Overloading __dict__() on python class

Question

I have a class where I want to get the object back as a dictionary, so I implemented this in the __dict__(). Is this correct?

I figured once I did that, I could then use the dict (custom object), and get back the object as a dictionary, but that does not work.

Should you overload __dict__()? How can you make it so a custom object can be converted to a dictionary using dict()?

Answer 1

__dict__ is not a special method on Python objects. It is used for the attribute dictionary; dict() never uses it.

Instead, you could support iteration; when dict() is passed an iterable that produces key-value pairs, a new dictionary object with those key-value pairs is produced.

You can provide an iterable by implementing a __iter__ method, which should return an iterator. Implementing that method as a generator function suffices:

class Foo(object):     def __init__(self, *values):         self.some_sequence = values      def __iter__(self):         for key in self.some_sequence:             yield (key, 'Value for {}'.format(key)) 

Demo:

>>> class Foo(object): ...     def __init__(self, *values): ...         self.some_sequence = values ...     def __iter__(self): ...         for key in self.some_sequence: ...             yield (key, 'Value for {}'.format(key)) ...  >>> f = Foo('bar', 'baz', 'eggs', 'ham') >>> dict(f) {'baz': 'Value for baz', 'eggs': 'Value for eggs', 'bar': 'Value for bar', 'ham': 'Value for ham'} 

You could also subclass dict, or implement the Mapping abstract class, and dict() would recognize either and copy keys and values over to a new dictionary object. This is a little more work, but may be worth it if you want your custom class to act like a mapping everywhere else too.

Answer 2

No. __dict__ is a method used for introspection - it returns object attributes. What you want is a brand new method, call it as_dict, for example - that's the convention. The thing to understand here is that dict objects don't need to be necessarily created with dict constructor.