residutal income

    Paramters
    ---------
        Px: array (1,ns) 
            projector onto nonprice characteristic space

        xk, z: arrays (J, 5) and (J, 12)
            nonprice char. and instruments

        invW: array (12, 12) 
            GMM weight matrix

    Returns
    -------
        float."""
    delta = invert(s1, s2, s3, s4, s5, a, delta0) # Invert market shares to get mean utility
    bmean = np.dot(Px, delta)                     # Project delta onto charancteristic space
    xihat = delta - np.dot(xk, bmean)             # Compute implied unobservable prod. quality
    temp1 = np.dot(xihat.T, z)
    if np.any(np.isnan(delta)) == True:
        value = 1e+10
    else:
        value = np.dot(np.dot(temp1, invW), temp1.T)
    return np.sqrt(value)

My question pertains to the variable delta bound inside of the function. Outside of the function I will set the initial value of delta0. Now, ultimately I will minimize this function. What I would like to have happen is that each time the function GMM evaluates, delta from the previous evaluation is used as the new delta0. I tried defining delta0 as a global variable, but it did not seem to work... likely this was my error though. Although, I have read here that generally this is a bad approach. Any suggestions?

Laurent Luce · Accepted Answer · 2010-12-23 23:16:51Z

25

There are multiple ways to achieve what you want. delta is saved across function calls in the following examples.

1- Class

class Example:
  def __init__(self, value):
    self.delta = value
  def gmm(self):
    self.delta += 1
    return self.delta

e = Example(0)
print e.gmm()

2- Generator

def gmm():
  delta = 0
  while True:
    delta += 1
    yield delta

for v in gmm():
  print v

3- Function attribute

def gmm():
  gmm.delta += 1
  return delta
gmm.delta = 0

4- Global variable (discouraged as you said):

delta = 0
def gmm():
  global delta
  delta += 1
  return delta

etc...

edited Dec 23, 2010 at 23:16

answered Dec 23, 2010 at 22:57

Laurent Luce

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pythonFoo · Accepted Answer · 2010-12-23 22:13:06Z

13

globalVariable = 0

def test():
    global globalVariable
    globalVariable = 10

test()
print globalVariable

You can edit a global variable in this way.

answered Dec 23, 2010 at 22:13

pythonFoo

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1 Comment

Martin v. Löwis Over a year ago

+1. It would have been better had you formulated the example code in terms of GMM, delta, and delta0

richo · Accepted Answer · 2010-12-23 23:17:07Z

4

When faced with this, a common kludge I use is stuff an object into a module, which then puts it into a namespace accessible by everything in the program. It is a big kludge, but I find it removes any ambiguity about what's global. For standalone stuff I put it into os, if it's an entire project I'll generally create an empty python file called my_globals and import it, ie

import my_globals
my_globals.thing = "rawp"
def func():
    my_globals.thing = "test"
func()
print my_globals.thing # "test"

answered Dec 23, 2010 at 23:17

richo

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