Python 2.5.2 (r252:60911, Sep 29 2008, 22:32:35) 
[GCC 4.3.2] on linux2
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\>>> foo = ()
\>>> bar = ()
\>>> foo == bar
True
\>>> foo is bar
True

I dislike it for that reason, but also because it’s not clear that the zero-item tuple might not mean something else. This also rules out, for me, other objects that seem to have a potential meaning in themselves.

So my preference is for a bald instance of the ‘object’ type:

\>>> foo = object()
\>>> bar = object()
\>>> foo == bar
False
\>>> foo is bar
False

The statement creating the object is pretty clearly not creating an object that is useful for anything other than being unique. This makes it distinct in a way that a tuple, of whatever length, is not; and so that makes it uniquely suitable for defining a sentinel value.

I tend to use (1,) as my sentinel.

>>> MARKER1 = (1,)

>>> MARKER2 = (1,)

>>> MARKER1 is MARKER2

False

Thus you can use it to give different sentinels distinct instances. I feel Ian’s solution is a bit .. much .. for such a simple thing.

class NoDefault(object):
    pass

def getuser(username, default=NoDefault):
    ...
    if default is NoDefault:
        ...

Schevo uses a metaclass to provide repr and str for the class itself, as well as some metaclass subclasses that also provide nonzero, cmp, len for Schevo’s special UNASSIGNED constant.

See http://github.com/gldnspud/schevo/tree/master/schevo/constant.py

When pickling an instance of a sentinel class, you have a situation where the in-memory singleton instance of a sentinel class is not the same object as an unpickled instance of the same class, therefore “is” comparisons won’t work.

By pickling the class itself, when the sentinel is unpickled it becomes a reference to the class itself, so “is” comparisons work as intended.

>>> a = ()

>>> b = ()

>>> a is b

True

Though it’s not a singleton like None, its immutability means that any variable set to the same value may point to the same object in memory. All the more reason to use your custom sentinel, or at least [] or object().