Escape newlines in result from assertrepr hook

The result from the pytest_assertrepr_compare hook should not include
any newlines since that will confuse the mini-formatting language used
by assertion.util.format_explanation.  So simply escape the included
newlines, this way hook writers do not have to worry about this at
all.

Fixes issue 453.

_pytest/assertion/__init__.py

@@ -88,22 +88,38 @@ def pytest_collection(session):
 
 
 def pytest_runtest_setup(item):
+    """Setup the pytest_assertrepr_compare hook
+
+    The newinterpret and rewrite modules will use util._reprcompare if
+    it exists to use custom reporting via the
+    pytest_assertrepr_compare hook.  This sets up this custom
+    comparison for the test.
+    """
     def callbinrepr(op, left, right):
+        """Call the pytest_assertrepr_compare hook and prepare the result
+
+        This uses the first result from the hook and then ensures the
+        following:
+        * Overly verbose explanations are dropped unles -vv was used.
+        * Embedded newlines are escaped to help util.format_explanation()
+          later.
+        * If the rewrite mode is used embedded %-characters are replaced
+          to protect later % formatting.
+
+        The result can be formatted by util.format_explanation() for
+        pretty printing.
+        """
         hook_result = item.ihook.pytest_assertrepr_compare(
             config=item.config, op=op, left=left, right=right)
         for new_expl in hook_result:
             if new_expl:
-                # Don't include pageloads of data unless we are very
-                # verbose (-vv)
                 if (sum(len(p) for p in new_expl[1:]) > 80*8
                         and item.config.option.verbose < 2):
                     new_expl[1:] = [py.builtin._totext(
                         'Detailed information truncated, use "-vv" to show')]
+                new_expl = [line.replace("\n", "\\n") for line in new_expl]
                 res = py.builtin._totext("\n~").join(new_expl)
                 if item.config.getvalue("assertmode") == "rewrite":
-                    # The result will be fed back a python % formatting
-                    # operation, which will fail if there are extraneous
-                    # '%'s in the string. Escape them here.
                     res = res.replace("%", "%%")
                 return res
     util._reprcompare = callbinrepr
@@ -145,4 +161,5 @@ def warn_about_missing_assertion(mode):
                          "(are you using python -O?)\n")
 
 
+# Expose this plugin's implementation for the pytest_assertrepr_compare hook
 pytest_assertrepr_compare = util.assertrepr_compare

testing/test_assertion.py

@@ -4,6 +4,7 @@ import sys
 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)")
 
 
@@ -199,6 +200,21 @@ class TestAssert_reprcompare:
         assert msg
 
 
+class TestFormatExplanation:
+
+    def test_speical_chars_full(self, testdir):
+        # Issue 453, for the bug this would raise IndexError
+        testdir.makepyfile("""
+            def test_foo():
+                assert u'\\n}' == u''
+        """)
+        result = testdir.runpytest()
+        assert result.ret == 1
+        result.stdout.fnmatch_lines([
+            "*AssertionError*",
+        ])
+
+
 def test_python25_compile_issue257(testdir):
     testdir.makepyfile("""
         def test_rewritten():