Vlastimil Zíma added the comment:
I encountered this problem when I tested code with cache, something like
def test_foo(self):
cache.set('a_key', sentinel.status) # Performs pickle
self.assertEqual(
cache.get('a_key'), # Performs unpickle
sentinel.status)
I spent some time searching for reason why the test failed. Since both
sentinels represent themselves as 'sentinel.status', I was quite surprised by
assertion failure. Only after I looked at the code I realized where is the
problem. Two sentinels are equal if and only if they are identical.
On my way home I realized this probably is and needs to be a feature of
sentinels. Consider testing this function:
def foo(value, do_copy):
if do_copy:
return bar(copy(value))
else:
return bar(value)
You would like to know whether `bar` was called with value or its copy.
Sentinel is in a test for such function a great option to make sure `value` is
not copied in process. This is especially usefull is you write tests for
wrapper-like function which should only pass arguments (or a subset) to other
interface.
After reading through comments and thinking about the problem I have following
opinions:
* Sentinels should not actually survive pickle/unpickle.
* Prevent sentinels from being pickled is a good idea, it is a valid way to
tell developer he does something he shouldn't. This might also apply to
copying, which would result in the same problem.
* It should be noted in documentation that sentinels are equal only if
identical. Since they represent themselves by their name it is hard to guess
the name is only for the representation. Also I find the documentation snippet
>>> assert result is sentinel.some_object
somewhat misleading as you don't usually use 'is' for comparison and it
provides no information about equality.
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