Catch error with assertError in test case
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I'd like to catch an error I am throwing with assertError in a test case. I tried doing a simple assertError, however, I get Too many output arguments.:
function fun()
error("test");
end
assertError(fun())
And if I try it in a class test suite setup it fails with the error being my error:
classdef test < matlab.unittest.TestCase
methods (Test)
function test_error(testCase)
testCase.assertError(fun());
end
end
end
3 Comments
which -all assertError
resolves to something inside unit testing framework, but I can find no documentation/reference to the function assertError as being user-callable function. I "know nuthink!" about the framework, but doesn't look to me as though this is a function that is supposed to be called by the user, but somehow is triggered by running unit tests...
Steven Lord
on 23 Mar 2025
assertError is part of the qualification API. See this table in the documentation; you can combine any of the "suffixes" that specify what behavior or quality you're testing for (-Equal, -Error, -Size, -Class, -Matches, etc.) with any of the "prefixes" that specify what happens if the qualification fails (assume-, verify-, assert-, fatalAssert-)
So assertError tests that the function handle throws an error (or a specific error) and if that qualification fails, it will throw an assertion. If it occurs inside a Test method, this will skip the rest of the method and move on to the next.
fatalAssertEqual tests that two things are equal and will stop all testing if it fails. [Think fatalAssert(tc, 1+1, 2) -- if that fails, there's no point in continuing to test as MATLAB is fundamentally broken.]
etc.
Accepted Answer
Remember, you must pass a matlab.unittest.TestCase Class object into assertError as an input argument. You also need to call it with a function handle that assertError can call.
tc = matlab.unittest.TestCase.forInteractiveUse; % Make a TestCase object for experimentation
If you don't use an error ID in your error call, tell assertError that as long as some error gets thrown the test should pass.
assertError(tc, @myfunctionThatThrowsAnErrorWithoutErrorID, ?MException)
If you do use an error ID, you can specify that error ID to check for that specific error.
assertError(tc, @myfunctionThatThrowsAnErrorWithErrorID, 'MATLABAnswers:specificErrorID')
If a different error gets thrown, this test case fails. I'm using verifyError instead of assertError from this point on so that MATLAB doesn't stop executing the code, and you can see the next three test cases fail. But assertError would behave the same (except throwing a hard error instead of simply recording a test failure and moving on.)
verifyError(tc, @myfunctionThatThrowsAnErrorWithErrorID, 'MATLABAnswers:incorrectErrorID')
If no error gets thrown, the test case also fails regardless of whether you're checking for any error or a specific error.
verifyError(tc, @myfunctionThatDoesNotError, ?MException)
verifyError(tc, @myfunctionThatDoesNotError, 'MATLABAnswers:specificErrorID')
If you call the function inside assertError, MATLAB would run that function to try to use its output as the function to check for an error. Yes, you can have a function handle that returns a function handle when evaluated. In this example, when f is run it returns a function handle that will create two vectors and try to add them together.
f = @(x) @() ones(1, 5) + ones(1, x);
verifyError(tc, f(2), 'MATLAB:sizeDimensionsMustMatch') % ones(1, 5) + ones(1, 2) will error
verifyError(tc, f(10), 'MATLAB:sizeDimensionsMustMatch') % ones(1, 5) + ones(1, 10) will error
verifyError(tc, f(5), 'MATLAB:sizeDimensionsMustMatch') % ones(1, 5) + ones(1, 5) will NOT error
function myfunctionThatThrowsAnErrorWithoutErrorID()
error('This is my error')
end
function myfunctionThatThrowsAnErrorWithErrorID()
error('MATLABAnswers:specificErrorID', 'This is my error')
end
function y = myfunctionThatDoesNotError()
y = 1+1;
end
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