feat(core): allow Triangle.drop() development dropping, label validation, and error options

[priyam0k]

Summary of Changes

completes Triangle.drop() functionality for #1052:

• drop development periods via development=, axis=3, or axis='development'. requires dropping first/last periods to prevent gaps.
• validates labels=None. raises ValueError if no labels or axis keywords are passed.
• adds level and errors parameters (errors='ignore' suppresses missing label KeyErrors).

Related GitHub Issue(s)

closes #1057 #1062 #1061
part of #1052

Additional Context for Reviewers

#1051 (index label dropping) remains

Checklist

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run --directory docs jb build . --builder=custom --custom-builder=doctest)

---

Note

Medium Risk
Changes core Triangle slicing/validation behavior; wrong edge cases could alter triangle shape, but rules mirror existing origin drops and are heavily tested.

Overview
Triangle.drop() now supports dropping development periods via development=, axis=3, or axis='development', using the same first/last-only rule as origin (interior drops raise ValueError). Missing labels on columns, origin, and development honor a new errors argument ('raise' vs 'ignore'); level is documented for API parity. Calling drop() with no labels/axis keywords now raises ValueError, and column drops check for missing names before slicing.

Docs gain a development-drop doctest; NotImplementedError text now lists column, origin, and development (index still unimplemented).

Minor cleanups: isinstance instead of type() == in hvplot and tests; remove an unused xp in groupby aggregation; test/style fixes (is_cumulative booleans, expanded drop coverage).

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[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.0% of exported symbols fully typed (201 / 1336)

	Known	Ambiguous	Unknown	Total
Project (head)	201	111	1024	1336

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 321
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 0.0% fully typed (0 / 9); 1 no longer exported

	Known	Ambiguous	Unknown	Total
Patch	0	0	9	9

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.test_drop_both_labels_and_alternative_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_development_axis_equivalents	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_development_first	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_development_interior_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_development_last	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_development_missing_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_errors_ignore	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_index_alternative_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_no_labels_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_index_origin_development_alternatives_raise	—	no longer exported (was ❌ unknown)

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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^{Reviewed by Cursor Bugbot for commit 23030c6. Configure here.}

[cursor]

Development drop skips valuation date

Medium Severity

Development drop() slices ddims through _slice and never refreshes valuation_date. On a valuation triangle, removing the latest period leaves valuation_date on a removed calendar date, so latest_diagonal and other valuation-date checks no longer match remaining cells.

 

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[codecov]

Codecov Report

❌ Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.46%. Comparing base (26a4a5c) to head (23030c6).
⚠️ Report is 317 commits behind head on main.

Files with missing lines	Patch %	Lines
chainladder/core/pandas.py	85.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1207      +/-   ##
==========================================
+ Coverage   91.25%   91.46%   +0.21%     
==========================================
  Files          91       91              
  Lines        5395     5577     +182     
  Branches      689      743      +54     
==========================================
+ Hits         4923     5101     +178     
- Misses        338      339       +1     
- Partials      134      137       +3     

Flag	Coverage Δ
unittests	91.46% <85.00%> (+0.21%)	⬆️

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[henrydingliu]

these reviews are gonna be challenging, with the mix of ruff debugging and actual code change.

@genedan would love to get your take