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.
filelike_objects_are_closed_on_exception function, the
will be an instance of a custom
FileLike class defined in the test suite. This
class implements the following methods:
close, for explicit opening and closing.
__exit__, for implicit opening and closing.
do_something, which may or may not throw an
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")
To run the tests, run
Alternatively, you can tell Python to run the pytest module:
python -m pytest error_handling_test.py
-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
Note that, when trying to submit an exercise, make sure the solution is in the
You can find your Exercism workspace by running
exercism debug and looking for the line that starts with
For more detailed information about running tests, code style and linting, please see Running the Tests.
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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()
def handle_error_by_throwing_exception(): raise Exception("Failed while doing something") def handle_error_by_returning_none(input_data): if (input_data == '1'): return 1 return None def handle_error_by_returning_tuple(input_data): if (input_data == '1'): return (True, 1) return (False, None) def filelike_objects_are_closed_on_exception(filelike_object): with filelike_object as f: f.do_something()
A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.
Here are some questions to help you reflect on this solution and learn the most from it.