ENH: Implement Styler.where

Author: Licht-T

pandas/io/formats/style.py

@@ -623,6 +623,40 @@ def applymap(self, func, subset=None, **kwargs):
                            (func, subset), kwargs))
         return self
 
+    def where(self, func, style1, style2=None, subset=None, **kwargs):
+        """
+        Apply a function elementwise, updating the HTML
+        representation with a style which is selected in
+        accordance with the return value of a function.
+
+        .. versionadded:: 0.21.0
+
+        Parameters
+        ----------
+        func : callable
+            ``func`` should take a scalar and return a boolean
+        style1 : str
+            applied when ``func`` returns true
+        style2 : str
+            applied when ``func`` returns false
+        subset : IndexSlice
+            a valid indexer to limit ``data`` to *before* applying the
+            function. Consider using a pandas.IndexSlice
+        kwargs : dict
+            pass along to ``func``
+
+        Returns
+        -------
+        self : Styler
+
+        """
+
+        if style2 is None:
+            style2 = ''
+
+        return self.applymap(lambda val: style1 if func(val) else style2,
+                             subset=subset, **kwargs)
+
     def set_precision(self, precision):
         """
         Set the precision used to render.