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to Error Handling in the Python Track

Published at Jan 28 2021 · 0 comments
Instructions
Test suite
Solution

Implement various kinds of error handling and resource management.

An important point of programming is how to handle errors and close resources even if errors occur.

This exercise requires you to handle various errors. Because error handling is rather programming language specific you'll have to refer to the tests for your track to see what's exactly required.

Hints

For the filelike_objects_are_closed_on_exception function, the filelike_object will be an instance of a custom FileLike class defined in the test suite. This class implements the following methods:

  • open and close, for explicit opening and closing.
  • __enter__ and __exit__, for implicit opening and closing.
  • do_something, which may or may not throw an Exception.

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run pytest error_handling_test.py

Alternatively, you can tell Python to run the pytest module: python -m pytest error_handling_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/error-handling directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see Running the Tests.

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

error_handling_test.py

import unittest

import error_handling as er


class FileLike:
    def __init__(self, fail_something=True):
        self.is_open = False
        self.was_open = False
        self.did_something = False
        self.fail_something = fail_something

    def open(self):
        self.was_open = False
        self.is_open = True

    def close(self):
        self.is_open = False
        self.was_open = True

    def __enter__(self):
        self.open()
        return self

    def __exit__(self, *args):
        self.close()

    def do_something(self):
        self.did_something = True
        if self.fail_something:
            raise Exception("Failed while doing something")


class ErrorHandlingTest(unittest.TestCase):
    def test_throw_exception(self):
        with self.assertRaisesWithMessage(Exception):
            er.handle_error_by_throwing_exception()

    def test_return_none(self):
        self.assertEqual(er.handle_error_by_returning_none('1'), 1,
                         'Result of valid input should not be None')
        self.assertIsNone(er.handle_error_by_returning_none('a'),
                          'Result of invalid input should be None')

    def test_return_tuple(self):
        successful_result, result = er.handle_error_by_returning_tuple('1')
        self.assertIs(successful_result, True,
                      'Valid input should be successful')
        self.assertEqual(result, 1, 'Result of valid input should not be None')

        failure_result, result = er.handle_error_by_returning_tuple('a')
        self.assertIs(failure_result, False,
                      'Invalid input should not be successful')

    def test_filelike_objects_are_closed_on_exception(self):
        filelike_object = FileLike(fail_something=True)
        with self.assertRaisesWithMessage(Exception):
            er.filelike_objects_are_closed_on_exception(filelike_object)
        self.assertIs(filelike_object.is_open, False,
                      'filelike_object should be closed')
        self.assertIs(filelike_object.was_open, True,
                      'filelike_object should have been opened')
        self.assertIs(filelike_object.did_something, True,
                      'filelike_object should call do_something()')

    def test_filelike_objects_are_closed_without_exception(self):
        filelike_object = FileLike(fail_something=False)
        er.filelike_objects_are_closed_on_exception(filelike_object)
        self.assertIs(filelike_object.is_open, False,
                      'filelike_object should be closed')
        self.assertIs(filelike_object.was_open, True,
                      'filelike_object should have been opened')
        self.assertIs(filelike_object.did_something, True,
                      'filelike_object should call do_something()')

    # Utility functions
    def assertRaisesWithMessage(self, exception):
        return self.assertRaisesRegex(exception, r".+")


if __name__ == '__main__':
    unittest.main()
class MyCustomError(Exception):
    """Base class for exceptions in this module."""
    pass


def handle_error_by_throwing_exception():
    raise MyCustomError("Meaningful message indicating the source of the error")


def handle_error_by_returning_none(input_data):
    try:
        return int(input_data)
    except Exception:
        return None


def handle_error_by_returning_tuple(input_data):
    try:
        return True, int(input_data)
    except Exception:
        return False, None


def filelike_objects_are_closed_on_exception(filelike_object):
    with filelike_object as f:
        f.do_something()

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