python - Print an error message without printing a traceback and close the program when a condition is not met

Question

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 1

You can turn off the traceback by limiting its depth.

Python 2.x

import sys
sys.tracebacklimit = 0

Python 3.x

In Python 3.5.2 and 3.6.1, setting tracebacklimit to 0 does not seem to have the intended effect. This is a known bug. Note that -1 doesn't work either. Setting it to None does however seem to work, at least for now.

>>> import sys

>>> sys.tracebacklimit = 0
>>> raise Exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception

>>> sys.tracebacklimit = -1
>>> raise Exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception

>>> sys.tracebacklimit = None
>>> raise Exception
Exception

Nevertheless, for better or worse, if multiple exceptions are raised, they can all still be printed. For example:

socket.gaierror: [Errno -2] Name or service not known

During handling of the above exception, another exception occurred:

urllib.error.URLError: <urlopen error [Errno -2] Name or service not known>