src/sage/structure/sequence.py: skip fewer cases when pbori is missing

[orlitzky]

When the (optional) sage.rings.polynomial.pbori.pbori fails to import, one of the cases in the Sequence() constructor is skipped, leading to certain polynomial sequences being returned as generic sequences instead of polynomial ones. In one egregious case, this causes a multivariate polynomial sequence with 324 elements to be printed to the screen rather than the short description that is intended:

File "...multi_polynomial_libsingular.pyx", line 4790...
Failed example:
    f.lift(I)
Expected:
    Polynomial Sequence with 324 Polynomials in 2 Variables
Got:
    [0,
     0,
     0,
     0,
     0, ...

We fix this by handling the (failure of the) pbori import separately, keeping the rest of the special case for polynomial sequences.

[orlitzky]

Can be reproduced/tested by passing -Dbrial=disabled to meson setup.

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[orlitzky]

@kiwifb can I borrow you for a second as the de-facto brial person?

This diff is confusing, but all it does is move something important out of the try ... except ImportError block for brial.

[kiwifb]

Yes, it seems keeping the try block minimal and not throwing the stuff you would need anyway looks much cleaner. But the other bits are interesting. You get rid of the else alternative and you alias the method you can't import to a generic one.
I'd say that else part was probably badly broken and giving funny behavior.

[kiwifb]

LGTM

[orlitzky]

But the other bits are interesting. You get rid of the else alternative and you alias the method you can't import to a generic one.

This was a trick to keep the existing conditional working without having to add cases. By setting BooleanMonomialMonoid = MPolynomialRing_base when the import fails, the subsequent isinstance() reduces to a check for one type (BooleanMonomialMonoid == MPolynomialRing_base). Without that we would have needed a special case (copy & paste of the "if" statement) to avoid checking for an instance of None.

The name BooleanMonomialMonoid is local and undefined when we set it (for only a few lines), so it shouldn't impact anything else. Thanks for taking a look.