types: add PyMapping

[davidhewitt]

Adds a PyMapping type - similar to PySequence, but specially to interact with the mapping protocol.

It doesn't have get_item, set_item, and del_item methods because they're already present because of Deref to PyAny. However it contains a few other helpful pieces such as .contains_key(), .keys(), .values(), and .items().

Seemed like it might be useful for davidhewitt/pythonize#16 (cc @i1i1)

WIP - needs tests.

[birkenfeld]

As I proposed in #1845 for sequences, the API here should mirror PyDict as closely as possible.

[birkenfeld]

If we use HasKey which suppresses exceptions, the method can return just bool?

Otherwise, use GetItem internally to propagate the exceptions...

[davidhewitt]

👍 will update this to follow what you've done in #1849 but for PyDict / PyMapping

[davidhewitt]

Updated to contain the other relevant PyDict APIs and added tests.

In the end I couldn't come up with a good decision for .contains_key. I felt it was potentially confusing to implement it using PyMapping_HasKey, because the suppressed exception might not be a KeyError but some other cause (e.g. failed comparison when comparing two keys).

So for now I left .contains_key out.

[davidhewitt]

Any objections if I merge this?

[birkenfeld]

Could add an entry in guide/src/conversions/tables.md I guess.

[davidhewitt]

👍 good idea, done!

[Gobot1234]

Is there a reason that contains_key wasn't implemented, it would be much better than obj.getattr("__contains__")?.call1((key,))?.is_true()?

[davidhewitt]

See #1844 (comment)

I just couldn't make an obvious decision on the semantics of contains_key in this initial PR. If you're willing to come up with an appropriate implementation and happy to open a PR for it, I'd be happy to add it to this type.

[birkenfeld]

Don't we have a method for the in operator on PyAny?

[Gobot1234]

Not that I could find?