error with the global variable in python

Question

I am just beginning in python and I am getting the following error:

global name 'data' not defined on line# 62

Following is my code:

class PriorityQueue(object):
    """
    Max PriorityQueue
    Binary heap based implementation
    """
    data = []

    def __init__(self):
        pass

    def insert(self,d):
        global data
        data.append(d)
        __siftup()

    def extractMax(self):
        global data
        max = -1

        if not len(data) == 0:
            max = data[0]

        __siftdown()
        return max

    def __siftup():
        global data
        index = len(data) - 1
        pIndex = (index - 1)/2

        while pIndex > 0 and data[pIndex] > data[index]:
            data[pIndex], data[index] = data[index], data[pIndex]
            index = pIndex
            pIndex = (index - 1)/2

    def __siftdown():
        global data
        data[0] = data.pop()
        index = 0

        while index *2 <= len(data) -1:
            minIndex = __getMinIndex(index)
            if data[minIndex] > data[index]:
                data[minIndex], data[index] = data[index], data[minIndex]
                index = minIndex
            else:
                break

    def __getMinIndex(i):
        global data
        lIndex = i*2 +1
        rIndex = lIndex + 1

        if rIndex >= len(data) or data[lIndex] > data[rIndex]:
            return lIndex
        else:
            return rIndex

    """Test Script"""

    q = PriorityQueue()
    q.insert(3)

Should I consider using instance variable instead of global. I have been mostly using Java and recently switched. Can anyone also provide some link for oops concepts in python.

I hope you put data at class level because it looked like the Java syntax for an instance field, and not because you would have used a static field in Java. — user2357112
– user2357112, Commented Feb 11, 2014 at 22:04
Once you fix that error, you'll have problems with __siftup() and __siftdown() as well. — Joel Cornett
– Joel Cornett, Commented Feb 11, 2014 at 22:04

voithos · Accepted Answer · 2014-02-11 22:05:08Z

4

When you write:

class MyClass(object):
    data = []

data becomes neither an instance variance for objects made via that class, nor a global variable in the module scope, but a class-level (static) variable.

>> instance = MyClass()
>> instance.data
[]
>> instance.data.append(1)
>> instance.data
[1]
>> other_instance = MyClass()
>> other_instance.data
[1]

This is generally not what you want.

The proper way to create instance variables is to assign them in your __init__ constructor.

class PriorityQueue(object):    
    def __init__(self):
        self.data = []

Then, in your other methods, you can reference data as self.data instead of trying to use the global statement. In fact, try to avoid using globals in general.

answered Feb 11, 2014 at 22:05

voithos

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Comments

jramirez · Accepted Answer · 2014-02-11 22:04:15Z

1

Use the instance varible Luke.

answered Feb 11, 2014 at 22:04

jramirez

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Comments

knx · Accepted Answer · 2014-02-11 22:15:15Z

Perhaps you find this helpfull. Especially the LGB-Part and the following chapters.

These two little examples always helped me to clear my confusion.

#!/usr/bin/python
# -*- coding: utf-8 -*-

class A(object):
    def __init__(self, a):
        self.a = a

    def f(self, v):
        return self.a + v

class B(A):
    def __init__(self, a, b):
        A.__init__(self,a)
        self.b = b

    def f(self, v):
        return A.f(self,v)*self.b

if __name__ == '__main__':
    b = B(2,3)
    print "18",b.f(4)
    b.a = 3
    print "21",b.f(4)

With the result:

18 18
21 21
[Finished in 0.1s]

And the second one:

n = 1

def A():
   n = 2
   def B():
       print n
   B()

def func1():
    n = 2
    print n

def func2():
    global n
    n = 2
    print n


if __name__ == '__main__':
    func1()
    func2()
    A()

With the result:

1
2
2
[Finished in 0.3s]

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