Return Unknown from ex.is_xyz() if Pynac returns false

[rwst]

Expected:

sage: x.is_integer()
Unknown
sage: if x.is_real(): True
sage: if not x.is_real(): False
False

To clarify, I think it's possible to separate the issue of
"why do I get False" from the issue of deciding between
True/False/Unknown, by simply returning Unknown if the
Pynac logic returns false.

It would be much less surprising to the user.

Later, when the complementary flags (irreal, noninteger, nonpositive) are implemented in Pynac these function can return True/Unknown/False, e.g. is_integer will return False if is_noninteger returns True, so both behave complementary.

CC: @pelegm

Component: symbolics

Author: Ralf Stephan

Branch/Commit: u/rws/22162-1 @ 079597d

Issue created by migration from https://trac.sagemath.org/ticket/22162

[rwst]

Branch: u/rws/return_unknown_from_ex_is_xyz___if_pynac_returns_false

[rwst]

Commit: 5918af7

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New commits:

5918af7

22162: return Unknown from is_integer()

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Author: Ralf Stephan

[rwst]

Description changed:

--- 
+++ 
@@ -8,4 +8,11 @@
 sage: 
 ```
 
+To clarify, I think it's possible to separate the issue of
+"why do I get False" from the issue of deciding between
+True/False/Unknown, by simply returning Unknown if the
+Pynac logic returns false.
+
+It would be much less surprising to the user.
+
 This ticket branch should change the behaviour of selected `is_xyz()` functions first, in order to see what's involved.

[rwst]

Description changed:

--- 
+++ 
@@ -10,7 +10,7 @@
 
 To clarify, I think it's possible to separate the issue of
 "why do I get False" from the issue of deciding between
-True/False/Unknown, by simply returning Unknown if the
+`True/False/Unknown`, by simply returning Unknown if the
 Pynac logic returns false.
 
 It would be much less surprising to the user.

[jdemeyer]

comment:5

Regarding Unknown, shouldn't we disallow conversion to bool? Currently, bool(Unknown) is False and not Unknown is True. What do you think about that?

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

5f5d134

22162: refine doctests

[sagetrac-git]

Changed commit from 5918af7 to 5f5d134

[rwst]

comment:7

Replying to @jdemeyer:

Regarding Unknown, shouldn't we disallow conversion to bool? Currently, bool(Unknown) is False and not Unknown is True. What do you think about that?

That would have to be changed to preserve previous usage. I don't think there is much of that in Sage (except is_zero which I don't want to deal with here).