Fix TypeError when stacking two headerless Datasets with stack_cols

[vineethsaivs]

Problem

Dataset.stack_cols() crashes with an opaque TypeError when both datasets have no headers, even though the method is written to allow that case:

>>> import tablib
>>> a = tablib.Dataset(); a.append(("x", "y")); a.append(("p", "q"))
>>> b = tablib.Dataset(); b.append(("1", "2", "3")); b.append(("4", "5", "6"))
>>> a.stack_cols(b)
TypeError: 'NoneType' object is not iterable

The method already anticipates two headerless datasets:

• the header guard raises HeadersNeeded only when exactly one side has headers, and
• the header concatenation catches None + None and sets new_headers = None.

But it then does for column in self.headers: to copy the columns, which iterates None and raises. Row-level stack() handles headerless datasets fine, so the column-level crash is an internal inconsistency rather than intended behavior.

Fix

When neither dataset has headers, copy the columns by index with get_col() instead of by header name. The existing headered path is unchanged (it is just moved under if self.headers:), so headered stacking behaves exactly as before.

Test

Added test_column_stacking_headerless, which stacks two headerless datasets and asserts the combined rows and headers is None. It fails on the current code with the TypeError above and passes with this change; the existing test_column_stacking (headered) still passes.

Note

This touches the same method as the open PR #649 ("Raise TypeError for invalid Dataset instance"), but a different line: #649 changes the isinstance(other, Dataset) guard, while this fixes the headerless column-iteration path. The two do not overlap and either order rebases trivially.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.28%. Comparing base (64f0201) to head (862a731).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #651      +/-   ##
==========================================
+ Coverage   93.19%   93.28%   +0.09%     
==========================================
  Files          29       29              
  Lines        3247     3263      +16     
==========================================
+ Hits         3026     3044      +18     
+ Misses        221      219       -2     

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