Improve assertion failure reporting on iterables, by using ndiff and pprint.

CHANGELOG

@@ -1,3 +1,8 @@
+Unreleased
+----------
+
+- Improve assertion failure reporting on iterables, by using ndiff and pprint.
+
 2.6.3
 -----------
 

_pytest/assertion/util.py

@@ -135,18 +135,32 @@ def assertrepr_compare(config, op, left, right):
     isdict = lambda x: isinstance(x, dict)
     isset = lambda x: isinstance(x, (set, frozenset))
 
+    def isiterable(obj):
+        try:
+            iter(obj)
+            return not istext(obj)
+        except TypeError:
+            return False
+
     verbose = config.getoption('verbose')
     explanation = None
     try:
         if op == '==':
             if istext(left) and istext(right):
                 explanation = _diff_text(left, right, verbose)
-            elif issequence(left) and issequence(right):
-                explanation = _compare_eq_sequence(left, right, verbose)
-            elif isset(left) and isset(right):
-                explanation = _compare_eq_set(left, right, verbose)
-            elif isdict(left) and isdict(right):
-                explanation = _compare_eq_dict(left, right, verbose)
+            else:
+                if issequence(left) and issequence(right):
+                    explanation = _compare_eq_sequence(left, right, verbose)
+                elif isset(left) and isset(right):
+                    explanation = _compare_eq_set(left, right, verbose)
+                elif isdict(left) and isdict(right):
+                    explanation = _compare_eq_dict(left, right, verbose)
+                if isiterable(left) and isiterable(right):
+                    expl = _compare_eq_iterable(left, right, verbose)
+                    if explanation is not None:
+                        explanation.extend(expl)
+                    else:
+                        explanation = expl
         elif op == 'not in':
             if istext(left) and istext(right):
                 explanation = _notin_text(left, right, verbose)
@@ -203,6 +217,19 @@ def _diff_text(left, right, verbose=False):
     return explanation
 
 
+def _compare_eq_iterable(left, right, verbose=False):
+    if not verbose:
+        return [u('Use -v to get the full diff')]
+    # dynamic import to speedup pytest
+    import difflib
+
+    left = pprint.pformat(left).splitlines()
+    right = pprint.pformat(right).splitlines()
+    explanation = [u('Full diff:')]
+    explanation.extend(line.strip() for line in difflib.ndiff(left, right))
+    return explanation
+
+
 def _compare_eq_sequence(left, right, verbose=False):
     explanation = []
     for i in range(min(len(left), len(right))):

testing/test_assertion.py

@@ -1,11 +1,14 @@
 # -*- coding: utf-8 -*-
 import sys
+import textwrap
 
 import py, pytest
 import _pytest.assertion as plugin
 from _pytest.assertion import reinterpret
 from _pytest.assertion import util
+
 needsnewassert = pytest.mark.skipif("sys.version_info < (2,6)")
+PY3 = sys.version_info >= (3, 0)
 
 
 @pytest.fixture
@@ -86,6 +89,48 @@ class TestAssert_reprcompare:
         expl = callequal([0, 1], [0, 2])
         assert len(expl) > 1
 
+    @pytest.mark.parametrize(
+        ['left', 'right', 'expected'], [
+            ([0, 1], [0, 2], """
+                Full diff:
+                - [0, 1]
+                ?     ^
+                + [0, 2]
+                ?     ^
+            """),
+            ({0: 1}, {0: 2}, """
+                Full diff:
+                - {0: 1}
+                ?     ^
+                + {0: 2}
+                ?     ^
+            """),
+            (set([0, 1]), set([0, 2]), """
+                Full diff:
+                - set([0, 1])
+                ?         ^
+                + set([0, 2])
+                ?         ^
+            """ if not PY3 else """
+                Full diff:
+                - {0, 1}
+                ?     ^
+                + {0, 2}
+                ?     ^
+            """)
+        ]
+    )
+    def test_iterable_full_diff(self, left, right, expected):
+        """Test the full diff assertion failure explanation.
+
+        When verbose is False, then just a -v notice to get the diff is rendered,
+        when verbose is True, then ndiff of the pprint is returned.
+        """
+        expl = callequal(left, right, verbose=False)
+        assert expl[-1] == 'Use -v to get the full diff'
+        expl = '\n'.join(callequal(left, right, verbose=True))
+        assert expl.endswith(textwrap.dedent(expected).strip())
+
     def test_list_different_lenghts(self):
         expl = callequal([0, 1], [0, 1, 2])
         assert len(expl) > 1