BUG: Don't ignore na_rep in DataFrame.to_html

pandas/io/formats/format.py

@@ -1533,7 +1533,14 @@ def format_values_with(float_format):
     def _format_strings(self) -> List[str]:
         # shortcut
         if self.formatter is not None:
-            return [self.formatter(x) for x in self.values]
+            if self.na_rep == "NaN":
+                return [self.formatter(x) for x in self.values]
+            else:
+                na_mask = isna(self.values)
+                return [
+                    self.formatter(x) if not m else self.na_rep
+                    for x, m in zip(self.values, na_mask)
+                ]
 
         return list(self.get_result_as_array())
 

pandas/tests/io/formats/test_format.py

@@ -3432,3 +3432,14 @@ def test_format_remove_leading_space_dataframe(input_array, expected):
     # GH: 24980
     df = pd.DataFrame(input_array).to_string(index=False)
     assert df == expected
+
+
+@pytest.mark.parametrize("na_rep, string", [("NaN", "nan"), ("Ted", "Ted")])
+def test_to_string_na_rep_and_float_format(na_rep, string):
+    # GH 13828
+    df = DataFrame([["A", 1.2225], ["A", None]], columns=["Group", "Data"])
+    result = df.to_string(na_rep=na_rep, float_format="{:.2f}".format)
+    expected = f"""  Group  Data
+0     A  1.22
+1     A   {string}"""
+    assert result == expected

pandas/tests/io/formats/test_to_html.py

@@ -820,3 +820,38 @@ def test_html_repr_min_rows(datapath, max_rows, min_rows, expected):
     with option_context("display.max_rows", max_rows, "display.min_rows", min_rows):
         result = df._repr_html_()
     assert result == expected
+
+
+@pytest.mark.parametrize("na_rep, string", [("NaN", "nan"), ("Ted", "Ted")])
+def test_to_html_na_rep_and_float_format(na_rep, string):
+    # https://github.com/pandas-dev/pandas/issues/13828
+    df = DataFrame(
+        [
+            ["A", 1.2225],
+            ["A", None],
+        ],
+        columns=["Group", "Data"],
+    )
+    result = df.to_html(na_rep=na_rep, float_format="{:.2f}".format)
+    expected = f"""<table border="1" class="dataframe">
+  <thead>
+    <tr style="text-align: right;">
+      <th></th>
+      <th>Group</th>
+      <th>Data</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <th>0</th>
+      <td>A</td>
+      <td>1.22</td>
+    </tr>
+    <tr>
+      <th>1</th>
+      <td>A</td>
+      <td>{string}</td>
+    </tr>
+  </tbody>
+</table>"""
+    assert result == expected

pandas/tests/io/formats/test_to_latex.py

@@ -1284,3 +1284,24 @@ def test_get_strrow_multindex_multicolumn(self, row_num, expected):
         )
 
         assert row_string_converter.get_strrow(row_num=row_num) == expected
+
+    @pytest.mark.parametrize("na_rep, string", [("NaN", "nan"), ("Ted", "Ted")])
+    def test_to_latex_na_rep_and_float_format(self, na_rep, string):
+        df = DataFrame(
+            [
+                ["A", 1.2225],
+                ["A", None],
+            ],
+            columns=["Group", "Data"],
+        )
+        result = df.to_latex(na_rep=na_rep, float_format="{:.2f}".format)
+        expected = f"""\\begin{{tabular}}{{llr}}
+\\toprule
+{{}} & Group &  Data \\\\
+\\midrule
+0 &     A &  1.22 \\\\
+1 &     A &   {string} \\\\
+\\bottomrule
+\\end{{tabular}}
+"""
+        assert result == expected