Column-level metadata/functional arguments to aq.matches

[bmschmidt]

This is an alternate solution related to the question at #33 about selecting certain subsets of columns based on types.

Arquero could have a metadata slot on each column that borrows from Apache Arrow's support of table and column-level metadata. If my arrow column is of type arrow.utf8 with a metadata field saying "language": "English", if would be useful to have an arquero table derived from it at some point declare table.metadata = {"language": "English", "arrow_type": "utf8"}. The pyarrow feather export functions do something similar with pandas frames: the feather metadata includes a description of the pandas dtypes.

If a function as argument to aq.matches worked with reference to the full column (not just the name as for strings and regexes) @ericemc3's case in #33 could be expressed something like this:

table.select(aq.matches(col => col.metadata.arrow_type.match(/int|float/)))

col.metadata would also conceivably be a useful place to expose information about what autotype inference in fromCSV and fromJSON decided to do.

Could try to find time for a pull request, but obviously this hinges on questions about how/whether you want to use slots on the column object other than data.

[jheer]

I had also thought about generally allowing a column metadata property, but it hasn’t (yet) been a priority. PRs welcome!