I have a strange bug in python code which only happens sometimes in CI.

We can't reproduce it.

Where is the test code:

response=self.admin_client.post(url, post)
self.assertEqual(200, response.status_code, response)

Sometimes we get a 302 which happens since the form gets saved.

My idea to debug this:

with some_magic_trace.trace() as trace:response=self.admin_client.post(url, post)self.assertEqual(200, response.status_code, trace)

The trace should contain the python lines (filename, line offset, line as string) executed by the interpreter.

How to implement some_magic_trace.trace()?

The trace module gives you a very simple solution (different from what you are asking for, but simple enough to have a try.)

from trace import Tracetracer = Trace()
response = tracer.runfunc(self.admin_client.post, url, post)
self.assertEqual(200, response.status_code, response)

A more complex solution that entails creating a context manager that saves the trace and prints it only on exceptions, requires the use of sys.settrace. Just a template for your own implementation could be:

class MyTracer():def __init__(self):self.trace = Nonedef newscope(self, frame, event, arg):## real work should be done here, just minimal exampleself.trace.append((frame, event, arg))return Nonedef pprint(self):## real pretty printing of trace info should be done hereprint(self.trace)def __enter__(self):self.trace = []sys.settrace(self.newscope)return selfdef __exit__(self, exc_type, exc_val, exc_tb):sys.settrace(None)if exc_type is not None:self.pprint()## print some info gathered from exc_type, exc_val, exc_tb

Then you can:

with MyTracer():response=self.admin_client.post(url, post)self.assertEqual(200, response.status_code, response)

The idea is that a MyTracer instance has a tracer method newscope that saves some useful info in self.trace. On an abnormal exit from the context the pprint method is called; on a normal exit the trace info is discarded.

Most of the work has to be done in the tracing method newscope. Some concrete examples of tracing functions can be found here.