Python嵌套类混淆

Question

I am studying the idea about lazy evaluation. This is the Stream class I am having trouble with: refer to 4.2.6 Stream

class Stream:
  """A lazily computed linked list.""" 

  class Empty:
      def __repr__(self):
          return 'Stream.empty'
  empty = Empty()

  def __init__(self, first, compute_rest= lambda: empty):
      assert callable(compute_rest), 'compute_rest must be callable.'
      self.first = first
      self._compute_rest = compute_rest

  @property
  def rest(self):
      """Return the rest of the stream, computing it if necessary."""
      if self._compute_rest is not None:
          self._rest = self._compute_rest()
          self._compute_rest = None
      return self._rest

  def __repr__(self):
      return 'Stream({0}, <...>)'.format(repr(self.first))

Then I create a toy Stream for testing:

s = Stream(1, lambda: Stream(2+3, lambda: Stream(9)))

I want to know what will happen when I reach the end of Stream , so I do:

s.rest.rest.rest

What I expect is the screen to print out Stream.empty since the last element is lambda: empty , but I got error traceback message:

NameError                                 Traceback (most recent call last)
<ipython-input-6-64cf45661094> in <module>()
----> 1 s.rest.rest.rest

<ipython-input-4-7cc49730db55> in rest(self)
     16         """Return the rest of the stream, computing it if necessary."""
     17         if self._compute_rest is not None:
---> 18             self._rest = self._compute_rest()
     19             self._compute_rest = None
     20         return self._rest

<ipython-input-4-7cc49730db55> in <lambda>()
      7     empty = Empty()
      8 
----> 9     def __init__(self, first, compute_rest= lambda: empty):
     10         assert callable(compute_rest), 'compute_rest must be callable.'
     11         self.first = first

NameError: name 'empty' is not defined

So my question is, I do have my empty defined as a class variable, but the interpreter says it is not defined. If I take the Empty class definition out of nested class to the global frame, the code works.

Do I understand how nested class work wrong? Anyone please give me a hint. Thanks for your time.

Answer 1

The code in that post is invalid and broken. The lambda will look up the name empty in a parent scope. The problem here then is that a class body is not a scope nestable scope , so only the global scope remains for the lookup.

From the Execution Model documentation :

The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods

It doesn't matter here that the lambda is not a method but a callable bound as a default value for an argument for a method.

You can reference it using the class name instead:

def __init__(self, first, compute_rest= lambda: Stream.empty):

because Stream is now a global (set by running the class statement).