Fixed return value from delete command.

The `delete()` command currently returns `1` if the response from the
server is either `'DELETED'` or `'NOT_FOUND'`. This differs from other
implementations, e.g. `pymemcache` and `pylibmc`, and doesn't seem to
make sense given that these are the only two documented return types
from the protocol for the delete command.

It seems that `delete()` was changed to explicitly consider both
responses as successful way back in 2010, but that change only seemed to
double down on the behavior that was being reported in the issue.
See https://bugs.launchpad.net/python-memcached/+bug/471727. The concern
was avoiding breaking backward compatibility.

It was possible to work around this by calling `_deletetouch()` and
passing in the `expected` responses, but that was broken by the change
to remove `time` in ab668ed.

Making the change to interpret `'NOT_FOUND'` as `0` doesn't cause any
test failures, is consistent with the docstring which states that that
the command returns non-zero on success, and also brings consistency
with other implementations.

Fixes #170.

Author: ngnpope

memcache.py

@@ -552,9 +552,9 @@ def delete(self, key, noreply=False):
             if noreply:
                 return 1
             line = server.readline()
-            if line and line.strip() in [b'DELETED', b'NOT_FOUND']:
+            if line and line.strip() == b'DELETED':
                 return 1
-            self.debuglog('delete expected DELETED or NOT_FOUND, got: {!r}'.format(line))
+            self.debuglog('delete expected DELETED, got: {!r}'.format(line))
         except OSError as msg:
             if isinstance(msg, tuple):
                 msg = msg[1]

tests/test_memcache.py

@@ -56,6 +56,8 @@ def test_delete(self):
         result = self.mc.delete("long")
         self.assertEqual(result, True)
         self.assertEqual(self.mc.get("long"), None)
+        result = self.mc.delete("<missing>")
+        self.assertEqual(result, False)
 
     def test_default(self):
         key = "default"