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`import sage.all` / cysignals dies with `TypeError: signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object`

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Source: Troubleshooting reference

Symptom. Importing cysignals.pysignals (directly, or transitively via passagemath’s sage.all) raises the TypeError above. Plain import cysignals.signals (and cypari2) works fine.

Root cause. This macOS release (Darwin 25.x) pre-installs C-level fault handlers (SIGILL, SIGABRT, SIGFPE, SIGBUS, SIGSEGV) in every process. signal.getsignal() reports a handler Python didn’t install as None, and cysignals’ pysignals init saves + re-installs existing handlers — re-setting None is rejected by CPython. Not sandbox-, uv-, or Python-version-specific: reproduced on uv’s python-build-standalone 3.12/3.13 and Homebrew 3.14.

Confirm.

Terminal window
python3 -c "import signal; print(signal.getsignal(signal.SIGSEGV))"   # → None

Fix. Reset the fault handlers from Python before anything imports cysignals.pysignals (~/dev/math-lab/sagefix.py does exactly this):

import signal
for s in (signal.SIGILL, signal.SIGABRT, signal.SIGFPE, signal.SIGBUS, signal.SIGSEGV):
    signal.signal(s, signal.SIG_DFL)

Related trap: pinning cysignals older than what passagemath wheels were built against fails later with cysignals.signals does not export expected C function _do_raise_exception — keep the resolver’s cysignals (1.12.x), fix the handlers instead.