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

Asked By: code base 5000

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Source

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.

Answered By: Martijn Pieters

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.

Answered By: letitbee

Answer 3

The following method will allow an object of the class to be casted to a dict:

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

Answered By: mehmet

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