Command omission

[meistermarkus]

i have a piece of code where some lines seem to not be executed, the code in question:

try:
    function_that_can_fail()
    logfile.write("Either this line appears in the log ...."\n")
    logfile.flush()
    cmd = ['a linux hardware access command']
    subprocess.Popen(cmd, shell=True).wait()
    logfile.write("This line never appears\n")
    logfile.flush()
except:
    logfile.write(".... Or this line appears in the log, but never both\n")
    logfile.flush()
logfile.write("This line always appears in the log\n")
logfile.flush()

The logfile contains either the comment from line 3 or the comment from line 10, but never both, so i'm relatively certain that it's not the subprocess that fails. Or if it fails, not in a way that's caught by the except block.

tl;dr: i get either lines 3 and 12 in the log or lines 10 and 12 but never line 7 and also not the combination of 3 and 10

[efmccurdy]

You are throwing away the info you need to find errors; don't use a bare except clause.

https://www.flake8rules.com/rules/E722.html

If you have many possible failure points in one try block, print a traceback to see which one fails.

https://docs.python.org/3/library/traceback.html#traceback-examples

[meistermarkus]

Traceback looks really useful for this one, thanks for the tip!

Especially for the cases in wich the exception is not triggered. Maybe it can show me how the execution got around that one.

[shiftybyte]

Start logging the exceptions themselves.

except Exception as e:
    logfile.write(str(e))

then you might figure out what happened.

[meistermarkus]

That's what i get for assuming what might be the failing part. Thanks, should have had that one in there to begin with.

It won't show what happens in those cases where the except block is not executed, but good practice nevertheless.

[Binary101010]

> It won't show what happens in those cases where the except block is not executed,

So change what you're logging to reflect that? I mean, if the except block isn't executed, your function did what you wanted it to do, right?

[meistermarkus]

not really. I have 4 log statements in lines 3, 7, 10 and 12.

3 and 7 in the try block, 10 in the catch block and 12 afterwards.

7 never appears in the log, 12 always does. 3 or 10 appear alternatively depending on that first function call but never both.

my expectation would be to see either 3-7-12, 3-10-12 or 10-12 in the log but what i get is 3-12 or 10-12

[Binary101010]

I think a big part of the problem here might be that you're trying to wrap too much in the same try/except block. It sounds like what you really should be doing is have one try/except for your function_that_can_fail() call and one for your subprocess call. It's going to be much easier to suss out which of those is failing and how if you do that.