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47 lines
1.2 KiB
Python
47 lines
1.2 KiB
Python
"""
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Construction of sentinel objects.
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Sentinel objects are used when you only care to check for object identity.
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"""
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import sys
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def sentinel(name, doc=None):
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try:
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return sentinel._cache[name, doc] # memoized
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except KeyError:
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pass
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@object.__new__ # bind a single instance to the name 'Sentinel'
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class Sentinel(object):
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__doc__ = doc
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__slots__ = ('__weakref__',)
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__name__ = name
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def __new__(cls):
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raise TypeError("Can't construct new instances of %r" % name)
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def __repr__(self):
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return 'sentinel(%r)' % name
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def __reduce__(self):
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return sentinel, (name, doc)
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def __deepcopy__(self, _memo):
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return self
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def __copy__(self):
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return self
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cls = type(Sentinel)
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try:
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# traverse up one frame to find the module where this is defined
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cls.__module__ = sys._getframe(1).f_globals['__name__']
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except (AttributeError, ValueError, KeyError):
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# Couldn't get the name from the calling scope, just use None.
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cls.__module__ = None
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sentinel._cache[name, doc] = Sentinel # cache result
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return Sentinel
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sentinel._cache = {}
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