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# Copyright (c) 2019 Ansible Project
# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
# Make coding more python3-ish
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
class Sentinel:
"""
Object which can be used to mark whether an entry as being special
A sentinel value demarcates a value or marks an entry as having a special meaning. In C, the
Null byte is used as a sentinel for the end of a string. In Python, None is often used as
a Sentinel in optional parameters to mean that the parameter was not set by the user.
You should use None as a Sentinel value any Python code where None is not a valid entry. If
None is a valid entry, though, then you need to create a different value, which is the purpose
of this class.
Example of using Sentinel as a default parameter value::
def confirm_big_red_button(tristate=Sentinel):
if tristate is Sentinel:
print('You must explicitly press the big red button to blow up the base')
elif tristate is True:
print('Countdown to destruction activated')
elif tristate is False:
print('Countdown stopped')
elif tristate is None:
print('Waiting for more input')
Example of using Sentinel to tell whether a dict which has a default value has been changed::
values = {'one': Sentinel, 'two': Sentinel}
defaults = {'one': 1, 'two': 2}
# [.. Other code which does things including setting a new value for 'one' ..]
values['one'] = None
# [..]
print('You made changes to:')
for key, value in values.items():
if value is Sentinel:
continue
print('%s: %s' % (key, value)
"""
def __new__(cls):
"""
Return the cls itself. This makes both equality and identity True for comparing the class
to an instance of the class, preventing common usage errors.
Preferred usage::
a = Sentinel
if a is Sentinel:
print('Sentinel value')
However, these are True as well, eliminating common usage errors::
if Sentinel is Sentinel():
print('Sentinel value')
if Sentinel == Sentinel():
print('Sentinel value')
"""
return cls
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