The Unknown truth value¶
The Unknown
object is used in Sage in several places as return value
in addition to True
and False
, in order to signal uncertainty
about or inability to compute the result. Unknown
can be identified
using is
, or by catching UnknownError
from a boolean operation.
Warning
Calling bool()
with Unknown
as argument will throw an
UnknownError
. This also means that in the following cases,
and
, not
, and or
fail or return a somewhat wrong value:
sage: not Unknown # should return Unknown
Traceback (most recent call last):
...
UnknownError: Unknown does not evaluate in boolean context
sage: Unknown and False # should return False
Traceback (most recent call last):
...
UnknownError: Unknown does not evaluate in boolean context
sage: Unknown or False # should return Unknown
Traceback (most recent call last):
...
UnknownError: Unknown does not evaluate in boolean context
EXAMPLES:
sage: def func(n):
....: if n > 0:
....: return True
....: elif n < 0:
....: return False
....: else:
....: return Unknown
Using direct identification:
sage: for n in [3, 0, 12]:
....: res = func(n)
....: if res is True:
....: print("n={} is positive".format(n))
....: elif res is False:
....: print("n={} is negative".format(n))
....: else:
....: print("n={} is neither positive nor negative".format(n))
n=3 is negative
n=0 is neither positive nor negative
n=12 is positive
Using UnknownError
:
sage: for n in [3, 0, 12]:
....: try:
....: if func(n):
....: print("n={} is positive".format(n))
....: else:
....: print("n={} is negative".format(n))
....: except UnknownError:
....: print("n={} is neither positive nor negative".format(n))
n=3 is negative
n=0 is neither positive nor negative
n=12 is positive
AUTHORS:
Florent Hivert (2010): initial version.
Ralf Stephan, Vincent Delecroix (20182020): redesign

sage.misc.unknown.
Unknown
= Unknown¶

class
sage.misc.unknown.
UnknownClass
¶ Bases:
sage.structure.unique_representation.UniqueRepresentation
The Unknown truth value
The
Unknown
object is used in Sage in several places as return value in addition toTrue
andFalse
, in order to signal uncertainty about or inability to compute the result.Unknown
can be identified usingis
, or by catchingUnknownError
from a boolean operation.Warning
Calling
bool()
withUnknown
as argument will throw anUnknownError
. This also means that applyingand
,not
, andor
toUnknown
might fail.