Public api change: rename adata to data and make it positional only

Makes sense to me! Make sure to add a / to the parameter specification to enforce that people aren’t actually using the name.

And put the / after all the “main” parameters if there are more than just data. Depends on your judgement what a “main” parameters are – it’s often the same list as the mandatory parameters, unless a function has too many mandatory parameters, or multiple mandatory parameters have the same type and the order could be confusing.

I think most good APIs have 1–2 positional-only parameters (might have defaults), and some keyword-only parameters that all have defaults. Most functions have zero positional-or-keyword parameters, but for some it makes sense.

e.g. var_by_distance would look like this:

def var_by_distance(
    adata: AnnData,
    groups: str | list[str] | NDArrayA,
    /,
    *,
    cluster_key: str | None = None,
    ...
) -> ...:

whereas nhood_enrichment would look like this, so people can call it either as nhood_enrichment(adata, "k") or nhood_enrichment(adata, cluster_key=..., library_key=...) to clarify which key is which.

def nhood_enrichment(
    data: AnnData | SpatialData,
    /,
    cluster_key: str,
    *,
    library_key: str | None = None,
    connectivity_key: str | None = None,
    ...
) -> ...:

Originally posted by @flying-sheep in https://github.com/scverse/squidpy/issues/1134#issuecomment-4458065013

Start by searching the Python source for public functions using adata, beginning with the var_by_distance and nhood_enrichment examples named in the issue. Review which parameters are main versus keyword-only, then consider the work complete when the affected public signatures consistently use positional-only data as specified.