Python cannot read global variable in function even though I used global keyword

Question

I want to access the variable number_of_messages in class A but I get "number_of_messages" is not defined error even though I used global keyword. Here is a code sample:

class A:
    number_of_messages=0;
    def inc(self):
        global number_of_messages
        number_of_messages+=1
    
print(A().inc())

It isn't defined. Why do you expect a global variable with that name to exist? — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 17, 2021 at 20:01
note: your inc method returns None, so I would never expect your print statement to shed light on what's happening with number_of_messages — Paul H
– Paul H, Commented Dec 17, 2021 at 20:07

Guinther Kovalski · Accepted Answer · 2022-01-03 11:37:29Z

3

Use the class attribute instead:

class A:
    def ___init__(self):
        self.number_of_messages=0

    def inc(self):
        self.number_of_messages+=1
a = A()
print(a.inc())
print(a.number_of_messages)

but you can also:

number_of_messages = 0

class A():
    def inc(self):
        global number_of_messages
        number_of_messages+=1

a = A()
a.inc()
print(number_of_messages)

you just forgot to declare the variable in the global scope

edited Jan 3, 2022 at 11:37

answered Dec 17, 2021 at 20:04

Guinther Kovalski

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Comments

Tim Roberts · Accepted Answer · 2021-12-17 20:01:17Z

1

That's not a global. That's a class attribute. Write

    def inc(self):
        A.number_of_messages += 1

You don't need the global statement.

answered Dec 17, 2021 at 20:01

Tim Roberts

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