| #!/usr/bin/env python3 |
| # |
| # Copyright (c) 2019, The OpenThread Authors. |
| # All rights reserved. |
| # |
| # Redistribution and use in source and binary forms, with or without |
| # modification, are permitted provided that the following conditions are met: |
| # 1. Redistributions of source code must retain the above copyright |
| # notice, this list of conditions and the following disclaimer. |
| # 2. Redistributions in binary form must reproduce the above copyright |
| # notice, this list of conditions and the following disclaimer in the |
| # documentation and/or other materials provided with the distribution. |
| # 3. Neither the name of the copyright holder nor the |
| # names of its contributors may be used to endorse or promote products |
| # derived from this software without specific prior written permission. |
| # |
| # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" |
| # AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE |
| # IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE |
| # ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE |
| # LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR |
| # CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF |
| # SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS |
| # INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN |
| # CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) |
| # ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE |
| # POSSIBILITY OF SUCH DAMAGE. |
| # |
| |
| nullField = None |
| |
| |
| class NullField(object): |
| """ |
| Represents a null field that does not exists. |
| """ |
| |
| def __new__(cls, *args, **kwargs): |
| global nullField |
| |
| if nullField is None: |
| nullField = object.__new__(cls, *args, **kwargs) |
| |
| return nullField |
| |
| def __init__(self): |
| assert self is nullField |
| |
| def __bool__(self): |
| """ |
| NullField is always treated as False. |
| """ |
| return False |
| |
| def __getattr__(self, item): |
| """ |
| Any sub field of the NullField is NullField itself. |
| """ |
| return self |
| |
| def __setattr__(self, key, value): |
| pass |
| |
| def __len__(self) -> 0: |
| return 0 |
| |
| def __eq__(self, other): |
| """ |
| NullField is always not equal to any other value. |
| """ |
| return False |
| |
| def __ne__(self, other): |
| return True |
| |
| def __lt__(self, other): |
| """ |
| Comparing NullField to any other value gets False. |
| """ |
| return False |
| |
| def __le__(self, other): |
| """ |
| Comparing NullField to any other value gets False. |
| """ |
| return False |
| |
| def __gt__(self, other): |
| """ |
| Comparing NullField to any other value gets False. |
| """ |
| return False |
| |
| def __ge__(self, other): |
| """ |
| Comparing NullField to any other value gets False. |
| """ |
| return False |
| |
| def __str__(self): |
| return "nullField" |
| |
| def __repr__(self): |
| return 'nullField' |
| |
| |
| NullField() |
| |
| if __name__ == '__main__': |
| assert nullField is NullField() |
| assert not nullField, repr(nullField) |
| assert nullField != nullField, repr(nullField) |
| assert nullField != 0 |
| assert not (nullField > 1) |
| assert not (nullField < 1) |
| assert not (nullField < nullField) |
| assert not (nullField > nullField) |
| assert bool(nullField) is False |
| assert nullField != "" |
| assert nullField != None # noqa |
| assert nullField is not None |