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Lib/signal.py

Source:

cpython 3.14 @ ab2d84fe1023/Lib/signal.py

signal provides Python-level signal handling. The Python layer (Lib/signal.py) is mostly a thin re-export of the C extension _signal. The actual signal handling logic, including the write-to-pipe mechanism that wakes the eval loop, lives in Modules/signalmodule.c and Python/ceval.c.

Map​

LinesSymbolRole
1-35Re-export from _signalsignal, getsignal, SIG_DFL, SIG_IGN, SIGINT, etc.
36-68Signals enum, convenience constantsSignals(IntEnum) wrapping each signal number

Reading​

Signal handler installation​

# CPython: Modules/signalmodule.c signal_signal_impl
import signal
old = signal.signal(signal.SIGINT, handler)

signal.signal(signum, handler) installs a Python callable as the handler for signum. The handler is called with (signum, frame) from the main thread between bytecode instructions (not from within the C signal handler itself).

How Python defers signal delivery​

The C signal handler (signal_handler in signalmodule.c) does not call Python code directly (that would be unsafe from an async signal context). Instead it sets a flag (Handlers[signum].set = 1) and writes a byte to the self-pipe (_Py_write_nocancel(wakeup_fd, ...)). The eval loop checks _Py_atomic_load(&eval_breaker) between instructions; when it sees a pending signal, it calls handle_signals() from the main thread.

Signals enum​

Python 3.5+ wraps the signal integer constants in a Signals(IntEnum):

# CPython: Lib/signal.py:40 Signals
class Signals(IntEnum):
    SIGABRT = _signal.SIGABRT
    SIGFPE = _signal.SIGFPE
    SIGILL = _signal.SIGILL
    SIGINT = _signal.SIGINT
    SIGSEGV = _signal.SIGSEGV
    SIGTERM = _signal.SIGTERM
    ...

This allows repr to show <Signals.SIGINT: 2> instead of just 2.

setitimer and getitimer​

Available on POSIX only. setitimer(which, seconds, interval=0.0) arms a timer that sends SIGALRM (or SIGVTALRM/SIGPROF). getitimer(which) returns the remaining time and interval.

gopy notes​

Status: partially relevant. Signal handling in gopy uses Go's os/signal.Notify channel mechanism rather than the C write-to-pipe approach. The signal.signal() function needs to bridge between a Python callable and a Go chan os.Signal. The eval-loop breaker mechanism in gopy uses a similar atomic flag checked between instructions.