BUG-17280 to_html follows display.precision for column numbers in notebooks

doc/source/whatsnew/v0.25.0.rst

@@ -353,6 +353,7 @@ I/O
 - Bug in :meth:`DataFrame.to_string` and :meth:`DataFrame.to_latex` that would lead to incorrect output when the ``header`` keyword is used (:issue:`16718`)
 - Bug in :func:`read_csv` not properly interpreting the UTF8 encoded filenames on Windows on Python 3.6+ (:issue:`15086`)
 - Improved performance in :meth:`pandas.read_stata` and :class:`pandas.io.stata.StataReader` when converting columns that have missing values (:issue:`25772`)
+- Bug in :meth:`DataFrame.to_html` where header numbers would ignore display options when rounding (:issue:`17280`)
 - Bug in :func:`read_hdf` not properly closing store after a ``KeyError`` is raised (:issue:`25766`)
 - Bug in ``read_csv`` which would not raise ``ValueError`` if a column index in ``usecols`` was out of bounds (:issue:`25623`)
 

pandas/io/formats/html.py

@@ -70,6 +70,9 @@ def row_levels(self):
         # not showing (row) index
         return 0
 
+    def _get_columns_formatted_values(self):
+        return self.columns
+
     @property
     def is_truncated(self):
         return self.fmt.is_truncated
@@ -292,7 +295,7 @@ def _write_col_header(self, indent):
                     row.append(self.columns.name or '')
                 else:
                     row.append('')
-            row.extend(self.columns)
+            row.extend(self._get_columns_formatted_values())
             align = self.fmt.justify
 
             if truncate_h:
@@ -494,6 +497,9 @@ class NotebookFormatter(HTMLFormatter):
     def _get_formatted_values(self):
         return {i: self.fmt._format_col(i) for i in range(self.ncols)}
 
+    def _get_columns_formatted_values(self):
+        return self.columns.format()
+
     def write_style(self):
         # We use the "scoped" attribute here so that the desired
         # style properties for the data frame are not then applied

pandas/tests/io/formats/test_to_html.py

@@ -633,3 +633,13 @@ def test_to_html_invalid_classes_type(classes):
 
     with pytest.raises(TypeError, match=msg):
         df.to_html(classes=classes)
+
+
+def test_to_html_round_column_headers():
+    # GH 17280
+    df = DataFrame([1], columns=[0.55555])
+    with pd.option_context('display.precision', 3):
+        html = df.to_html(notebook=False)
+        notebook = df.to_html(notebook=True)
+    assert "0.55555" in html
+    assert "0.556" in notebook