Add len to Fetch and add test case for Fetch repr

Author: Edgar Y. Walker

datajoint/fetch.py

@@ -174,11 +174,13 @@ def __repr__(self):
         repr_string += ' '.join(['+' + '-' * (width - 2) + '+' for _ in columns]) + '\n'
         for tup in rel.fetch(limit=limit):
             repr_string += ' '.join([template % column for column in tup]) + '\n'
-        if len(self._relation) > limit:
+        if len(rel) > limit:
             repr_string += '...\n'
-        repr_string += ' (%d tuples)\n' % len(self._relation)
+        repr_string += ' (%d tuples)\n' % len(rel)
         return repr_string
 
+    def __len__(self):
+        return len(self._relation)
 
 class Fetch1:
     def __init__(self, relation):

tests/test_fetch.py

@@ -125,3 +125,11 @@ def test_copy(self):
         f2 = f.order_by('name')
         assert_true(f.behavior['order_by'] is None and len(f2.behavior['order_by']) == 1, 'Object was not copied')
 
+    def test_repr(self):
+        """Test string representation of fetch, returning table preview"""
+        repr = self.subject.fetch.__repr__()
+        n = len(repr.strip().split('\n'))
+        limit = dj.config['display.limit']
+        # 3 lines are used for headers (2) and summary statement (1)
+        assert_true(n - 3 <= limit)
+