Backport PR #46057: bug: styler latex longtable column count (#46157)

Author: meeseeksmachine

doc/source/whatsnew/v1.4.2.rst

@@ -23,7 +23,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Fixed "longtable" formatting in :meth:`.Styler.to_latex` when ``column_format`` is given in extended format (:issue:`46037`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/io/formats/templates/latex_longtable.tpl

@@ -59,7 +59,7 @@
 {% if midrule is not none %}
 \{{midrule}}
 {% endif %}
-\multicolumn{% raw %}{{% endraw %}{{column_format|length}}{% raw %}}{% endraw %}{r}{Continued on next page} \\
+\multicolumn{% raw %}{{% endraw %}{{body[0]|length}}{% raw %}}{% endraw %}{r}{Continued on next page} \\
 {% if midrule is not none %}
 \{{midrule}}
 {% endif %}

pandas/tests/io/formats/style/test_to_latex.py

@@ -990,3 +990,10 @@ def test_clines_multiindex(clines, expected, env):
     styler.hide(level=1)
     result = styler.to_latex(clines=clines, environment=env)
     assert expected in result
+
+
+def test_col_format_len(styler):
+    # gh 46037
+    result = styler.to_latex(environment="longtable", column_format="lrr{10cm}")
+    expected = r"\multicolumn{4}{r}{Continued on next page} \\"
+    assert expected in result