Python - Why is this class variable not defined in the method?

Question

I have a python application which is as follows:

global_counter = 0
connections = {}

class SocketHandler():
    currentid = 0
    def open(self):
        global global_counter
        global connections
        currentid = global_counter
        global_counter += 1
        connections[currentid] = self
        print "WebSocket " + str(currentid) + " opened"

    def on_close(self):
        global connections
        print "WebSocket " + str(currentid) + " closed"
        del connections[currentid]

I'm getting the error:

NameError: global name 'currentid' is not defined

on the lines of "open" and "on_close" where I print that I am opening/closing the connection. I defined it in the class, why is it not in scope. Also, I have read that using global variables is bad, but I don't see a way around this. Can someone point out what I should do? Thanks.

Answer 1

You don't have implicit access to attributes inside methods, in Python.

A bare name like currentid in the line:

del connections[currentid]

always looks up a name in the local function scope, then in each enclosing function scope, before trying the global module scope (and then looks at built-ins as a last resort). currentid is a class attribute, which won't be found in any of those scopes.

To look up an attribute in Python you always need to specify an object in which to look. Though the lookup protocol means the object need not necessarily have the attribute itself; attribute lookup will fall back to the class of the object you specified (and the base classes, if inheritance is involved).

So this would work:

del connections[self.currentid]

However, I don't think the rest of your code is doing what you think it is either. This line in the open method:

currentid = global_counter

doesn't set the currentid attribute of your SocketHandler object. Assigning to a bare name always assigns to a local variable, unless you explicitly declare it global (you appear to be aware of this, since you've used the global keyword). So in the open method, currentid is a local function variable; its value is lost at the end of the open method.

In fact, your SocketHandler objects do not have a currentid attribute at all (unless there's more code you haven't shown us). Putting currentid = 0 in the class block doesn't give all the SocketHandler instances a currentid attribute. It gives the SocketHandler class itself an attribute currentid; this is just as the def open(self): block creates an open attribute (storing a function) on the class object, not on each individual instance.

Reading self.currentid in the on_close method will fail to find a currentid attribute in the object self, so Python will look at the class of self which is SocketHandler. That object does have a currentid value, so the result of reading self.currentid will be 0, whether or not you've previously run open on that SocketHandler.

If you meant to store the currentid as an instance variable in each SocketHandler, then the line in open would need to be:

self.currentid = global_counter

This assigns to the currentid attribute of the object referred to by self. You would also then need to change all the other references to currentid in your methods to self.currentid.

Answer 2

currentid should be self.currentid since it is a class variable.