Fixed to_string with line_width and without index (#13998)

Author: cr3

doc/source/whatsnew/v0.19.0.txt

@@ -990,3 +990,4 @@ Bug Fixes
 - Bug in ``Index`` raises ``KeyError`` displaying incorrect column when column is not in the df and columns contains duplicate values (:issue:`13822`)
 - Bug in ``Period`` and ``PeriodIndex`` creating wrong dates when frequency has combined offset aliases (:issue:`13874`)
 - Bug in ``pd.to_datetime()`` did not cast floats correctly when ``unit`` was specified, resulting in truncated datetime (:issue:`13845`)
+- Bug in ``.to_string()`` when called with an integer ``line_width`` and ``index=False`` raises an UnboundLocalError exception because ``idx`` referenced before assignment.

pandas/formats/format.py

@@ -68,7 +68,9 @@
         Set to False for a DataFrame with a hierarchical index to print every
         multiindex key at each row, default True
     index_names : bool, optional
-        Prints the names of the indexes, default True"""
+        Prints the names of the indexes, default True
+    line_width : int, optional
+        Width to wrap a line in characters, default no wrap"""
 
 justify_docstring = """
     justify : {'left', 'right'}, default None
@@ -632,7 +634,8 @@ def _join_multiline(self, *strcols):
         st = 0
         for i, ed in enumerate(col_bins):
             row = strcols[st:ed]
-            row.insert(0, idx)
+            if self.index:
+                row.insert(0, idx)
             if nbins > 1:
                 if ed <= len(strcols) and i < nbins - 1:
                     row.append([' \\'] + ['  '] * (nrows - 1))

pandas/tests/formats/test_format.py

@@ -1964,6 +1964,14 @@ def test_to_string_no_index(self):
 
         self.assertEqual(df_s, expected)
 
+    def test_to_string_line_width_no_index(self):
+        df = DataFrame({'x': [1, 2, 3], 'y': [4, 5, 6]})
+
+        df_s = df.to_string(line_width=1, index=False)
+        expected = "x  \\\n1   \n2   \n3   \n\ny  \n4  \n5  \n6"
+
+        self.assertEqual(df_s, expected)
+
     def test_to_string_float_formatting(self):
         self.reset_display_options()
         fmt.set_option('display.precision', 5, 'display.column_space', 12,