BUG: Fix float formatting when a string is passed as float_format arg (pandas-dev#22308)

Author: tomneep

doc/source/whatsnew/v0.24.0.rst

@@ -1399,6 +1399,7 @@ Notice how we now instead output ``np.nan`` itself instead of a stringified form
 - Bug in :meth:`read_excel()` in which extraneous header names were extracted, even though none were specified (:issue:`11733`)
 - Bug in :meth:`read_excel()` in which ``index_col=None`` was not being respected and parsing index columns anyway (:issue:`20480`)
 - Bug in :meth:`read_excel()` in which ``usecols`` was not being validated for proper column names when passed in as a string (:issue:`20480`)
+- :func:`DataFrame.to_string()`, :func:`DataFrame.to_html()`, :func:`DataFrame.to_latex()` will correctly format output when a string is passed as the ``float_format`` argument (:issue:`21625`, :issue:`22270`)
 
 Plotting
 ^^^^^^^^

pandas/io/formats/format.py

@@ -960,6 +960,8 @@ def __init__(self, *args, **kwargs):
         # float_format is expected to be a string
         # formatter should be used to pass a function
         if self.float_format is not None and self.formatter is None:
+            # GH21625, GH22270
+            self.fixed_width = False
             if callable(self.float_format):
                 self.formatter = self.float_format
                 self.float_format = None

pandas/tests/io/formats/data/gh21625_expected_output.html

@@ -0,0 +1,14 @@
+<table border="1" class="dataframe">
+  <thead>
+    <tr style="text-align: right;">
+      <th></th>
+      <th>x</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <th>0</th>
+      <td>0.200</td>
+    </tr>
+  </tbody>
+</table>

pandas/tests/io/formats/data/gh22270_expected_output.html

@@ -0,0 +1,14 @@
+<table border="1" class="dataframe">
+  <thead>
+    <tr style="text-align: right;">
+      <th></th>
+      <th>x</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <th>0</th>
+      <td>100</td>
+    </tr>
+  </tbody>
+</table>

pandas/tests/io/formats/test_format.py

@@ -1359,6 +1359,18 @@ def test_to_string_float_formatting(self):
                         '1  2.512000e-01')
         assert df_s == expected
 
+    def test_to_string_float_format_no_fixed_width(self):
+
+        # GH 21625
+        df = DataFrame({'x': [0.19999]})
+        expected = '      x\n0 0.200'
+        assert df.to_string(float_format='%.3f') == expected
+
+        # GH 22270
+        df = DataFrame({'x': [100.0]})
+        expected = '    x\n0 100'
+        assert df.to_string(float_format='%.0f') == expected
+
     def test_to_string_small_float_values(self):
         df = DataFrame({'a': [1.5, 1e-17, -5.5e-7]})
 

pandas/tests/io/formats/test_to_html.py

@@ -465,3 +465,15 @@ def test_to_html_with_id(self):
                                                   name='myindexname'))
         result = df.to_html(index_names=False, table_id="TEST_ID")
         assert ' id="TEST_ID"' in result
+
+    def test_to_html_float_format_no_fixed_width(self, datapath):
+
+        # GH 21625
+        df = DataFrame({'x': [0.19999]})
+        expected = expected_html(datapath, 'gh21625_expected_output')
+        assert df.to_html(float_format='%.3f') == expected
+
+        # GH 22270
+        df = DataFrame({'x': [100.0]})
+        expected = expected_html(datapath, 'gh22270_expected_output')
+        assert df.to_html(float_format='%.0f') == expected

pandas/tests/io/formats/test_to_latex.py

@@ -708,3 +708,29 @@ def test_to_latex_multiindex_empty_name(self):
 \end{tabular}
 """
         assert observed == expected
+
+    def test_to_latex_float_format_no_fixed_width(self):
+
+        # GH 21625
+        df = DataFrame({'x': [0.19999]})
+        expected = r"""\begin{tabular}{lr}
+\toprule
+{} &     x \\
+\midrule
+0 & 0.200 \\
+\bottomrule
+\end{tabular}
+"""
+        assert df.to_latex(float_format='%.3f') == expected
+
+        # GH 22270
+        df = DataFrame({'x': [100.0]})
+        expected = r"""\begin{tabular}{lr}
+\toprule
+{} &   x \\
+\midrule
+0 & 100 \\
+\bottomrule
+\end{tabular}
+"""
+        assert df.to_latex(float_format='%.0f') == expected