Question

So let’s say you’ve got an application with a variable that you will be creating an instance of when you load it independently (ie when you use if __name__ == '__main__').

Also, there is a method that is to be called for when a client imports the application for use within another application. This method will also instantiate this variable.

What I want to do is test whether the variable has already been instantiated before defining it (so I don’t have to go through the creation of the object twice). My intuition told me to use if SOME_VARIABLE is not None: #instantiate here but this yields the error

local variable ‘SOME_VARIABLE’
referenced before assignment

What gives?

Answer 1

It’s an error to access a variable before it is initialized. An uninitialized variable’s value isn’t None; accessing it just raises an exception.

You can catch the exception if you like:

>>> try:
...    foo = x
... except NameError:
...    x = 5
...    foo = 1

In a class, you can provide a default value of None and check for that to see if it has changed on a particular instance (assuming that None isn’t a valid value for that particular variable):

class Foo(object):
    bar = None
    def foo(self):
        if self.bar is None:
            self.bar = 5
        return self.bar