Produce consistent string for __repr__() of NoValue

The :attr:`.api.NO_VALUE` constant now has a fixed ``__repr__()``
output, so that scenarios where this constant's string value
ends up being used as a cache key do not create multiple values.
Pull request courtesy Paul Brown.

Change-Id: Iecc9633d6be562bc6821bf4ee2ff3f29d1b31e80
Pull-request: https://bitbucket.org/zzzeek/dogpile.cache/pull-requests/61

docs/build/changelog.rst

@@ -14,6 +14,14 @@ Changelog
       problems for backends like that of Redis.  Pull request courtesy
       Tobias Sauerwein.
 
+    .. change::
+      :tags: bug
+
+      The :attr:`.api.NO_VALUE` constant now has a fixed ``__repr__()``
+      output, so that scenarios where this constant's string value
+      ends up being used as a cache key do not create multiple values.
+      Pull request courtesy Paul Brown.
+
 .. changelog::
     :version: 0.6.3
     :released: Thu May 18, 2017

dogpile/cache/api.py

@@ -13,6 +13,13 @@ class NoValue(object):
     def payload(self):
         return self
 
+    def __repr__(self):
+        """Ensure __repr__ is a consistent value in case NoValue is used to
+        fill another cache key.
+
+        """
+        return '<dogpile.cache.api.NoValue object>'
+
     if py3k:
         def __bool__(self):  # pragma NO COVERAGE
             return False
@@ -20,6 +27,7 @@ class NoValue(object):
         def __nonzero__(self):  # pragma NO COVERAGE
             return False
 
+
 NO_VALUE = NoValue()
 """Value returned from ``get()`` that describes
 a  key not present."""

tests/cache/test_region.py

@@ -17,6 +17,11 @@ def key_mangler(key):
     return "HI!" + key
 
 
+class APITest(TestCase):
+    def test_no_value_str(self):
+        eq_(str(NO_VALUE), "<dogpile.cache.api.NoValue object>")
+
+
 class RegionTest(TestCase):
 
     def _region(self, init_args={}, config_args={}, backend="mock"):