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