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llllllllll
61 lines
1.6 KiB
Python
61 lines
1.6 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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from textwrap import dedent
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def sentinel(name, doc=None):
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try:
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value = sentinel._cache[name] # memoized
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except KeyError:
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pass
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else:
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if doc == value.__doc__:
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return value
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raise ValueError(dedent(
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"""\
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New sentinel value %r conflicts with an existing sentinel of the
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same name.
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Old sentinel docstring: %r
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New sentinel docstring: %r
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Resolve this conflict by changing the name of one of the sentinels.
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""",
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) % (name, value.__doc__, doc))
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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('cannot create %r instances' % 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 (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] = Sentinel # cache result
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return Sentinel
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sentinel._cache = {}
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