datajoint · dimitri-yatsenko · Jul 29, 2015 · Jul 28, 2015 · Jul 28, 2015 · Jul 28, 2015
diff --git a/datajoint/fetch.py b/datajoint/fetch.py
@@ -8,6 +8,7 @@
 from datajoint import DataJointError
 from . import key as PRIMARY_KEY
 from collections import abc
+from . import config
 
 
 def prepare_attributes(relation, item):
@@ -35,7 +36,9 @@ def ret(*args, **kwargs):
 
     return ret
 
+
 class Fetch:
+
     def __init__(self, relation):
         if isinstance(relation, Fetch): # copy constructor
             self.behavior = dict(relation.behavior)
@@ -46,7 +49,6 @@ def __init__(self, relation):
             )
             self._relation = relation
 
-
     @copy_first
     def order_by(self, *args):
         if len(args) > 0:
@@ -162,6 +164,23 @@ def __getitem__(self, item):
             ]
         return return_values[0] if single_output else return_values
 
+    def __repr__(self):
+        limit = config['display.limit']
+        width = config['display.width']
+        rel = self._relation.project(*self._relation.heading.non_blobs)  # project out blobs
+        template = '%%-%d.%ds' % (width, width)
+        columns = rel.heading.names
+        repr_string = ' '.join([template % column for column in columns]) + '\n'
+        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(rel) > limit:
+            repr_string += '...\n'
+        repr_string += ' (%d tuples)\n' % len(rel)
+        return repr_string
+
+    def __len__(self):
+        return len(self._relation)
 
 class Fetch1:
     def __init__(self, relation):

diff --git a/datajoint/relation.py b/datajoint/relation.py
@@ -106,6 +106,9 @@ def descendants(self):
                      for table in self.connection.erm.get_descendants(self.full_table_name))
         return [relation for relation in relations if relation.is_declared]
 
+    def _repr_helper(self):
+        return "%s.%s()" % (self.__module__, self.__class__.__name__)
+
     # --------- SQL functionality --------- #
     @property
     def is_declared(self):
@@ -282,6 +285,9 @@ def __init__(self, connection, full_table_name, definition=None, context=None):
         self._definition = definition
         self._context = context
 
+    def __repr__(self):
+        return "FreeRelation(`%s`.`%s`)" % (self.database, self._table_name)
+
     @property
     def definition(self):
         return self._definition

diff --git a/datajoint/relational_operand.py b/datajoint/relational_operand.py
@@ -143,7 +143,17 @@ def __sub__(self, restriction):
         """
         inverted restriction aka antijoin
         """
-        return self & Not(restriction)
+        return \
+            self & Not(restriction)
+
+    def _repr_helper(self):
+        return "None"
+
+    def __repr__(self):
+        ret = self._repr_helper()
+        if self._restrictions:
+            ret += ' & %r' % self._restrictions
+        return ret
 
     # ------ data retrieval methods -----------
 
@@ -190,21 +200,6 @@ def cursor(self, offset=0, limit=None, order_by=None, as_dict=False):
         logger.debug(sql)
         return self.connection.query(sql, as_dict=as_dict)
 
-    def __repr__(self):
-        limit = config['display.limit']
-        width = config['display.width']
-        rel = self.project(*self.heading.non_blobs)  # project out blobs
-        template = '%%-%d.%ds' % (width, width)
-        columns = rel.heading.names
-        repr_string = ' '.join([template % column for column in columns]) + '\n'
-        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) > limit:
-            repr_string += '...\n'
-        repr_string += ' (%d tuples)\n' % len(self)
-        return repr_string
-
     @property
     def fetch1(self):
         return Fetch1(self)
@@ -274,6 +269,9 @@ def __init__(self, arg1, arg2):
         self._arg2 = Subquery(arg1) if arg2.heading.computed else arg2
         self._restrictions = self._arg1.restrictions + self._arg2.restrictions
 
+    def _repr_helper(self):
+        return "(%r) * (%r)" % (self._arg1, self._arg2)
+
     @property
     def connection(self):
         return self._arg1.connection
@@ -352,6 +350,10 @@ def _restrict(self, restriction):
         else:
             return super()._restrict(restriction)
 
+    def _repr_helper(self):
+        # TODO: create better repr
+        return "project(%r, %r)" % (self._arg, self._attributes)
+
 
 class Subquery(RelationalOperand):
     """
@@ -379,3 +381,6 @@ def from_clause(self):
     @property
     def heading(self):
         return self._arg.heading.resolve()
+
+    def _repr_helper(self):
+        return "%r" % self._arg
diff --git a/tests/test_fetch.py b/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)
+