Question

I came across a piece of code today that looks like this:

class ClassName(object):
     def __init__(self):
         self._vocabulary = None

     def vocabulary(self):
         self._vocabulary = self._vocabulary or self.keys()
         return self._vocabulary

What exactly is the line self._vocabulary = self._vocabulary or self.keys() doing?

Answer 1

A line like this:

self._vocabulary = self._vocabulary or self.keys()

Is what’s called lazy initialization, when you are retrieving the value for the first time it initializes. So if it has never been initialized self._vocabulary will be None (because the __init__ method had set this value) resulting in the evaluation of the second element of the or so self.keys() will be executed, assigning the returning value to self._vocabulary, thus initializing it for future requests.

When a second time vocabulary is called self._vocabulary won’t be None and it will keep that value.