to_html formatter not called for float values in a mixed-type column (#25983)

Author: simonjayhawkins

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/io/formats/format.py

@@ -981,19 +981,24 @@ 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/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])