SimpleLazyObject doesn't implement __radd__

[Keryn Knight]

Technically, there's a whole bunch of magic methods it doesn't implement, compared to a complete proxy implementation, like that of wrapt.ObjectProxy, but __radd__ being missing is the one that's biting me at the moment.

As far as I can tell, the implementation can't just be

__radd__ = new_method_proxy(operator.radd)

because that doesn't exist, which is rubbish.

__radd__ = new_method_proxy(operator.attrgetter("__radd__"))

also won't work because types may not have that attr, and attrgetter doesn't supress the exception (correctly)

The minimal implementation I've found that works for me is:

    def __radd__(self, other):
        if self._wrapped is empty:
            self._setup()
        return other + self._wrapped

[Tim Graham]

Could you please give some sample code with your use case?

[Keryn Knight]

In a boiled-down nutshell:

def lazy_consumer():
    # something more complex, obviously.
    return [1, 3, 5]
consumer = SimpleLazyObject(lazy_consumer)

# inside third party code ...
def some_func(param):
    third_party_code = [...]
    # then, through parameter passing, my value is provided to be used.
    # param is at  this point, `consumer`
    third_party_code_plus_mine = third_party_code + param

which ultimately yields:

TypeError: unsupported operand type(s) for +: 'list' and 'SimpleLazyObject'

[Tim Graham]

Seems okay, although I'm not an expert on the SimpleLazyObject class.

[Attila Tovt]

Replying to kezabelle:

def lazy_consumer():
    # something more complex, obviously.
    return [1, 3, 5]
consumer = SimpleLazyObject(lazy_consumer)

If you know what is the resulting type or possible resulting types of your expression, I think you better use django.utils.functional.lazy which will provide all the necessary methods.

[Debanshu Kundu]

Replying to kezabelle:

As far as I can tell, the implementation can't just be

__radd__ = new_method_proxy(operator.radd)

because that doesn't exist, which is rubbish.

__radd__ = new_method_proxy(operator.attrgetter("__radd__"))

also won't work because types may not have that attr, and attrgetter doesn't supress the exception (correctly)

Wouldn't the following code work?

    __add__ = new_method_proxy(operator.add)
    __radd__ = new_method_proxy(lambda a, b: operator.add(b, a))

I have tested this and it seems to work as excepted.

[Theofilos Alexiou]

PR here: ​https://github.com/django/django/pull/15400. Tried the suggestion by @Debanshu Kundu and it seems to work.

[Mariusz Felisiak]

RFC should be set by reviewer, see ​docs.

[Mariusz Felisiak <felisiak.mariusz@…>]

In f9ec777a:

Fixed #26287 -- Added support for addition operations to SimpleLazyObject.