to_html formatter not called for float values in a mixed-type column (2)

doc/source/whatsnew/v0.25.0.rst

@@ -347,6 +347,7 @@ I/O
 ^^^
 
 - Bug in :func:`DataFrame.to_html()` where values were truncated using display options instead of outputting the full content (:issue:`17004`)
+- Bug in :meth:`DataFrame.to_html` that would ignore ``formatters`` argument for float values in a column with ``dtype=object`` (:issue:`13021`)
 - Fixed bug in missing text when using :meth:`to_clipboard` if copying utf-16 characters in Python 3 on Windows (:issue:`25040`)
 - Bug in :func:`read_json` for ``orient='table'`` when it tries to infer dtypes by default, which is not applicable as dtypes are already defined in the JSON schema (:issue:`21345`)
 - Bug in :func:`read_json` for ``orient='table'`` and float index, as it infers index dtype by default, which is not applicable because index dtype is already defined in the JSON schema (:issue:`25433`)

pandas/core/arrays/interval.py

@@ -804,6 +804,10 @@ def value_counts(self, dropna=True):
 
     # Formatting
 
+    def _formatter(self, boxed=False):
+        # Defer to GenericArrayFormatter's formatter.
+        return None
+
     def _format_data(self):
 
         # TODO: integrate with categorical and make generic

pandas/io/formats/format.py

@@ -938,6 +938,10 @@ def get_result(self):
         return _make_fixed_width(fmt_values, self.justify)
 
     def _format_strings(self):
+        # shortcut
+        if self.formatter is not None:
+            return [' {}'.format(self.formatter(x)) for x in self.values]
+
         if self.float_format is None:
             float_format = get_option("display.float_format")
             if float_format is None:

pandas/tests/io/formats/data/html/gh13021_expected_output.html

@@ -0,0 +1,26 @@
+<table border="1" class="dataframe">
+  <thead>
+    <tr style="text-align: right;">
+      <th></th>
+      <th>x</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <th>0</th>
+      <td>a</td>
+    </tr>
+    <tr>
+      <th>1</th>
+      <td>$0</td>
+    </tr>
+    <tr>
+      <th>2</th>
+      <td>$10</td>
+    </tr>
+    <tr>
+      <th>3</th>
+      <td>$3</td>
+    </tr>
+  </tbody>
+</table>

pandas/tests/io/formats/test_to_html.py

@@ -635,6 +635,17 @@ def test_to_html_invalid_classes_type(classes):
         df.to_html(classes=classes)
 
 
+def test_to_html_formatters_object_type(datapath):
+    # GH 13021
+    def f(x):
+        return x if isinstance(x, str) else '${:,.0f}'.format(x)
+
+    df = pd.DataFrame([['a'], [0], [10.4], [3]], columns=['x'])
+    result = df.to_html(formatters=dict(x=f))
+    expected = expected_html(datapath, 'gh13021_expected_output')
+    assert result == expected
+
+
 def test_to_html_round_column_headers():
     # GH 17280
     df = DataFrame([1], columns=[0.55555])