Pandas annotations

[genedan]

Summary of Changes

Add type annotations to pandas.py. Add API reference examples to TrianglePandas.dropna(). This method really needed examples to illustrate its complex behavior. I'll try to stick to just 1 or 2 methods next time if they turn out to be as complex.

Related GitHub Issue(s)

#486
#704

Additional Context for Reviewers

Removed line 303:

self.values = np.where((xp.nan_to_num(self.values) == 0) * (self.nan_triangle == 1), self.nan_triangle * 0, self.values)

This line didn't do anything, it simplifies to self.values = self.values, so I removed it.

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

---

Note

Medium Risk
fillna no longer accepts implicit None and uses a different inplace value merge, which can change runtime behavior for callers that relied on the old path; other changes are mostly typing and documentation.

Overview
Adds static typing across TrianglePandas by typing mixin methods with TriangleProtocol, tightening signatures (e.g. _get_axis, plot/hvplot), and extending TriangleProtocol in typing.py with the properties and methods those pandas mirrors need.

Triangle.dropna() gains a large doctest-backed API reference that spells out edge-only trimming (origins/development at the ends, multi-index aggregation), plus small inline comments in the implementation. to_frame(keepdims=True) gets minor cleanup: COO cast for sparse values, valuation built via a named Series, and removal of invalid variable type annotations on DataFrame column assignments.

fillna now requires an explicit fill value (TypeError if value is None) and simplifies inplace updates to self.values = (self + fill).values instead of the prior frame = self + value * 0 path. fillzero drops a redundant np.where assignment that was effectively a no-op and keeps the real NaN-mask fill logic.

^{Reviewed by Cursor Bugbot for commit 53fffc4. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.33%. Comparing base (468f11a) to head (53fffc4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
chainladder/core/pandas.py	95.34%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1035      +/-   ##
==========================================
- Coverage   89.38%   89.33%   -0.05%     
==========================================
  Files          91       89       -2     
  Lines        5266     5177      -89     
  Branches      678      662      -16     
==========================================
- Hits         4707     4625      -82     
+ Misses        391      387       -4     
+ Partials      168      165       -3     

Flag	Coverage Δ
unittests	89.33% <95.34%> (-0.05%)	⬇️

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[genedan]

I'll fix that codecov comment in another PR, I think this one's already kind of big.

[henrydingliu]

two casts (i guess technically 3) in 2 lines. why the differences?

• one upcasts to object but ones doesn't
• one casts to triangleprotocal one casts to triangle

[genedan]

The double-cast pattern is used to eliminate a certain IDE warning. If I were to do a single cast:

cast("TriangleProtocol", obj)

I'd get this warning from my IDE:

Cast of type 'Triangle | Series' to type 'TriangleProtocol' may be a mistake because they are not in the same inheritance hierarchy. If this was intentional, cast the expression to 'object' first.

The next cast doesn't throw the warning since we're casting to Triangle, which is one of Triangle | Series. I wonder if casting is a crutch to cover up for some weaknesses in the code. I have a hunch that if we had clearer branching in the functions, the return type would be unambiguous and the type checker would know which one of Triangle | Series it'd be. But that's just a hunch, I guess I've gotten used to dynamic typing for far too long...

[henrydingliu]

hmmm, where is your IDE getting Triangle|Series from? self is typed to TriangleProtocol at the top.

and when should we type to Triangle and when should we type to TriangleProtocol?