BUG: fix floating precision formatting in presence of inf (#24863)

doc/source/whatsnew/v0.24.0.rst

@@ -1751,6 +1751,7 @@ I/O
 - Bug in :meth:`DataFrame.to_stata`, :class:`pandas.io.stata.StataWriter` and :class:`pandas.io.stata.StataWriter117` where a exception would leave a partially written and invalid dta file (:issue:`23573`)
 - Bug in :meth:`DataFrame.to_stata` and :class:`pandas.io.stata.StataWriter117` that produced invalid files when using strLs with non-ASCII characters (:issue:`23573`)
 - Bug in :class:`HDFStore` that caused it to raise ``ValueError`` when reading a Dataframe in Python 3 from fixed format written in Python 2 (:issue:`24510`)
+- Bug in :func:`DataFrame.to_string()` and more generally in the floating ``repr`` formatter. Zeros were not trimmed if ``inf`` was present in a columns while it was the case with NA values. Zeros are now trimmed as in the presence of NA (:issue:`24861`).
 
 Plotting
 ^^^^^^^^

pandas/io/formats/format.py

@@ -1414,16 +1414,20 @@ def _trim_zeros(str_floats, na_rep='NaN'):
     """
     trimmed = str_floats
 
+    def _is_number(x):
+        return (x != na_rep and not x.endswith('inf'))
+
     def _cond(values):
-        non_na = [x for x in values if x != na_rep]
-        return (len(non_na) > 0 and all(x.endswith('0') for x in non_na) and
-                not (any(('e' in x) or ('E' in x) for x in non_na)))
+        finite = [x for x in values if _is_number(x)]
+        return (len(finite) > 0 and all(x.endswith('0') for x in finite) and
+                not (any(('e' in x) or ('E' in x) for x in finite)))
 
     while _cond(trimmed):
-        trimmed = [x[:-1] if x != na_rep else x for x in trimmed]
+        trimmed = [x[:-1] if _is_number(x) else x for x in trimmed]
 
     # leave one 0 after the decimal points if need be.
-    return [x + "0" if x.endswith('.') and x != na_rep else x for x in trimmed]
+    return [x + "0" if x.endswith('.') and _is_number(x) else x
+            for x in trimmed]
 
 
 def _has_names(index):

pandas/tests/io/formats/test_format.py

@@ -1465,6 +1465,39 @@ def test_to_string_format_na(self):
                     '4  4.0     bar')
         assert result == expected
 
+    def test_to_string_format_inf(self):
+        # Issue #24861
+        tm.reset_display_options()
+        df = DataFrame({
+            'A': [-np.inf, np.inf, -1, -2.1234, 3, 4],
+            'B': [-np.inf, np.inf, 'foo', 'foooo', 'fooooo', 'bar']
+        })
+        result = df.to_string()
+
+        expected = ('        A       B\n'
+                    '0    -inf    -inf\n'
+                    '1     inf     inf\n'
+                    '2 -1.0000     foo\n'
+                    '3 -2.1234   foooo\n'
+                    '4  3.0000  fooooo\n'
+                    '5  4.0000     bar')
+        assert result == expected
+
+        df = DataFrame({
+            'A': [-np.inf, np.inf, -1., -2., 3., 4.],
+            'B': [-np.inf, np.inf, 'foo', 'foooo', 'fooooo', 'bar']
+        })
+        result = df.to_string()
+
+        expected = ('     A       B\n'
+                    '0 -inf    -inf\n'
+                    '1  inf     inf\n'
+                    '2 -1.0     foo\n'
+                    '3 -2.0   foooo\n'
+                    '4  3.0  fooooo\n'
+                    '5  4.0     bar')
+        assert result == expected
+
     def test_to_string_decimal(self):
         # Issue #23614
         df = DataFrame({'A': [6.0, 3.1, 2.2]})