Group list members optimization when scanning keys

[alimanfoo]

When a store does not implement listdir and keys need to be scanned, there is a possible optimization for listing members of a group because child arrays and groups could be discovered within the key scan, rather than requiring additional __contains__ tests.

[marynaggita]

Hi @joshmoore, I am Mary, an outreachy candidate. Iam interested in this issue.

[marynaggita]

Kindly provide pointers which i can look at

[joshmoore]

@marynaggita: the listdir() method that is being referred to can be found here: https://github.com/zarr-developers/zarr-python/blob/main/zarr/storage.py#L214-L220

It uses _listdir_from_keys() method here:
https://github.com/zarr-developers/zarr-python/blob/main/zarr/_storage/store.py#L409

Maybe the first thing to do is to find a test that uses the listdir method.