What is the best way to compare two objects within a PyIterator (PyAny)

[tgross35]

Hello all,

I have a handful of existing functions that require PartialEq on items in an iterator. What is a good way to transform from a PyIterator to this? It seems like this type has PyAny as its item - and even though this has an eq function, it can't be directly compared.

I am thinking that two options are either to try to downcast into various python types that might be comparable, or to implement PartialEq by unwrapping the eq and ne methods on PyAny, but it seems like there must be a reason those options aren't available by default.

[davidhewitt]

implement PartialEq by unwrapping the eq and ne methods on PyAny,

You probably want to wrap your PyAny objects in a newtype which does exactly this, or alternatively could return None from PartialEq::cmp if there's an exception raised during comparison. For now we're undecided what the correct error handling strategy is if PyO3 were to provide a PartialEq implementation.

Some brief history - PyAny used to implement PartialEq using pointer comparison (i.e. Python is), which was almost certainly a footgun. We removed that implementation a couple of versions ago, and now in a cooling off period to take stock before we consider if we add a more correct implementation back.

[tgross35]

Interesting, thank you for the context!

On a similar note, is there a good way to get a impl DoubleEndedIterator from a PySequence?

[davidhewitt]

I think you'd need to again create a newtype DoubleEndedIterator<'a>(&'a PySequence) and implement it yourself. Should be straightforward enough; only thing to bear in mind is that because Python objects are freely mutable you might want to check length on each iterator (& panic if changed?)