warn if Link objects are instantiated with unicode urls

scrapy/link.py

@@ -11,6 +11,11 @@ class Link(object):
     __slots__ = ['url', 'text', 'fragment', 'nofollow']
 
     def __init__(self, url, text='', fragment='', nofollow=False):
+        if isinstance(url, unicode):
+            import warnings
+            warnings.warn("Do not instantiate Link objects with unicode urls. " \
+                "Assuming utf-8 encoding (which could be wrong)")
+            url = url.encode('utf-8')
         self.url = url
         self.text = text
         self.fragment = fragment

scrapy/tests/test_link.py

@@ -1,4 +1,5 @@
 import unittest
+import warnings
 
 from scrapy.link import Link
 
@@ -41,3 +42,10 @@ class LinkTest(unittest.TestCase):
         l1 = Link("http://www.example.com", text="test", fragment='something', nofollow=True)
         l2 = eval(repr(l1))
         self._assert_same_links(l1, l2)
+
+    def test_unicode_url(self):
+        with warnings.catch_warnings(record=True) as w:
+            l = Link(u"http://www.example.com/\xa3")
+            assert isinstance(l.url, str)
+            assert l.url == 'http://www.example.com/\xc2\xa3'
+            assert len(w) == 1, "warning not issued"