BUG: Fix float formatting when a string is passed as float_format arg

doc/source/whatsnew/v0.24.0.txt

@@ -633,7 +633,9 @@ I/O
 
 - :func:`read_html()` no longer ignores all-whitespace ``<tr>`` within ``<thead>`` when considering the ``skiprows`` and ``header`` arguments. Previously, users had to decrease their ``header`` and ``skiprows`` values on such tables to work around the issue. (:issue:`21641`)
 - :func:`read_excel()` will correctly show the deprecation warning for previously deprecated ``sheetname`` (:issue:`17994`)
--
+- :func:`DataFrame.to_string()`, :func:`DataFrame.to_html()` and
+  :func:`DataFrame.to_latex()` will correctly format output when a string is
+  passed as the ``float_format`` argument (:issue:`21625` and :issue:`22270`)
 
 Plotting
 ^^^^^^^^

pandas/io/formats/format.py

@@ -974,6 +974,7 @@ 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:
+            self.fixed_width = False
             if callable(self.float_format):
                 self.formatter = self.float_format
                 self.float_format = None

pandas/tests/io/formats/test_format.py

@@ -1328,6 +1328,18 @@ 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]})