BUG: Fix TypeError caused by GH13374 (pandas-dev#17465)

Author: matthax

doc/source/whatsnew/v0.21.0.txt

@@ -411,6 +411,7 @@ I/O
 - Bug in :func:`read_csv` when called with a single-element list ``header`` would return a ``DataFrame`` of all NaN values (:issue:`7757`)
 - Bug in :func:`read_stata` where value labels could not be read when using an iterator (:issue:`16923`)
 - Bug in :func:`read_html` where import check fails when run in multiple threads (:issue:`16928`)
+- Bug in :func:`read_csv` where automatic delimiter detection caused a ``TypeError`` to be thrown when a bad line was encountered rather than the correct error message (:issue:`13374`)
 
 Plotting
 ^^^^^^^^

pandas/io/parsers.py

@@ -2836,7 +2836,9 @@ def _rows_to_cols(self, content):
             for row_num, actual_len in bad_lines:
                 msg = ('Expected %d fields in line %d, saw %d' %
                        (col_len, row_num + 1, actual_len))
-                if len(self.delimiter) > 1 and self.quoting != csv.QUOTE_NONE:
+                if (self.delimiter and
+                        len(self.delimiter) > 1 and
+                        self.quoting != csv.QUOTE_NONE):
                     # see gh-13374
                     reason = ('Error could possibly be due to quotes being '
                               'ignored when a multi-char delimiter is used.')

pandas/tests/io/parser/python_parser_only.py

@@ -218,6 +218,25 @@ def test_multi_char_sep_quotes(self):
                 self.read_csv(StringIO(data), sep=',,',
                               quoting=csv.QUOTE_NONE)
 
+    def test_none_delimiter(self):
+        # see gh-13374 and gh-17465
+
+        data = "a,b,c\n0,1,2\n3,4,5,6\n7,8,9"
+        expected = DataFrame({'a': [0, 7],
+                              'b': [1, 8],
+                              'c': [2, 9]})
+
+        # We expect the third line in the data to be
+        # skipped because it is malformed,
+        # but we do not expect any errors to occur.
+        result = self.read_csv(StringIO(data), header=0,
+                               sep=None,
+                               error_bad_lines=False,
+                               warn_bad_lines=True,
+                               engine='python',
+                               tupleize_cols=True)
+        tm.assert_frame_equal(result, expected)
+
     def test_skipfooter_bad_row(self):
         # see gh-13879
         # see gh-15910