Python global variable and class functionality

Question

Im creating a simple python program that gives basic functionality of an SMS_Inbox. I have created an SMS_Inbox method.

store = []
message_count = 0
class sms_store:
    def add_new_arrival(self,number,time,text):
        store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
        **message_count += 1**
    def delete(self,i):
        if i > len(store-1):
            print("Index does not exist")
        else:
            del store[i]
            message_count -= 1

In the bolded bit I am getting an error:

UnboundLocalError: local variable 'message_count' referenced before assignment.

I created a global variable store which is an empty list and this works when I use the add_new_variable object. However for some reason it is not adding values to my global message_count variable.

Please help

Answer 1

You are trying to assign to a global variable message_count without declaring it as such:

message_count = 0

class sms_store:
    def add_new_arrival(self,number,time,text):
        store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
        global message_count
        message_count += 1

Try to avoid using globals, or at least encapsulate the variable as a class attribute:

class sms_store:
    message_count = 0
    store = []

    def add_new_arrival(self,number,time,text):
        sms_store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
        sms_store.message_count += 1

However, your class instances have no state anymore, so there is no point in creating a class here. It only serves to confuse your purpose.

Either store state in instances, or use global functions (so don't use a class at all); the former is preferable to the latter.

Transforming your setup to a class whose instances hold state, using proper PEP-8 styleguide naming and string formatting:

class SMSStore(object):
    def __init__(self):
        self.store = []
        self.message_count = 0

    def add_new_arrival(self, number, time, text):
        self.store.append('From: {}, Received: {}, Msg: {}'.format(number, time, text))
        self.message_count += 1

You are then free to create one instance and use that as a global if needs be:

sms_store = SMSStore()

Other code just uses sms_store.add_new_arrival(...), but the state is encapsulated in one instance.

Answer 2

If the variable you are referring to is message_count, the error is because in Python, you have to specify a variable as global before you can make edits with it.

This should work.

store = []
message_count = 0
class sms_store:
    def add_new_arrival(self,number,time,text):
        global message_count
        store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
        message_count += 1
    def delete(self,i):
        if i > len(store-1):
            print("Index does not exist")
        else:
            global message_count
            del store[i]
            message_count -= 1

As written above, you'd be better off encapsulating it in the __init__ function instead of declaring it global.

Answer 3

That's not how classes work. Data should be stored within the class instance, not globally.

class SMSStore(object):
    def __init__(self):
        self.store = []
        self.message_count = 0

    def add_new_arrival(self,number,time,text):
        self.store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
        self.message_count += 1

    def delete(self, i):
        if i >= len(store):
            raise IndexError
        else:
            del self.store[i]
            self.message_count -= 1

sms_store = SMSStore()
sms_store.add_new_arrival("1234", "now", "lorem ipsum")
try:
    sms_store.delete(20)
except IndexError:
    print("Index does not exist")

print sms_store.store

# multiple separate stores
sms_store2 = SMSStore()
sms_store2.add_new_arrival("4321", "then", "lorem ipsum")
print sms_store2.store