Fix call for FiniteEnumeratedSet's of plain Python objects

[nthiery]

Problem

Before this ticket, a FiniteEnumeratedSet could not be called on its
own elements whenever they were not Element's:

sage: F = FiniteEnumeratedSet(["a", 1])
sage: F(1)
1
sage: F("a")
...
TypeError: Cannot convert str to sage.structure.element.Element

This prevented the use of F(x) as generic idiom to convert x into
F while checking that it's in F.

And indeed:

sage: TestSuite(F).run()
Failure in _test_an_element:
...
TypeError: Cannot convert str to sage.structure.element.Element
------------------------------------------------------------
The following tests failed: _test_an_element

Analysis

Parent.__call__ enforces that _element_constructor_ return an
Element (more precisely, it calls _element_constructor_ through a
DefaultConvertMap, and any Map requires its results to be
instances of Element).

Proposed solution

Since FiniteEnumeratedSets is often a facade over plain Python
objects, this ticket works around this limitation by a custom
FiniteEnumeratedSets.__call__ that calls directly
_element_constructor_ whenever el is not an Element. Otherwise
Parent.__call__ is called as usual.

Limitation

This workaround prevents conversions or coercions from facade parents
over plain Python objects. But it's already much better than before!

CC: @sagetrac-sage-combinat @nathanncohen @videlec

Component: combinatorics

Author: Nicolas M. Thiéry

Branch/Commit: 4e27454

Reviewer: Nathann Cohen

Issue created by migration from https://trac.sagemath.org/ticket/16280

[nthiery]

Description changed:

--- 
+++ 
@@ -1 +1,48 @@
+*Problem*
 
+Before this ticket, a FiniteEnumeratedSet could not be called on it's
+own elements whenever they were not Element's:
+
+```
+sage: F = FiniteEnumeratedSet(["a", 1])
+sage: F(1)
+1
+sage: F("a")
+...
+TypeError: Cannot convert str to sage.structure.element.Element
+```
+
+This prevented the use of `F(x)` as generic idiom to convert `x` into
+`F` while checking that it's in `F`.
+
+And indeed:
+
+```
+sage: TestSuite(F).run()
+Failure in _test_an_element:
+...
+TypeError: Cannot convert str to sage.structure.element.Element
+------------------------------------------------------------
+The following tests failed: _test_an_element
+```
+
+*Analysis*
+
+`Parent.__call__` enforces that `_element_constructor_` return an
+`Element` (more precisely, it calls `_element_constructor_` through a
+`DefaultConvertMap`, and any `Map` requires its results to be
+instances of `Element`).
+
+*Proposed solution*
+
+Since `FiniteEnumeratedSets` is often a facade over plain Python
+objects, this ticket works around this limitation by a custom
+`FiniteEnumeratedSets.__call__` that calls directly
+`_element_constructor_` whenever ``el`` is not an `Element`. Otherwise
+`Parent.__call__` is called as usual.
+
+*Limitation*
+
+This workaround prevents conversions or coercions from facade parents
+over plain Python objects. But it's already much better than before!
+

[nthiery]

Author: Nicolas M. Thiéry

[nthiery]

Branch: u/nthiery/fix_call_for_finite_enumeratedset_s_over_plain_python_objects

[nthiery]

Commit: 8ac32c2

[nthiery]

New commits:

8ac32c2

#16280: Fix call for FiniteEnumeratedSet's of plain Python objects

[nthiery]

Description changed:

--- 
+++ 
@@ -1,7 +1,7 @@
 *Problem*
 
-Before this ticket, a FiniteEnumeratedSet could not be called on it's
-own elements whenever they were not Element's:
+Before this ticket, a `FiniteEnumeratedSet` could not be called on its
+own elements whenever they were not `Element`'s:
 
 ```
 sage: F = FiniteEnumeratedSet(["a", 1])

[nathanncohen]

comment:5

sage -t --long sets/totally_ordered_finite_set.py  # 1 doctest failed
sage -t --long sets/finite_enumerated_set.py  # 3 doctests failed

Nothing serious, but there is something weird O_O;;

File "sets/finite_enumerated_set.py", line 345, in sage.sets.finite_enumerated_set.FiniteEnumeratedSet.__call__
Failed example:
    psi('a')
Expected:
    2
Got:
    1