Question

I have a Python class that stores some fields and has some properties, like

class A(object):
  def __init__(self, x, y):
    self.x = x
    self.y = y

  @property
  def z(self):
    return self.x+1

What changes do I need to make to the class so that I can do

>>> a = A(1,5)
>>> dict(a)
{'y':5, 'z':2}

where I specify that I want to return y and z? I can’t just use a.__dict__ because it would contain x but not z. I would like to be able to specify anything that can be accessed with __getattribute__.

Answer 1

Add an __iter__() method to your class that returns an iterator of the object’s items as key-value pairs. Then you can pass your object instance directly to the dict() constructor, as it accepts a sequence of key-value pairs.

def __iter__(self):
    for key in "y", "z":
        yield key, getattr(self, key)

If you want this to be a little more flexible, and allow the list of attributes to be easily overridden on subclasses (or programmatically) you could store the key list as an attribute of your class:

_dictkeys = "y", "z"

def __iter__(self):
    for key in self._dictkeys:
        yield key, getattr(self, key)

If you want the dictionary to contain all attributes (including those inherited from parent classes) try:

def __iter__(self):
    for key in dir(self):
        if not key.startswith("_"):
            value = getattr(self, key)
            if not callable(value):
                yield key, value

This excludes members that start with “_” and also callable objects (e.g. classes and functions).