ENH: add named based column css styling to pandas Styler

Author: attack68

doc/source/user_guide/style.ipynb

@@ -793,7 +793,8 @@
    "source": [
     "The next option you have are \"table styles\".\n",
     "These are styles that apply to the table as a whole, but don't look at the data.\n",
-    "Certain sytlings, including pseudo-selectors like `:hover` can only be used this way."
+    "Certain stylings, including pseudo-selectors like `:hover` can only be used this way.\n",
+    "These can also be used to set specific row or column based class selectors, as will be shown."
    ]
   },
   {
@@ -831,9 +832,31 @@
     "The value for `props` should be a list of tuples of `('attribute', 'value')`.\n",
     "\n",
     "`table_styles` are extremely flexible, but not as fun to type out by hand.\n",
-    "We hope to collect some useful ones either in pandas, or preferable in a new package that [builds on top](#Extensibility) the tools here."
+    "We hope to collect some useful ones either in pandas, or preferable in a new package that [builds on top](#Extensibility) the tools here.\n",
+    "\n",
+    "`table_styles` can be used to add column and row based class descriptors. For large tables this can increase performance by avoiding repetitive individual css for each cell, and it can also simplify style construction in some cases.\n",
+    "`Styler.extend_column_styles` is a specific function to set styles to individual columns.\n",
+    "\n",
+    "Note that `Styler.set_table_styles` will overwrite existing styles (including those column based) whilst the preferred `Styler.extend_table_styles` will add to the current items."
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "outputs": [],
+   "source": [
+    "html = html.extend_column_styles({\n",
+    "    'B': [dict(selector='', props=[('color', 'blue')])]\n",
+    "})\n",
+    "html"
+   ],
+   "metadata": {
+    "collapsed": false,
+    "pycharm": {
+     "name": "#%%\n"
+    }
+   }
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -922,10 +945,12 @@
     "- DataFrame only `(use Series.to_frame().style)`\n",
     "- The index and columns must be unique\n",
     "- No large repr, and performance isn't great; this is intended for summary DataFrames\n",
-    "- You can only style the *values*, not the index or columns\n",
+    "- You can only style the *values*, not the index or columns (except with `table_styles` above)\n",
     "- You can only apply styles, you can't insert new HTML entities\n",
     "\n",
-    "Some of these will be addressed in the future.\n"
+    "Some of these will be addressed in the future.\n",
+    "Performance can suffer when adding styles to each cell in a large DataFrame.\n",
+    "It is recommended to apply table or column based styles where possible to limit overall HTML length, as well as setting a shorter UUID to avoid unnecessary repeated data transmission. \n"
    ]
   },
   {
@@ -1237,8 +1262,17 @@
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
    "version": "3.7.0"
+  },
+  "pycharm": {
+   "stem_cell": {
+    "cell_type": "raw",
+    "source": [],
+    "metadata": {
+     "collapsed": false
+    }
+   }
   }
  },
  "nbformat": 4,
  "nbformat_minor": 1
-}
+}

pandas/io/formats/style.py

@@ -939,6 +939,86 @@ def set_table_styles(self, table_styles) -> "Styler":
         self.table_styles = table_styles
         return self
 
+    def extend_table_styles(self, table_styles) -> "Styler":
+        """
+        Extend the existing table styles on a Styler.
+
+        These are placed in a ``<style>`` tag before the generated HTML table.
+
+        Parameters
+        ----------
+        table_styles : list
+            Each individual table_style should be a dictionary with
+            ``selector`` and ``props`` keys. ``selector`` should be a CSS
+            selector that the style will be applied to (automatically
+            prefixed by the table's UUID) and ``props`` should be a list of
+            tuples with ``(attribute, value)``.
+
+        Returns
+        -------
+        self : Styler
+
+        Examples
+        --------
+        >>> df = pd.DataFrame(np.random.randn(10, 4))
+        >>> df.style.set_table_styles(
+        ...     [{'selector': 'tr:hover',
+        ...       'props': [('background-color', 'yellow')]}]
+        ... ).extend_table_styles(
+        ...     [{'selector': '.col2',
+        ...       'props': [('background-color', 'blue')]}]
+        ...)
+        """
+        if self.table_styles is None:
+            return self.set_table_styles(table_styles)
+        self.table_styles.extend(table_styles)
+        return self
+
+    def extend_column_styles(self, column_styles) -> "Styler":
+        """
+        Sets class styles for each column on a Styler.
+
+        These are placed in a ``<style>`` tag before the generated HTML table.
+
+        Parameters
+        ----------
+        column_styles : dict
+            Each key of the dict should be a column header with the value
+            being a list of individual styles. Each style is a dictionary with
+            ``selector`` and ``props`` keys. ``selector`` should be a CSS
+            selector that the style will be applied to (automatically
+            prefixed by the table's UUID) and ``props`` should be a list of
+            tuples with ``(attribute, value)``.
+
+        Returns
+        -------
+        self : Styler
+
+        Notes
+        -----
+        Where the ``selector`` is an empty string or `None` the ``props`` will
+        be applied to the column class generically.
+
+        Examples
+        --------
+        >>> df = pd.DataFrame(np.random.randn(3, 2), columns=['a', 'b'])
+        >>> df.style.extend_column_styles({
+        ...        'a': [{'selector': '',
+        ...               'props': [('font-size', '20px')]},
+        ...              {'selector': 'td:hover',
+        ...               'props': [('color', 'pink')]}],
+        ...        'b': [{'selector': '',
+        ...               'props': [('font-size', '10px')]}]
+        ... })
+        """
+        _styles = []
+        for col, styles in column_styles.items():
+            for s in styles:
+                c = str(self.data.columns.get_loc(col))
+                _styles.append({'selector': s['selector'] + '.col' + c,
+                                'props': s['props']})
+        return self.extend_table_styles(_styles)
+
     def set_na_rep(self, na_rep: str) -> "Styler":
         """
         Set the missing data representation on a Styler.

pandas/tests/io/formats/test_style.py

@@ -1691,6 +1691,28 @@ def test_no_cell_ids(self):
         s = styler.render()  # render twice to ensure ctx is not updated
         assert s.find('<td  class="data row0 col0" >') != -1
 
+    def test_extending_table_styles(self):
+        df = pd.DataFrame(data=[[0, 1], [1, 2]], columns=['A', 'B'])
+        styler = df.style.set_table_styles([{
+            'selector': '',
+            'props': [('background-color', 'yellow')]}]
+        ).extend_table_styles([{
+            'selector': '.col0',
+            'props': [('background-color', 'blue')]}]
+        )
+        assert len(styler.table_styles) == 2
+
+    def test_column_styling(self):
+        df = pd.DataFrame(data=[[0, 1], [1, 2]], columns=['A', 'B'])
+        s = Styler(df, uuid='_')
+        s = s.extend_column_styles({
+            'A': [{
+                'selector': '',
+                'props': [('color', 'blue')]
+            }]
+        })
+        assert '#T__ .col0 {\n          color: blue;\n    }' in s.render()
+
 
 @td.skip_if_no_mpl
 class TestStylerMatplotlibDep: