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We'll need to bump the version requirement to >= 1.4 in pyproject.toml, too.

We'll need to bump the version requirement to >= 1.4 in pyproject.toml, too.

Well, the idea is that this should work with older mpmath too. Otherwise it doesn't make sense to split it from the sage-the-distro upgrade. I think it's too soon to require 1.4, which was just released.

Well, the idea is that this should work with older mpmath too.

I tried this in #41014, and while I'm sure it can be done (I got it working with the meson build but gave up due to repeated CI failures I couldn't reproduce), it requires a lot more work. The tests are not too bad, but a different set of C extensions are compiled when v1.3 is installed, and utils.pyx has a different set of cimports.

Well, the idea is that this should work with older mpmath too.

I tried this in #41014, and while I'm sure it can be done (I got it working with the meson build but gave up due to repeated CI failures I couldn't reproduce), it requires a lot more work. The tests are not too bad, but a different set of C extensions are compiled when v1.3 is installed, and utils.pyx has a different set of cimports.

CI is fine with 1.3. Can you give more details about that different set of C extensions you mention, and what does it break?

There is a circular dependency between mpmath-1.3 and the portion of sage.libs.mpmath.utils that was deleted, but in the end it's not doing much and it looks like I was wasting my time trying to preserve it for backwards-compatibility. I just spent a while trying to cause problems and couldn't do it without essentially setting CAUSE_PROBLEMS=1 in my environment.

Someone added a guard to mpmath a few years ago to disable the sage backend when SAGE_ROOT is not set, and SAGE_ROOT is obsolete / meaningless outside of the sage distro. Inside the sage distro, MPMATH_NOSAGE disables it.
In either case, with the sage backend disabled in v1.3, there is a GMP backend that will be used instead unless you explicitly disable that. (I only ran trivial addition and multiplication tests, but the GMP backend didn't look any slower than sage). After upgrading from 1.3 -> 1.4, the sage backend ceases to exist, so you'll get the GMP backend there too unless you've explicitly disabled it.