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

doc/source/whatsnew/v0.25.0.rst

@@ -344,6 +344,7 @@ 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/io/formats/format.py

@@ -980,19 +980,24 @@ def _format(x):
         if leading_space is None:
             leading_space = is_float_type.any()
 
+        if leading_space is False:
+            # False specifically, so that the default is
+            # to include a space if we get here.
+            tpl = '{v}'
+        else:
+            tpl = ' {v}'
+
+        # shortcut
+        if self.formatter is not None:
+            return [tpl.format(v=self.formatter(x)) for x in self.values]
+
         fmt_values = []
         for i, v in enumerate(vals):
             if not is_float_type[i] and leading_space:
                 fmt_values.append(' {v}'.format(v=_format(v)))
             elif is_float_type[i]:
                 fmt_values.append(float_format(v))
             else:
-                if leading_space is False:
-                    # False specifically, so that the default is
-                    # to include a space if we get here.
-                    tpl = '{v}'
-                else:
-                    tpl = ' {v}'
                 fmt_values.append(tpl.format(v=_format(v)))
 
         return fmt_values

pandas/tests/io/formats/test_to_html.py

@@ -633,3 +633,13 @@ def test_to_html_invalid_classes_type(classes):
 
     with pytest.raises(TypeError, match=msg):
         df.to_html(classes=classes)
+
+
+def test_to_html_formatters_object_type():
+    # GH 13021
+    def f(x):
+        return x if type(x) is str else '${:,.0f}'.format(x)
+
+    df = pd.DataFrame([['a'], [0], [10.4], [3]], columns=['x'])
+    result = df.to_html(formatters=dict(x=f))
+    assert '$10' in result