Add "not in" to detailed explanations

This simply uses difflib to compare the text without the offending
string to the full text.

Also ensures the summary line uses all space available.  But the
terminal width is still hardcoded.

_pytest/assertion.py

@@ -50,8 +50,9 @@ except NameError:
 
 def pytest_assertrepr_compare(op, left, right):
     """return specialised explanations for some operators/operands"""
-    left_repr = py.io.saferepr(left, maxsize=30)
-    right_repr = py.io.saferepr(right, maxsize=30)
+    width = 80 - 15 - len(op) - 2 # 15 chars indentation, 1 space around op
+    left_repr = py.io.saferepr(left, maxsize=width/2)
+    right_repr = py.io.saferepr(right, maxsize=width-len(left_repr))
     summary = '%s %s %s' % (left_repr, op, right_repr)
 
     issequence = lambda x: isinstance(x, (list, tuple))
@@ -71,6 +72,9 @@ def pytest_assertrepr_compare(op, left, right):
             elif isdict(left) and isdict(right):
                 explanation = _diff_text(py.std.pprint.pformat(left),
                                          py.std.pprint.pformat(right))
+        elif op == 'not in':
+            if istext(left) and istext(right):
+                explanation = _notin_text(left, right)
     except py.builtin._sysex:
         raise
     except:
@@ -154,3 +158,11 @@ def _compare_eq_set(left, right):
         for item in diff_right:
             explanation.append(py.io.saferepr(item))
     return explanation
+
+
+def _notin_text(term, text):
+    index = text.find(term)
+    head = text[:index]
+    tail = text[index+len(term):]
+    correct_text = head + tail
+    return _diff_text(correct_text, text)

doc/example/assertion/failure_demo.py

@@ -77,6 +77,22 @@ class TestSpecialisedExplanations(object):
     def test_in_list(self):
         assert 1 in [0, 2, 3, 4, 5]
 
+    def test_not_in_text_multiline(self):
+        text = 'some multiline\ntext\nwhich\nincludes foo\nand a\ntail'
+        assert 'foo' not in text
+
+    def test_not_in_text_single(self):
+        text = 'single foo line'
+        assert 'foo' not in text
+
+    def test_not_in_text_single_long(self):
+        text = 'head ' * 50 + 'foo ' + 'tail ' * 20
+        assert 'foo' not in text
+
+    def test_not_in_text_single_long_term(self):
+        text = 'head ' * 50 + 'f'*70 + 'tail ' * 20
+        assert 'f'*70 not in text
+
 
 def test_attribute():
     class Foo(object):

testing/test_assertion.py

@@ -114,6 +114,11 @@ class TestAssert_reprcompare:
         expl = callequal(A(), '')
         assert not expl
 
+def test_reprcompare_notin():
+    detail = plugin.pytest_assertrepr_compare('not in', 'foo', 'aaafoobbb')[1:]
+    assert '- aaabbb' in detail
+    assert '+ aaafoobbb' in detail
+
 @needsnewassert
 def test_pytest_assertrepr_compare_integration(testdir):
     testdir.makepyfile("""