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.

• user2357112: 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.
• Joel Cornett: Once you fix that error, you'll have problems with __siftup() and __siftdown() as well.

Answer 1

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.

Answer 2

Use the instance varible Luke.

Answer 3

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]