Don't let the default delimiter override a caller-supplied CSV dialect

src/tablib/formats/_csv.py

@@ -18,7 +18,8 @@ def export_stream_set(cls, dataset, **kwargs):
         """Returns CSV representation of Dataset as file-like."""
         stream = StringIO()
 
-        kwargs.setdefault('delimiter', cls.DEFAULT_DELIMITER)
+        if 'dialect' not in kwargs:
+            kwargs.setdefault('delimiter', cls.DEFAULT_DELIMITER)
 
         _csv = csv.writer(stream, **kwargs)
 
@@ -40,7 +41,8 @@ def import_set(cls, dset, in_stream, headers=True, skip_lines=0, **kwargs):
 
         dset.wipe()
 
-        kwargs.setdefault('delimiter', cls.DEFAULT_DELIMITER)
+        if 'dialect' not in kwargs:
+            kwargs.setdefault('delimiter', cls.DEFAULT_DELIMITER)
 
         rows = csv.reader(in_stream, **kwargs)
         for i, row in enumerate(rows):

tests/test_tablib.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 """Tests for Tablib."""
 
+import csv
 import datetime as dt
 import doctest
 import json
@@ -1157,6 +1158,19 @@ def test_csv_formatter_support_kwargs(self):
         d2 = tablib.import_set(_csv, format="csv", **kwargs)
         self.assertEqual(3, len(d2.headers))
 
+    def test_csv_dialect_is_not_overridden_by_default_delimiter(self):
+        """A caller-supplied dialect must win over DEFAULT_DELIMITER."""
+        text = 'test:test2\nvalue1:value2\n'
+        dialect = csv.Sniffer().sniff(text.splitlines()[0])
+        self.assertEqual(':', dialect.delimiter)
+
+        imported = tablib.Dataset().load(text, format='csv', dialect=dialect)
+        self.assertEqual(['test', 'test2'], imported.headers)
+        self.assertEqual(('value1', 'value2'), imported[0])
+
+        exported = imported.export('csv', dialect=dialect)
+        self.assertEqual('test:test2\r\nvalue1:value2\r\n', exported)
+
 
 class TSVTests(BaseTestCase):
     def test_tsv_import_set(self):