If Prolog is embedded into Python, SWI-Prolog is started with the
--no-signals, i.e., SWI-Prolog does not install any
signal handlers. This implies that signals are handled by Python. Python
handles signals synchronously (as SWI-Prolog) when executing byte code.
As Prolog execution does not involve Prolog execution, running a program
like below cannot be in interrupted
import janus_swi as janus
janus.query_once("repeat,fail")
If your program makes possibly slow Prolog queries and you want
signal handling, you can enable a heartbeat.
- None janus.heartbeat(count=10000)
- Ask Prolog to call a dummy function every count
inferences. This allows Python to handle signals. Lower numbers
for count improve responsiveness at the cost of slowing down
Prolog. Note that Prolog calls to foreign code count as one
inference. Signal handling is completely blocked if Prolog is blocked in
foreign code.
To complete the picture, some Python exceptions are propagated
through Prolog by mapping them into a Prolog exception and back again.
This notably concerns
- SystemExit(code)
- This Python exception is mapped to the Prolog exception
unwind(halt(code)) and back again when Prolog
returns control back to Python.
- KeyboardInterrupt
- This Python exception is mapped to the Prolog exception
unwind(keyboard_interrupt)
