How Can I Reveal the Stack Trace of a Live Python Application on the Fly?

Revealing the Stack Trace of a Live Python Application

Despite the stability of Python, applications can occasionally encounter deadlocks or freeze unexpectedly. Identifying the root cause of these issues can be challenging.

On-the-Fly Stack Trace Inspection

To gain insight into the current state of a running Python program, a useful tool is the "on-the-fly" stack trace. This allows you to inspect the exact code being executed, potentially pinpointing the source of any problems.

Implementing an On-the-Fly Stack Trace

A module can be employed to enable on-the-fly stack trace inspection. This module operates only on Unix systems and utilizes signals:

<code class="python">import code, traceback, signal

def debug(sig, frame):
    d={'_frame':frame}
    d.update(frame.f_globals)
    d.update(frame.f_locals)

    i = code.InteractiveConsole(d)
    message  = "Signal received : entering python shell.\nTraceback:\n"
    message += ''.join(traceback.format_stack(frame))
    i.interact(message)

def listen():
    signal.signal(signal.SIGUSR1, debug)</code>

Usage

To use this module, simply call listen() during program initialization:

<code class="python">listen()</code>

When the program encounters an issue, send a SIGUSR1 signal using kill or within Python:

<code class="python">os.kill(pid, signal.SIGUSR1)</code>

This will interrupt the program and open an interactive Python console at the execution point. You can examine the stack trace and explore variables to identify the cause of the problem.

Note that this approach may disrupt I/O operations when the signal is triggered.

For more extensive debugging, a separate script can be used to establish communication via a pipe, enabling the debugging of backgrounded processes.

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