Index: Lib/csv.py
===================================================================
--- Lib/csv.py	(revision 87151)
+++ Lib/csv.py	(working copy)
@@ -181,11 +181,11 @@
 
         quotechar, doublequote, delimiter, skipinitialspace = \
                    self._guess_quote_and_delimiter(sample, delimiters)
-        if not delimiter:
+        if delimiter is None:
             delimiter, skipinitialspace = self._guess_delimiter(sample,
                                                                 delimiters)
 
-        if not delimiter:
+        if delimiter is None:
             raise Error("Could not determine delimiter")
 
         class dialect(Dialect):
@@ -218,7 +218,7 @@
         matches = []
         for restr in ('(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\']).*?(?P=quote)(?P=delim)', # ,".*?",
                       '(?:^|\n)(?P<quote>["\']).*?(?P=quote)(?P<delim>[^\w\n"\'])(?P<space> ?)',   #  ".*?",
-                      '(?P<delim>>[^\w\n"\'])(?P<space> ?)(?P<quote>["\']).*?(?P=quote)(?:$|\n)',  # ,".*?"
+                      '(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\']).*?(?P=quote)(?:$|\n)',  # ,".*?"
                       '(?:^|\n)(?P<quote>["\']).*?(?P=quote)(?:$|\n)'):                            #  ".*?" (no delim, no space)
             regexp = re.compile(restr, re.DOTALL | re.MULTILINE)
             matches = regexp.findall(data)
Index: Lib/test/test_csv.py
===================================================================
--- Lib/test/test_csv.py	(revision 87151)
+++ Lib/test/test_csv.py	(working copy)
@@ -833,6 +833,29 @@
         self.assertEqual(dialect.delimiter, "|")
         self.assertEqual(dialect.quotechar, "'")
 
+    # Issue 10515.
+    for i, (sample, expected_delimiter) in enumerate((
+        # XXX: I'm not sure if first two make sense but they are
+        # the existing behavior.
+        ('abcde', 'e'),
+        ('a', 'a'),
+        ('a,"b,c",d\ne,f,g', ','),
+        ('"a,b",c,d\ne,f,g', ','),
+        ('a,b,"c,d"\ne,f,g', ','),
+        ('a,b,"c,d"\r\ne,f,g', ','),
+        # XXX: I don't know what the correct behavior should be for these.
+        # Currently the first one raises an error that the delimiter can't
+        # be determined while the second one returns '\r'.  The second
+        # is obviously.
+        ('"a,b,c,d"\ne',  ''),
+        ('"a,b,c,d"\r\ne', ''),
+        )):
+        def f(self, sample=sample, expected_delimiter=expected_delimiter):
+            dialect = csv.Sniffer().sniff(sample)
+            self.assertEqual(dialect.delimiter, expected_delimiter, 
+                msg="sample was: {!r}".format(sample))
+        locals()['test_delimiters_extended_{}'.format(i)] = f
+
     def test_doublequote(self):
         sniffer = csv.Sniffer()
         dialect = sniffer.sniff(self.header)