Implement PartialEq for PyBytes and [u8]

[codeguru42]

This is my attempt to implement PartialEq for PyBytes and [u8]. See #4250.

[codeguru42]

This is my first draft for this PR. At this point, I just want to be sure I'm heading in the right general direction. I realize the meat of the implementation is incomplete. I have a couple comments/questions to fill in some gaps for me.

[codeguru42]

From what I can tell, all the other impls are just permutations of different types that ultimate call this impl. I have two questions here:

1. I copied this from types/string.py. But I removed the special implementation for 3.10 or ABI. I assume that is specific to the string implementation. Is that correct? Or is there an analogous implementation for bytes that is special for 3.10 or ABI?
2. I'm pretty sure to_cow() isn't a thing for PyBytes. I will work on an implementation here. Do I manually map (or loop) over the two parameters and compare each element? Or is there something more idiomatic in Rust?

[Icxolu]

I think you can use self.as_bytes() == other here (which should then use the PartialEq impl on slices on std)

[codeguru42]

@Icxolu Thanks for the feedback. I tried your suggestion and tests fail with

no implementation for `instance::Bound<'_, bytes::PyBytes> == &[u8; 12]

I guess we need the length of the array as part of the type? Or should I use Vec<u8> instead?

[Icxolu]

Ah, this is because let b = b"hello, world"; is creating an array (&[u8, 12]), not a slice (&[u8]). To test these impl you can turn that array into a slice like this: let b = b"hello, world".as_slice();

If we want we can also add implementations for arrays directly as well, using const generics, something like the following:

impl <const N: usize> PartialEq<[u8; N]> for Borrowed<'_, '_, PyString> { ... }

[codeguru42]

...or are my tests wrong and need changed?

[Icxolu]

They currently test comparison with arrays (which are nor implemented) instead of the comparison with slices. This can be a bit surprising, since often arrays can be automatically coerced to slices by the compiler. In this case the compiler is not smart enough to see that this is what we want.

If you change your test to use let b = b"hello, world".as_slice(); or let b: &[_] = b"hello, world";, we explicitly specify that we want b to be a slice as opposed to an array. This way it should then also use the PartialEq impl that you have implemented here.

[codeguru42]

For context, your first response wasn't showing when I typed my previous comment.

[codeguru42]

From what I can tell you have to specify the length of the array like [u8; 12]. Is there a way to make the length generic or should I use Vec<u8> instead?

[Icxolu]

I think you can use self.as_bytes() == other here (which should then use the PartialEq impl on slices on std)

[codeguru42]

This is working for PartialEq for PyBytes and [u8]. Do we want to merge this as-is or should I also add impls for [u8; N] as suggested above by icxolu?

[codeguru42]

I think I fixed one of the examples in the doc comment. I'm not sure how to fix the second one, though.

[Icxolu]

This is working for PartialEq for PyBytes and [u8]. Do we want to merge this as-is or should I also add impls for [u8; N] as suggested above by icxolu?

I would suggest we start with what we have here and if there is demand for the array comparisons, we can add them later as a followup.

[davidhewitt]

Looks great, thanks! Will merge now and rebase the 0.22 release on it tomorrow 👍

[alex]

Suggested change

	Implement `PartialEq<str>` for `Bound<'py, PyBytes>`.
	Implement `PartialEq<[u8]>` for `Bound<'py, PyBytes>`.

[codeguru42]

I saw this but wasn't able to accept it in time before the merge.

[codeguru42]

I'll fix it on my next PR for #4245

[davidhewitt]

No prob; I got it in #4266 thanks to @alex

[alex]

Changelog is wrong, so let's remember to fix when we do the release!