Create assert_that function and use it from TestCase.assertThat.

This will allow matchers to be used outside of the TestCase context.
This is useful for people who can't buy in to testtools.TestCase fully
(for whatever reason), or who like using function tests (as supported by
nose).

testtools/assertions.py

@@ -0,0 +1,12 @@
+from testtools.matchers import (
+    Annotate,
+    MismatchError,
+    )
+
+
+def assert_that(matchee, matcher, message='', verbose=False):
+    matcher = Annotate.if_message(message, matcher)
+    mismatch = matcher.match(matchee)
+    if not mismatch:
+        return
+    raise MismatchError(matchee, matcher, mismatch, verbose)

testtools/testcase.py

@@ -29,6 +29,9 @@ from extras import (
 from testtools import (
     content,
     )
+from testtools.assertions import (
+    assert_that,
+)
 from testtools.compat import (
     advance_iterator,
     reraise,
@@ -401,14 +404,13 @@ class TestCase(unittest.TestCase):
         :param matcher: An object meeting the testtools.Matcher protocol.
         :raises MismatchError: When matcher does not match thing.
         """
-        matcher = Annotate.if_message(message, matcher)
-        mismatch = matcher.match(matchee)
-        if not mismatch:
-            return
-        existing_details = self.getDetails()
-        for (name, content) in mismatch.get_details().items():
-            self.addDetailUniqueName(name, content)
-        raise MismatchError(matchee, matcher, mismatch, verbose)
+        try:
+            assert_that(matchee, matcher, message, verbose)
+        except MismatchError as error:
+            mismatch = error.mismatch
+            for (name, content) in mismatch.get_details().items():
+                self.addDetailUniqueName(name, content)
+            raise
 
     def addDetailUniqueName(self, name, content_object):
         """Add a detail to the test, but ensure it's name is unique.