ENH: Add built-in function for Styler to format the text displayed for missing values

doc/source/reference/style.rst

@@ -52,6 +52,7 @@ Builtin styles
    Styler.highlight_max
    Styler.highlight_min
    Styler.highlight_null
+   Styler.format_null
    Styler.background_gradient
    Styler.bar
 

doc/source/user_guide/style.ipynb

@@ -492,6 +492,22 @@
     "df.style.highlight_max(axis=0)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "You can format the text displayed for missing values by `.format_null`."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "df.style.highlight_max(axis=0).format_null(na_rep='-')"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

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