BUG: fix duplicate entries in LaTeX List of Tables when using longtable environments (#36297)

Author: jeschwar

doc/source/whatsnew/v1.2.0.rst

@@ -323,6 +323,7 @@ I/O
 - :meth:`to_csv` did not support zip compression for binary file object not having a filename (:issue:`35058`)
 - :meth:`to_csv` and :meth:`read_csv` did not honor `compression` and `encoding` for path-like objects that are internally converted to file-like objects (:issue:`35677`, :issue:`26124`, and :issue:`32392`)
 - :meth:`to_picke` and :meth:`read_pickle` did not support compression for file-objects (:issue:`26237`, :issue:`29054`, and :issue:`29570`)
+- Bug in :func:`LongTableBuilder.middle_separator` was duplicating LaTeX longtable entires in the List of Tables of a LaTeX document (:issue:`34360`)
 - Bug in :meth:`read_csv` with `engine='python'` truncating data if multiple items present in first row and first element started with BOM (:issue:`36343`)
 
 Plotting

pandas/io/formats/latex.py

@@ -431,13 +431,18 @@ class LongTableBuilder(GenericTableBuilder):
     >>> from pandas.io.formats import format as fmt
     >>> df = DataFrame({"a": [1, 2], "b": ["b1", "b2"]})
     >>> formatter = fmt.DataFrameFormatter(df)
-    >>> builder = LongTableBuilder(formatter, caption='caption', label='lab',
-    ...                            column_format='lrl')
+    >>> builder = LongTableBuilder(formatter, caption='a long table',
+    ...                            label='tab:long', column_format='lrl')
     >>> table = builder.get_result()
     >>> print(table)
     \\begin{longtable}{lrl}
-    \\caption{caption}
-    \\label{lab}\\\\
+    \\caption{a long table}
+    \\label{tab:long}\\\\
+    \\toprule
+    {} &  a &   b \\\\
+    \\midrule
+    \\endfirsthead
+    \\caption[]{a long table} \\\\
     \\toprule
     {} &  a &   b \\\\
     \\midrule
@@ -476,7 +481,16 @@ def _caption_and_label(self) -> str:
     @property
     def middle_separator(self) -> str:
         iterator = self._create_row_iterator(over="header")
+
+        # the content between \endfirsthead and \endhead commands
+        # mitigates repeated List of Tables entries in the final LaTeX
+        # document when dealing with longtable environments; GH #34360
         elements = [
+            "\\midrule",
+            "\\endfirsthead",
+            f"\\caption[]{{{self.caption}}} \\\\" if self.caption else "",
+            self.top_separator,
+            self.header,
             "\\midrule",
             "\\endhead",
             "\\midrule",

pandas/tests/io/formats/test_to_latex.py

@@ -411,6 +411,11 @@ def test_to_latex_longtable(self):
         df = DataFrame({"a": [1, 2], "b": ["b1", "b2"]})
         withindex_result = df.to_latex(longtable=True)
         withindex_expected = r"""\begin{longtable}{lrl}
+\toprule
+{} &  a &   b \\
+\midrule
+\endfirsthead
+
 \toprule
 {} &  a &   b \\
 \midrule
@@ -430,6 +435,11 @@ def test_to_latex_longtable(self):
 
         withoutindex_result = df.to_latex(index=False, longtable=True)
         withoutindex_expected = r"""\begin{longtable}{rl}
+\toprule
+ a &  b \\
+\midrule
+\endfirsthead
+
 \toprule
  a &  b \\
 \midrule
@@ -525,6 +535,9 @@ def test_to_latex_longtable_caption_label(self):
 
         df = DataFrame({"a": [1, 2], "b": ["b1", "b2"]})
 
+        # test when no caption and no label is provided
+        # is performed by test_to_latex_longtable()
+
         # test when only the caption is provided
         result_c = df.to_latex(longtable=True, caption=the_caption)
 
@@ -533,6 +546,11 @@ def test_to_latex_longtable_caption_label(self):
 \toprule
 {} &  a &   b \\
 \midrule
+\endfirsthead
+\caption[]{a table in a \texttt{longtable} environment} \\
+\toprule
+{} &  a &   b \\
+\midrule
 \endhead
 \midrule
 \multicolumn{3}{r}{{Continued on next page}} \\
@@ -552,6 +570,11 @@ def test_to_latex_longtable_caption_label(self):
 
         expected_l = r"""\begin{longtable}{lrl}
 \label{tab:longtable}\\
+\toprule
+{} &  a &   b \\
+\midrule
+\endfirsthead
+
 \toprule
 {} &  a &   b \\
 \midrule
@@ -578,6 +601,11 @@ def test_to_latex_longtable_caption_label(self):
 \toprule
 {} &  a &   b \\
 \midrule
+\endfirsthead
+\caption[]{a table in a \texttt{longtable} environment} \\
+\toprule
+{} &  a &   b \\
+\midrule
 \endhead
 \midrule
 \multicolumn{3}{r}{{Continued on next page}} \\
@@ -623,6 +651,11 @@ def test_to_latex_longtable_position(self):
         result_p = df.to_latex(longtable=True, position=the_position)
 
         expected_p = r"""\begin{longtable}[t]{lrl}
+\toprule
+{} &  a &   b \\
+\midrule
+\endfirsthead
+
 \toprule
 {} &  a &   b \\
 \midrule