Example -

class Example:
    def __init__(self):
        self.producer = None

    def run(self):
        foo(None, self.producer)
        # Do we need to poll for the producer?

When placing the cursor over the word 'producer' in the comment, the variable in __init__ is highlighted. The variable is not highlighted in the call to foo.

Changing the value of the variable fixes it -

class Example:
    def __init__(self):
        self.producer = None

    def run(self):
        foo(None, self.producer)
        self.producer = None
        # Do we need to poll for the producer?