How to access and modify global variables in python class

Question

I am new to classes in Python. I was trying to write a linked-list program where I needed a global variable to do the counting of number of nodes instead of a function. So, when I did as follows:

class DoublyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None
        self.count = 0

Then outside the linked list class I was unable to access this self.count (in the part where I created the object of this class etc.) I thought that since it is a local variable to this class I was getting the error. So, I tried this:

count = 0
class DoublyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None
        global count
        self.count=0

I was thinking that if I made the global variable a datafield of this class, then I don't need to write:

global count

in every function under this class. But outside the class whenever I am accessing count, it's value is zero. Can someone please help.

Edit: The display function doesn't need this count so I can see my list being created perfectly. I just want to access the number of nodes using count outside the class so that I can check for position validity before calling insert or delete functions, etc. I am attaching the snippet where it is giving me error, if it helps:

pos = int(input("Enter the position : \n"))
if (pos>(count+1))or(pos<1):
    print("Invalid Position")

Elhadede · Accepted Answer · 2022-01-09 09:07:49Z

1

there is no need to make a global variable in the class instead you can make a self.[variable name] however for changing the variable value you can self.[var name] = [new value] for example :

class myclass:
    def __init__(self):
        self.counter = 1

classvar = myclass();
print(classvar.counter)

answered Jan 9, 2022 at 9:07

Elhadede

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Surya Majumder Over a year ago

Thanks a lot, @Elhadede, it solved my problem completely.

sputnick567 · Accepted Answer · 2022-01-09 08:56:53Z

0

I think you misunderstood the meaning of the keyword global. global is used when you want to access / modify a variable which was defined in the normal script, not in the function, e.g.

#test.py
c = 1

def test1():
    c = 3 #this will not modify the c which we declared earlier

test1()
print(c) #will print 1

def test2():
     global c #this tells the interpreter to look for the previously defined c
     c = 3
test2()
print(c) #will print 3

Now to access members of an object, you just need to use objectname.variablename

answered Jan 9, 2022 at 8:56

sputnick567

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Heang Sok · Accepted Answer · 2022-01-09 10:22:16Z

0

It is unusual to use global inside a class. To access your class property, you just need to instantiate a new object.

class Car:
    def __init__(self):
    self.color = 'red'
    self.number_of_door = 5

# let's say I want to get the number of doors and plus one for Toyota

Toyota = Car()
print(Toyota.number_of_door + 1)

answered Jan 9, 2022 at 10:22

Heang Sok

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