I'm using GLib.MainLoop() from PyGObject in my Python application and have a question.
Is it possible to handle Python exception that raises in loop.run()?
For example I'm calling some function using GLib.MainContext.invoke_full():
import traceback, gi
from gi.repository import GLib
try:
loop = GLib.MainLoop()
def handler(self):
print('handler')
raise Exception('from handler with love')
loop.get_context().invoke_full(GLib.PRIORITY_DEFAULT, handler, None)
loop.run()
except Exception:
print('catched!')
I thought that handler() should be called somewhere inside loop.run() so raise Exception('from handler with love') should be catched by except Exception:. However, it is not:
$ python test.py
handler
Traceback (most recent call last):
File "test.py", line 9, in handler
raise Exception('from handler with love')
Exception: from handler with love
It seems that handler() called in the middle of nowhere (called from GLib's C code?), and not catched by except Exception:.
Is it possible to catch all Python exceptions that raises in GLib.MainLoop.run()? I have a dozen of handlers called like that so I have to add same try: ... except OneException: ... exceptAnotherException: ... wrapper into each handler.
No, the exception is not propagated. It is caught and printed. No exception in a Python callback causes the loop to exit.
You can handle these types of errors through sys.excepthook
Related
I am using the Python APScheduler and I wish to catch the raised exceptions from the callback method, however unsure as to how to do this. I have provided my code so far however I am stuck on how to do this appropriately. Any help would be appreciated.
import time
from apscheduler.schedulers.background import BackgroundScheduler
def expiry_callback():
raise ValueError
print("inside job")
sched = BackgroundScheduler(daemon=True)
sched.add_job(expiry_callback,'interval',seconds=1)
try:
sched.start()
except ValueError as e:
print(f'ran into an issue!! {e}')
try:
while True:
time.sleep(5)
except (KeyboardInterrupt, SystemExit):
sched.shutdown()
stacktrace:
/Users/me/Documents/environments/my_env/bin/python3.9 /Users/me/PycharmProjects/pythonProject2/run.py
Job "expiry_callback (trigger: interval[0:00:01], next run at: 2021-08-24 22:33:26 MDT)" raised an exception
Traceback (most recent call last):
File "/Users/me/Documents/environments/my_env/lib/python3.9/site-packages/apscheduler/executors/base.py", line 125, in run_job
retval = job.func(*job.args, **job.kwargs)
File "/Users/me/PycharmProjects/pythonProject2/run.py", line 6, in expiry_callback
raise ValueError
ValueError
Process finished with exit code 0
Calling sched.start() only starts the background thread that executes the callback function but does not call the callback function itself so it is never going to produce the exception.
If you're looking to handle exceptions from callback functions in a consistent way, you can instead call the via a wrapper function that catches a given exception and outputs the error in a definite manner:
# insert definition of expiry_callback before this line
def catch_exception(func, exception):
def wrapper():
try:
func()
except exception as e:
print(f'ran into an issue!! {e}')
return wrapper
sched = BackgroundScheduler(daemon=True)
sched.add_job(catch_exception(expiry_callback, ValueError),'interval',seconds=1)
sched.start()
# insert idle code after this line
Demo: https://replit.com/#blhsing/BlueCreepyMethod
I have a parent class who has a try clause in it, and a child class overrides a method inside of the try clause. Normal exceptions can be caught when the child raises it. However, the keyboard interrupt exception cannot be caught. Moreover, it can be caught inside the child method, but not the parent method.
Please see the example code like the following, where the interrupt cannot be caught in bar1, but can be caught after bar2 changes it into an assertion error. I produced this problem in python 3.6 in both Linux and Windows.
import time
class foo:
def body(self):
pass
def run(self):
try:
self.body()
except Exception as ex:
print("caught exception:", str(ex))
class bar1(foo):
def body(self):
while(1):
print(1)
time.sleep(0.1)
class bar2(foo):
def body(self):
interrupted = False
while(1):
assert not interrupted, "assert not interrupted"
try:
print(1)
time.sleep(0.1)
except KeyboardInterrupt as ex:
print("received interrupt")
interrupted = True
Interrupting the run method of class bar1 gets
1
....
1
Traceback (most recent call last):
File "tmp.py", line 34, in <module>
b.run()
File "tmp.py", line 7, in run
self.body()
File "tmp.py", line 15, in body
time.sleep(0.1)
KeyboardInterrupt
However, interrupting bar2 gets
1
...
1
received interrupt
caught exception: assert not interrupted
I have searched over StackOverflow and found some problems regarding the keyboard interruption handling with threads and stdIO, but I did not found some problems like this.
I want to access the traceback of a python programm running in a subprocess.
The documentation says:
Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent. Additionally, the exception object will have one extra attribute called child_traceback, which is a string containing traceback information from the child’s point of view.
Contents of my_sub_program.py:
raise Exception("I am raised!")
Contents of my_main_program.py:
import sys
import subprocess
try:
subprocess.check_output([sys.executable, "my_sub_program.py"])
except Exception as e:
print e.child_traceback
If I run my_main_program.py, I get the following error:
Traceback (most recent call last):
File "my_main_program.py", line 6, in <module>
print e.child_traceback
AttributeError: 'CalledProcessError' object has no attribute 'child_traceback'
How can I access the traceback of the subprocess without modifying the subprocess program code? This means, I want to avoid adding a large try/except clause around my whole sub-program code, but rather handle error logging from my main program.
Edit: sys.executable should be replaceable with an interpreter differing from the one running the main program.
As you're starting another Python process, you can also try to use the multiprocessing Python module ; by sub-classing the Process class it is quite easy to get exceptions from the target function:
from multiprocessing import Process, Pipe
import traceback
import functools
class MyProcess(Process):
def __init__(self, *args, **kwargs):
Process.__init__(self, *args, **kwargs)
self._pconn, self._cconn = Pipe()
self._exception = None
def run(self):
try:
Process.run(self)
self._cconn.send(None)
except Exception as e:
tb = traceback.format_exc()
self._cconn.send((e, tb))
# raise e # You can still rise this exception if you need to
#property
def exception(self):
if self._pconn.poll():
self._exception = self._pconn.recv()
return self._exception
p = MyProcess(target=functools.partial(execfile, "my_sub_program.py"))
p.start()
p.join() #wait for sub-process to end
if p.exception:
error, traceback = p.exception
print 'you got', traceback
The trick is to have the target function executing the Python sub-program, this is done by using functools.partial.
I know the "signal" module in python 2.7, in general, we can use the following code to timeout a main-thread function call:
import signal
def signal_handle_timeout:
raise Exception("Timeout!")
signal.signal(signal.SIGALRM, signal_handle_timeout)
signal.alarm(seconds)
try:
long_time_function()
except Exception as ex:
print ex
But it only can run in main-thread, if try to run in sub-thread, it throw this error: 'ValueError: signal only works in main thread'
In Python, how can we timeout a function call in sub-thread?
I'm trying to do some exception handling in python3 / pygobject with a property inside one of my custom gobject classes. The code I had was something like this
try:
label = foo.label # This is a GObject.Property
except Exception:
label = "fallback"
I had noticed that the interpreter never got around to the except block, after trying to figure out the problem I came up with this test case
from gi.repository import Gtk, GObject
class foo(GObject.Object):
#GObject.Property
def bar(self):
raise NotImplementedError
fish = foo()
print("Bar: ", fish.bar)
The output
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/gi/_gobject/propertyhelper.py", line 403, in obj_get_property
return prop.fget(self)
File "test.py", line 6, in bar
raise NotImplementedError
NotImplementedError
Bar: None
As you can see, even though there is an exception the property returns None and the program continues.
I don't get it either.
Anyone knows a workaround or a solution for this?
GObject properties don't support exceptions, so it's understandable that exceptions wouldn't work here. The workaround is to use getter/setter methods.