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

>>> from sage.all import *
>>> not Unknown         # should return Unknown
Traceback (most recent call last):
...
UnknownError: Unknown does not evaluate in boolean context
>>> Unknown and False   # should return False
Traceback (most recent call last):
...
UnknownError: Unknown does not evaluate in boolean context
>>> 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

>>> from sage.all import *
>>> def func(n):
...     if n > Integer(0):
...         return True
...     elif n < Integer(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

>>> from sage.all import *
>>> for n in [-Integer(3), Integer(0), Integer(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

>>> from sage.all import *
>>> for n in [-Integer(3), Integer(0), Integer(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 (2018-2020): redesign

class sage.misc.unknown.UnknownClass[source]#

Bases: UniqueRepresentation

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 applying and, not, and or to Unknown might fail.

exception sage.misc.unknown.UnknownError[source]#

Bases: TypeError

Raised whenever Unknown is used in a boolean operation.

EXAMPLES:

sage: not Unknown
Traceback (most recent call last):
...
UnknownError: Unknown does not evaluate in boolean context

>>> from sage.all import *
>>> not Unknown
Traceback (most recent call last):
...
UnknownError: Unknown does not evaluate in boolean context