user2560035 user2560035 - 1 year ago 103
Python Question

In Python, how do I print an error message without printing a traceback and close the program when a condition is not met?

I've seen similar questions to this one but none of them really address the trackback.
If I have a class like so

class Stop_if_no_then():
def __init__(self, value one, operator, value_two, then, line_or_label, line_number):
self._firstvalue = value_one
self._secondvalue = value_two
self._operator = operator
self._gohere = line_or_label
self._then = then
self._line_number = line_number

def execute(self, OtherClass):
"code comparing the first two values and making changes etc"

What I want my execute method to be able to do is if self._then is not equal to the string "THEN" (in allcaps) then I want it to raise a custom error message and terminate the whole program while also not showing a traceback.

If the error is encountered the only thing that should print out would look something like (I'm using 3 as an example, formatting is not a problem) this.

`Syntax Error (Line 3): No -THEN- present in the statement.`

I'm not very picky about it actually being an exception class object, so there's no issue in that aspect. Since I will be using this in a while loop, simple if, elif just repeats the message over and over (because obviously I am not closing the loop). I have seen sys.exit() but that also prints out a giant block of red text, unless I am not using it correctly. I don't want to catch the exception in my loop because there are other classes in the same module in which I need to implement something like this.

Answer Source

You can use a try: and then except Exception as inst: What that will do is give you your error message in a variable named inst and you can print out the arguments on the error with inst.args. Try printing it out and seeing what happens, and is any item in inst.args is the one you are looking for.

EDIT Here is an example I tried with pythons IDLE:

>>> try:
except Exception as inst:
    d = inst

>>> d
FileNotFoundError(2, 'No such file or directory')
>>> d.args
(2, 'No such file or directory')
>>> d.args[1]
'No such file or directory'

EDIT 2: as for closing the program you can always raise and error or you can use sys.exit()

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