Dead code in TriangleDunders._prep_columns()

[genedan]

Are you on the latest chainladder version?

• Yes, this bug occurs on the latest version.

Describe the bug in words

Lines 147 - 150 can't be reached:

chainladder-python/chainladder/core/dunders.py

Lines 114 to 152 in 6f80200

	def _prep_columns(self, x, y):
	x_backend, y_backend = x.array_backend, y.array_backend
	if len(x.columns) == 1 and len(y.columns) > 1:
	x.vdims = y.vdims
	elif len(y.columns) == 1 and len(x.columns) > 1:
	y.vdims = x.vdims
	elif len(y.columns) == len(x.columns) == 1 and x.columns != y.columns:
	y.vdims = x.vdims
	elif x.shape[1] == y.shape[1] and np.array_equal(x.columns, y.columns):
	return x, y
	else:
	# Find columns to add to each triangle
	cols_to_add_to_x = [col for col in y.columns if col not in x.columns]
	cols_to_add_to_y = [col for col in x.columns if col not in y.columns]
	# Start with case with no new columns, y simply has a different order
	new_x_cols = list(x.columns)
	# Then, if there are new columns, add them.
	if cols_to_add_to_x:
	new_x_cols = list(x.columns) + list(cols_to_add_to_x)
	x = x.reindex(columns=new_x_cols, fill_value=0)
	if cols_to_add_to_y:
	new_y_cols = list(y.columns) + list(cols_to_add_to_y)
	y = y.reindex(columns=new_y_cols, fill_value=0)
	# Ensure both triangles have the same column order
	x = x[new_x_cols]
	y = y[new_x_cols]
	# Reset backends only if they've changed
	if x.array_backend != x_backend:
	x = x.set_backend(x_backend, inplace=True)
	if y.array_backend != y_backend:
	y = y.set_backend(y_backend, inplace=True)
	return x, y

How can the bug be reproduced?

This was discovered when trying to write unit tests for them. This diagnostic was generated by Claude, so I want to double check whether it's correct before we remove the lines:

● These lines are dead code — the backend never changes inside _prep_columns. Here's why:

  1. _prep_columns is always called after _compatibility_check, which calls set_common_backend to align both triangles to the same backend before we get here.
  2. None of the operations inside _prep_columns (vdims assignment, reindex, column slicing) change the backend — I verified this empirically with a monkey-patch that would detect any change.
  So x.array_backend != x_backend and y.array_backend != y_backend are always False, and lines 146/148 can never execute. There's no test that can reach them.

What is the expected behavior?

My IDE detects that there's only 1 usage of TriangleDunders._prep_columns(), and it does occur after _compatibility_check(), so item 1 is correct. We just need to check item 2 before we remove the lines.

Would you be willing to contribute this ticket?

• Yes, absolutely!
• Yes, but I would like some help.
• No.