42 lines
976 B
Python
42 lines
976 B
Python
"""
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This module provides a ``Sentinel`` class that can be used directly as a
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sentinel singleton, or subclassed if a distinct sentinel singleton is needed.
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The ``repr`` of a ``Sentinel`` class is its name::
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>>> class Missing(Sentinel): pass
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>>> Missing
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Missing
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If a different ``repr`` is required,
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you can define it as a class attribute::
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>>> class CustomRepr(Sentinel):
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... repr = '<CustomRepr>'
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...
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>>> CustomRepr
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<CustomRepr>
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``Sentinel`` classes cannot be instantiated::
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>>> Missing()
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Traceback (most recent call last):
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...
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TypeError: 'Missing' is a sentinel and cannot be instantiated
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"""
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class _SentinelMeta(type):
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def __repr__(cls):
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try:
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return cls.repr
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except AttributeError:
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return cls.__name__
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class Sentinel(metaclass=_SentinelMeta):
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def __new__(cls):
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msg = 'is a sentinel and cannot be instantiated'
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raise TypeError(f"'{cls!r}' {msg}")
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