CODEWITHSUNDEEP
python
code_2_peep
code_2_peep

Reputation: 25

error with the global variable in python

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.

Upvotes: 1

Views: 402

Answers (3)

knx
knx

Reputation: 398

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]

Upvotes: 0

voithos
voithos

Reputation: 70632

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.

Upvotes: 4

jramirez
jramirez

Reputation: 8685

Use the instance varible Luke.

Upvotes: 1

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