Hey Denis!
On Saturday May 29 2010 10:29:43 spir ☣ wrote:
> I'll try to clarify the purpose and use of sentinels with an example.
> Please, advanced programmers correct me. A point is that, in languages
> like python, sentinels are under-used, because everybody tends to une None
> instead, or as all-purpose sentinel.
>
> Imagine you're designing a kind of database of books; with a user interface
> to enter new data. What happens when an author is unknown? A proper way, I
> guess, to cope with this case, is to define a sentinel object, eg:
> UNKNOWN_AUTHOR = Object()
> There are many ways to define a sentinel; one could have defined "=0" or
> "=False" or whatever. But this choice is simple, clear, and secure because
> a custom object in python will only compare equal to itself -- by default.
> Sentinels are commonly written upercase because they are constant,
> predefined, elements.
I waited for a thread like this to appear, because I have a quirky, but IMHO
elegant, solution for those kinds of variables:
class EnumMeta(type):
def __repr__(self):
return self.__name__
class Enum(object):
__metaclass__ = EnumMeta
Objects are created by inheriting from the Enum class. (Not by instantiating
it.)
>>> class EAST(Enum): pass
>>> class WEST(Enum): pass
>>> class NORTH(Enum): pass
>>> class SOUTH(Enum): pass
The objects know their name, and when printed their name is printed. In this
respect they behave similarly to None.
>>> print NORTH, SOUTH, EAST, WEST
NORTH SOUTH EAST WEST
I call the class Enum, but this is certainly the wrong term since the values
are not enumerated. But is Sentinel the right term for something like this? I
thought a sentinel is a soldier who guards something. Hello English native
speakers! What is a good name?
Eike.
P.S. By the way Denis, an earlier thread from you on the subject got me
thinking about it.
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