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faulthandler Module¶
The faulthandler module dumps Python tracebacks when the interpreter crashes
on a fatal signal (SIGSEGV, SIGFPE, SIGABRT, SIGBUS, SIGILL), on a
timeout, or on a user signal. It is the tool for diagnosing hangs and hard
crashes, where a normal exception traceback never gets a chance to print.
The handlers write directly to a file descriptor using pre-allocated buffers, so dumping is async-signal-safe and allocates no memory.
Complexity Reference¶
| Operation | Time | Space | Notes |
|---|---|---|---|
enable(file=sys.stderr, all_threads=True) |
O(1) | O(1) | Installs signal handlers |
disable() |
O(1) | O(1) | Removes handlers |
is_enabled() |
O(1) | O(1) | Flag check |
dump_traceback(file, all_threads=True) |
O(t*d) | O(1) | t = threads, d = stack depth |
dump_traceback_later(timeout, repeat=False) |
O(1) | O(1) | Starts a watchdog thread |
cancel_dump_traceback_later() |
O(1) | O(1) | Stops the watchdog |
register(signum, file, all_threads=True) |
O(1) | O(1) | Handler for a user signal |
unregister(signum) |
O(1) | O(1) | Removes that handler |
Dump cost is proportional to what is printed: one line per frame, across the
threads selected. With all_threads=False it is O(d) for the current thread.
Enabling the Handler¶
import faulthandler
# O(1) - installs handlers for the fatal signals
faulthandler.enable()
# Equivalent, without touching code:
# python -X faulthandler script.py
# PYTHONFAULTHANDLER=1 python script.py
Diagnosing a Hang¶
dump_traceback_later() starts a watchdog: if the timeout elapses before it is
cancelled, every thread's stack is dumped. This is how you find out where a
process is stuck.
import faulthandler
# Dump all thread stacks if the work below takes over 30 seconds - O(1) to arm
faulthandler.dump_traceback_later(30, exit=True)
try:
do_slow_work()
finally:
faulthandler.cancel_dump_traceback_later() # O(1)
Dumping on Demand¶
import faulthandler
import signal
# Send SIGUSR1 to the process to print all stacks without stopping it
faulthandler.register(signal.SIGUSR1) # O(1)
# Or dump immediately - O(threads * depth)
faulthandler.dump_traceback()
Not a substitute for exception handling
faulthandler reports the interpreter state at the moment of a fault; it
does not recover from it. After a fatal signal the process still dies.
Cheap to leave on
Enabling the handler costs one signal-handler installation and nothing per operation afterwards, so there is no steady-state overhead in production.
Version Notes¶
- Python 3.3+: module introduced
- Python 3.5+: handlers are installed with
SA_ONSTACKwhere available, so stack-overflow crashes can still be dumped - Python 3.6+:
dump_traceback_later()is available on all platforms with threads - All versions: dumping allocates no memory and is signal-safe