How can I replace a class @property's len(..) return with a custom __len__() procedure?

Question

When a class is defined (ex. class MyClass), it is possible to replace what it returns to a len(my_class_instance) using def __len__(self).

• How can I apply this to a custom class @property in the below scenario?

  class CarWash(object):
      def __init__(self):
          self._queue = [1, 2, 3]
  
      @property
      def queue(self):
          return self._queue
  

when len(car_wash.queue) is called, I don't want the list's len to be returned but would like to replace it with a custom __len__ procedure, since I want to query the db first.

How can I do this?

Answer 1

If you don't want list.__len__ to be called, don't return a list:

def CarQueue(list):
    def __len__(self):
        return something_dubious()
    # if you're going to override len, make sure to overide `__iter__`
    # and `__getitem__`

class CarWash(object):
    def __init__(self):
        self._queue = CarQueue([1, 2, 3])

    @property
    def queue(self):
        return self._queue