<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def exists(self, value=None):
 """ Return True if the given pk value exists for the given class. If no value is given, we use the value of the current field, which is the value of the "_pk" attribute of its instance. """

        try:
            if not value:
                value = self.get()
        except (AttributeError, DoesNotExist):
            # If the instance is deleted, the _pk attribute doesn't exist
            # anymore. So we catch the AttributeError to return False (this pk
            # field doesn't exist anymore) in this specific case
            return False
        else:
            return self.connection.sismember(self.collection_key, value)