Let default_field_resolver check "typing.Mapping" instead of the more restrictive "dict"

[jstlaurent]

Feature requests

The current default_field_resolver checks against instances of dict specifically to decide whether to use get or getattr to access the field on the source. Checking against typing.Mapping would be more flexible, allowing, for example, a ChainMap to be used as the underlying source.

[Cito]

Sounds like a good idea. But we need to explore these questions:

• Maybe we should check against collections.abc.Mapping instead? What are the pros/cons?
• Performance implications: A quick test showed that using collections.abc.Mapping is ~ 6x slower, typing.Mapping is ~ 14x slower than using dict. This may be problematic, since the checks happen so frequently.

[jstlaurent]

Those are very good questions. Thank you for testing the performance. I wonder if there is a good way to benchmark the impact on the whole process of resolving an object structure.

[Cito]

It is hardly measurable with our standard benchmarks and probably doesn't matter much in real life as well.

So I've implemented this now and added a test using ChainMap. I've used abc.collections.Mapping because typing.Mapping is intended to be used for type checking, not for runtime checks, and more or less only proxies the checks anyway.

Will be available in the next release.

[jstlaurent]

Thank you very much!