ENH: add set_td_classes method for CSS class addition to data cells

doc/source/whatsnew/v1.2.0.rst

@@ -104,7 +104,7 @@ Other enhancements
 - Added :meth:`~DataFrame.set_flags` for setting table-wide flags on a ``Series`` or ``DataFrame`` (:issue:`28394`)
 - :class:`Index` with object dtype supports division and multiplication (:issue:`34160`)
 - :meth:`DataFrame.explode` and :meth:`Series.explode` now support exploding of sets (:issue:`35614`)
--
+- `Styler` now allows direct CSS class name addition to individual data cells (:issue:`36159`)
 
 .. _whatsnew_120.api_breaking.python:
 

pandas/io/formats/style.py

@@ -171,6 +171,8 @@ def __init__(
         self.cell_ids = cell_ids
         self.na_rep = na_rep
 
+        self.cell_context: Dict[str, Any] = {}
+
         # display_funcs maps (row, col) -> formatting function
 
         def default_display_func(x):
@@ -262,7 +264,7 @@ def format_attr(pair):
         idx_lengths = _get_level_lengths(self.index)
         col_lengths = _get_level_lengths(self.columns, hidden_columns)
 
-        cell_context = dict()
+        cell_context = self.cell_context
 
         n_rlvls = self.data.index.nlevels
         n_clvls = self.data.columns.nlevels
@@ -499,6 +501,49 @@ def format(self, formatter, subset=None, na_rep: Optional[str] = None) -> "Style
                 self._display_funcs[(i, j)] = formatter
         return self
 
+    def set_data_classes(self, classes: DataFrame) -> "Styler":
+        """
+        Add string based CSS class names to data cells that will appear in the
+        `Styler` HTML result.
+
+        Parameters
+        ----------
+        classes : DataFrame
+            DataFrame containing strings that will be translated to CSS classes. Empty
+            strings, None, or NaN values will be ignored. DataFrame must have
+            identical rows and columns to the underlying `Styler` data.
+
+        Returns
+        -------
+        self : Styler
+
+        Examples
+        --------
+        >>> df = pd.DataFrame(data=[[1, 2, 3], [4, 5, 6]], columns=['A', 'B', 'C'])
+        >>> classes = pd.DataFrame([
+        ...     ['min-num red', '', 'blue'],
+        ...     ['red', None, 'blue max-num']
+        ... ], index=df.index, columns=df.columns)
+        >>> df.style.set_data_classes(classes)
+        """
+        if not (
+            self.columns.equals(classes.columns) and self.index.equals(classes.index)
+        ):
+            raise ValueError(
+                "CSS classes DataFrame must have identical column and index labelling "
+                "to underlying."
+            )
+
+        mask = (classes.isna()) | (classes.eq(""))
+        self.cell_context["data"] = {
+            r: {c: [classes.iloc[r, c]]}
+            for r, rn in enumerate(classes.index)
+            for c, cn in enumerate(classes.columns)
+            if not mask.iloc[r, c]
+        }
+
+        return self
+
     def render(self, **kwargs) -> str:
         """
         Render the built up styles to HTML.
@@ -609,6 +654,7 @@ def clear(self) -> None:
         Returns None.
         """
         self.ctx.clear()
+        self.cell_context = {}
         self._todo = []
 
     def _compute(self):

pandas/tests/io/formats/test_style.py

@@ -1691,6 +1691,20 @@ 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_set_data_classes(self):
+        # GH 36159
+        df = pd.DataFrame(data=[[0, 1], [2, 3]])
+        classes = pd.DataFrame(
+            data=[["test-class", ""], [np.nan, None]],
+            columns=df.columns,
+            index=df.index,
+        )
+        s = Styler(df, uuid="_", cell_ids=False).set_data_classes(classes).render()
+        assert '<td  class="data row0 col0 test-class" >0</td>' in s
+        assert '<td  class="data row0 col1" >1</td>' in s
+        assert '<td  class="data row1 col0" >2</td>' in s
+        assert '<td  class="data row1 col1" >3</td>' in s
+
 
 @td.skip_if_no_mpl
 class TestStylerMatplotlibDep: