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unknown.py
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unknown.py
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"""
The Unknown truth value
AUTHORS:
- Florent Hivert (2010): initial version.
"""
from __future__ import print_function
from sage.structure.sage_object import SageObject
from sage.structure.unique_representation import UniqueRepresentation
class UnknownClass(UniqueRepresentation, SageObject):
"""
TESTS::
sage: TestSuite(Unknown).run()
"""
def __init__(self):
"""
The ``Unknown`` truth value
EXAMPLES::
sage: l = [False, Unknown, True]
sage: for a in l: print([a and b for b in l])
[False, False, False]
[Unknown, Unknown, Unknown]
[False, Unknown, True]
sage: for a in l: print([a or b for b in l])
[False, Unknown, True]
[False, Unknown, True]
[True, True, True]
..warning:: Unless PEP 335 is accepted, in the following cases,
``and``, ``not`` and ``or`` return a somewhat wrong value::
sage: not Unknown # should return Unknown
True
sage: Unknown and False # should return False
Unknown
sage: Unknown or False # should return Unknown
False
"""
def _repr_(self):
"""
TESTS::
sage: Unknown
Unknown
"""
return "Unknown"
def __nonzero__(self):
"""
When evaluated in a boolean context ``Unknown()`` is evalutated into
``False``.
EXAMPLES::
sage: bool(Unknown)
False
sage: not Unknown
True
"""
return False
def __and__(self, other):
"""
The ``and`` logical connector.
..warning:: This is not used by ``and`` unless PEP 335 is accepted.
EXAMPLES::
sage: Unknown & False
False
sage: Unknown & Unknown
Unknown
sage: Unknown & True
Unknown
Compare with::
sage: Unknown and False # should return False
Unknown
"""
if other is False: return False
else: return self
def __or__(self, other):
"""
The ``or`` logical connector.
..warning:: This is not used by ``or`` unless PEP 335 is accepted.
EXAMPLES::
sage: Unknown | False
Unknown
sage: Unknown | Unknown
Unknown
sage: Unknown | True
True
Compare with::
sage: Unknown or False # should return Unknown
False
"""
if other: return True
else: return self
def __not__(self):
"""
The ``not`` logical connector.
..warning:: This is not used by ``not`` unless PEP 335 is accepted.
EXAMPLES::
sage: Unknown.__not__()
Unknown
Compare with::
sage: not Unknown # should return Unknown
True
"""
return self
def __cmp__(self, other):
"""
Comparison of truth value.
EXAMPLES::
sage: l = [False, Unknown, True]
sage: for a in l: print([a < b for b in l])
[False, True, True]
[False, False, True]
[False, False, False]
sage: for a in l: print([a <= b for b in l])
[True, True, True]
[False, True, True]
[False, False, True]
"""
if other is self:
return 0
if isinstance(other, bool):
if other:
return -1
else:
return +1
else:
raise ValueError("Unable to compare {} with {}".format(self, other))
Unknown = UnknownClass()