Give more context when nodes collected different tests.

testing/test_dsession.py

@@ -1,4 +1,9 @@
-from xdist.dsession import DSession, LoadScheduling, EachScheduling
+from xdist.dsession import (
+    DSession,
+    LoadScheduling,
+    EachScheduling,
+    report_collection_diff,
+)
 from _pytest import main as outcome
 import py
 import execnet
@@ -165,3 +170,36 @@ class TestDistReporter:
         linecomp.assert_contains_lines([
             "[X1,X2] rsyncing: hello",
         ])
+
+
+def test_report_collection_diff_equal():
+    """Test reporting of equal collections."""
+    from_collection = to_collection = ['aaa', 'bbb', 'ccc']
+    assert report_collection_diff(from_collection, to_collection, 1, 2)
+
+
+def test_report_collection_diff_different():
+    """Test reporting of different collections."""
+    from_collection = ['aaa', 'bbb', 'ccc', 'YYY']
+    to_collection = ['aZa', 'bbb', 'XXX', 'ccc']
+    error_message = (
+        u'Different tests were collected between 1 and 2. The difference is:\n'
+        u'--- 1 \n'
+        u'\n'
+        u'+++ 2 \n'
+        u'\n'
+        u'@@ -1,4 +1,4 @@\n'
+        u'\n'
+        u'-aaa\n'
+        u'+aZa\n'
+        u' bbb\n'
+        u'+XXX\n'
+        u' ccc\n'
+        u'-YYY'
+    )
+
+    try:
+        report_collection_diff(from_collection, to_collection, 1, 2)
+    except AssertionError as e:
+
+        assert unicode(e) == error_message

xdist/dsession.py

@@ -1,8 +1,14 @@
-import pytest, py
 import sys
+import difflib
+
+import pytest
+import py
 from xdist.slavemanage import NodeManager
+
+
 queue = py.builtin._tryimport('queue', 'Queue')
 
+
 class EachScheduling:
 
     def __init__(self, numnodes, log=None):
@@ -127,9 +133,11 @@ class LoadScheduling:
         assert not hasattr(self, 'item2nodes')
         self.item2nodes = {}
         # XXX allow nodes to have different collections
-        col = list(self.node2collection.values())[0]
+        first_node, col = list(self.node2collection.items())[0]
         for node, collection in self.node2collection.items():
-            assert collection == col
+            if collection != col:
+                report_collection_diff(col, collection, first_node.gateway.id, node.gateway.id)
+
         self.pending = col
         if not col:
             return
@@ -146,6 +154,34 @@ class LoadScheduling:
                 break
         del self.pending[:i + 1]
 
+
+def report_collection_diff(from_collection, to_collection, from_id, to_id):
+    """Report the collected test difference between two nodes.
+
+    :returns: True if collections are equal.
+
+    :raises: AssertionError with a detailed error message describing the
+             difference between the collections.
+
+    """
+    if from_collection == to_collection:
+        return True
+
+    diff = difflib.unified_diff(
+        from_collection,
+        to_collection,
+        fromfile=from_id,
+        tofile=to_id,
+    )
+    error_message = (
+        u'Different tests were collected between {from_id} and {to_id}. '
+        u'The difference is:\n'
+        u'{diff}'
+    ).format(from_id=from_id, to_id=to_id, diff='\n'.join(diff))
+
+    raise AssertionError(error_message)
+
+
 class Interrupted(KeyboardInterrupt):
     """ signals an immediate interruption. """