Add built-in funcion for Styler to format the text displayed for missing values

As described in GH pandas-dev#28358, user who wants to control how NA values are printed
while applying styles to the output will have to implement their own formatter.
(so that the underlying data will not change and can be used for styling)

Since the behavior is common in styling (for reports etc.), suggest to add this
shortcut function to enable users format their NA values as something like '--'
or 'Not Available' easily.

example usage: `df.style.highlight_max().format_null('--')`

Author: immaxchen

doc/source/whatsnew/v1.0.0.rst

@@ -110,6 +110,7 @@ Other enhancements
 - :meth:`DataFrame.to_json` now accepts an ``indent`` integer argument to enable pretty printing of JSON output (:issue:`12004`)
 - :meth:`read_stata` can read Stata 119 dta files. (:issue:`28250`)
 - Added ``encoding`` argument to :func:`DataFrame.to_html` for non-ascii text (:issue:`28663`)
+- :meth:`Styler.format_null` is now added into the built-in functions to help formatting missing values (:issue:`28358`)
 
 Build Changes
 ^^^^^^^^^^^^^

pandas/io/formats/style.py

@@ -930,6 +930,25 @@ def hide_columns(self, subset):
     # A collection of "builtin" styles
     # -----------------------------------------------------------------------
 
+    def format_null(self, na_rep="-"):
+        """
+        Format the text displayed for missing values.
+
+        .. versionadded:: 1.0.0
+
+        Parameters
+        ----------
+        na_rep : str
+
+        Returns
+        -------
+        self : Styler
+        """
+        self.format(
+            lambda x: na_rep if pd.isna(x) else self._display_funcs.default_factory()(x)
+        )
+        return self
+
     @staticmethod
     def _highlight_null(v, null_color):
         return (

pandas/tests/io/formats/test_style.py

@@ -990,6 +990,14 @@ def test_bar_bad_align_raises(self):
         with pytest.raises(ValueError):
             df.style.bar(align="poorly", color=["#d65f5f", "#5fba7d"])
 
+    def test_format_null(self, na_rep="-"):
+        # GH 28358
+        df = pd.DataFrame({"A": [0, np.nan]})
+        ctx = df.style.format_null()._translate()
+        result = ctx["body"][1][1]["display_value"]
+        expected = "-"
+        assert result == expected
+
     def test_highlight_null(self, null_color="red"):
         df = pd.DataFrame({"A": [0, np.nan]})
         result = df.style.highlight_null()._compute().ctx