这是在做什么（Python）

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.

Answer 2

In a nutshell, if self._vocabulary evaluates to a logical false (eg if it's None , 0 , False , etc), then it will be replaced with self.keys() .

The or operator in this case, returns whichever value evaluates to a logical true.

Also, your code should look more like this:

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

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

     def keys(self):
         return ['a', 'b']

ex = Example()
print ex.vocabulary()
ex._vocabulary = 'Hi there'
print ex.vocabulary()

Answer 3

Hard to say, the code will not run for a number of reasons. To guess though, I'd say it looks like it would be evaluated as a logical expression, self._vocabulary would be evaluated False by python as type None , and self.keys() is a method that would (hopefully) return something to be evaluated as well. Then it's just a logical OR between the two and the result gets put into self._vocabulary .