BUG: Improve error message for skipfooter malformed rows in Python engine

Python's native CSV library does not respect the
skipfooter parameter, so if one of those skipped
rows is malformed, it will still raise an error.

Closes pandas-devgh-13879.

Author: gfyoung

doc/source/whatsnew/v0.19.2.txt

@@ -31,6 +31,7 @@ Bug Fixes
 - Allow ``nanoseconds`` in ``Timestamp.replace`` as a kwarg (:issue:`14621`)
 - Bug in ``pd.read_csv`` where reading files fails, if the number of headers is equal to the number of lines in the file (:issue:`14515`)
 - Bug in ``pd.read_csv`` for the Python engine in which an unhelpful error message was being raised when multi-char delimiters were not being respected with quotes (:issue:`14582`)
+- Bug in ``pd.read_csv`` for the Python engine in which an unhelpful error message was being raised when ``skipfooter`` was not being respected by Python's CSV library (:issue:`13879`)
 
 
 

pandas/io/parsers.py

@@ -2327,14 +2327,21 @@ def _next_line(self):
                 try:
                     orig_line = next(self.data)
                 except csv.Error as e:
+                    msg = str(e)
+
                     if 'NULL byte' in str(e):
-                        raise csv.Error(
-                            'NULL byte detected. This byte '
-                            'cannot be processed in Python\'s '
-                            'native csv library at the moment, '
-                            'so please pass in engine=\'c\' instead.')
-                    else:
-                        raise
+                        msg = ('NULL byte detected. This byte '
+                               'cannot be processed in Python\'s '
+                               'native csv library at the moment, '
+                               'so please pass in engine=\'c\' instead')
+
+                    if self.skipfooter > 0:
+                        reason = ('Error could possibly be due to '
+                                  'skipfooter being ignored in Python\'s '
+                                  'native csv library.')
+                        msg += '. ' + reason
+
+                    raise csv.Error(msg)
                 line = self._check_comments([orig_line])[0]
                 self.pos += 1
                 if (not self.skip_blank_lines and

pandas/io/tests/parser/python_parser_only.py

@@ -221,3 +221,18 @@ def test_multi_char_sep_quotes(self):
             with tm.assertRaisesRegexp(ValueError, msg):
                 self.read_csv(StringIO(data), sep=',,',
                               quoting=csv.QUOTE_NONE)
+
+    def test_skipfooter_bad_row(self):
+        # see gh-13879
+
+        data = 'a,b,c\ncat,foo,bar\ndog,foo,"baz'
+        msg = 'skipfooter being ignored'
+
+        with tm.assertRaisesRegexp(csv.Error, msg):
+            self.read_csv(StringIO(data), skipfooter=1)
+
+        # We expect no match, so there should be an assertion
+        # error out of the inner context manager.
+        with tm.assertRaises(AssertionError):
+            with tm.assertRaisesRegexp(csv.Error, msg):
+                self.read_csv(StringIO(data))