Add value counts aggregation to select.group_by

[ebhills]

Summary

• add a counts aggregation to select.group_by
• return each distinct value and its frequency as a dictionary
• convert dictionary keys to JSON-safe strings
• represent missing values as null and booleans as lowercase true or false
• support existing per-column output renaming behavior

Why

Grouped recipes sometimes need the distribution of values within each group rather than only a scalar count or a list of the original values. Without this aggregation, recipes need additional transformations to construct a value-to-frequency mapping.

Impact

Recipes can now request value counts directly:

wrangles:
  - select.group_by:
      by: Group
      counts:
        - Selection: Selection Counts
      auto_rename_columns: false

The resulting Selection Counts value is a JSON-safe dictionary such as {"Primary": 2, "None": 1}.

Validation

• complete TestGroupBy class: 30 passed
• grouped smoke coverage for strings, nulls, and booleans passed
• git diff --check passed
• two existing Python invalid-escape SyntaxWarning warnings were emitted; no test failures
• branch differs from current main by one commit and two files

Workflow

This is the first pull request for the feature and targets the dev staging destination. After staging validation, open a second pull request from the same recovery/select-group-by-counts branch into main. Do not delete the branch after the dev merge.

[Copilot]

Pull request overview

Adds a new counts aggregation option to select.group_by so grouped recipes can return per-value frequency distributions (as JSON-safe dictionaries) instead of only scalar aggregates.

Changes:

• Extend select.group_by to support a counts aggregation that returns {value_as_string: frequency} per group.
• Normalize keys for JSON-safety (string conversion, lowercase boolean strings, and a "null" key for missing values).
• Add a unit test validating the new aggregation and per-column output renaming with auto_rename_columns: false.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
wrangles/recipe_wrangles/select.py	Adds schema documentation and implements the counts aggregation using value_counts(dropna=False) with JSON-safe key normalization.
tests/recipes/wrangles/test_select.py	Adds a test covering counts output structure and column renaming behavior.