unittest: assertRaises() with an instance instead of a type
ulrich.eckhardt at dominolaser.com
Wed Mar 28 14:28:08 CEST 2012
I'm currently writing some tests for the error handling of some code. In
this scenario, I must make sure that both the correct exception is
raised and that the contained error code is correct:
self.fail('exception not raised')
catch MyException as e:
self.fail('unexpected exception raised')
This is tedious to write and read. The docs mention this alternative:
with self.assertRaises(MyException) as cm:
This is shorter, but I think there's an alternative syntax possible that
would be even better:
Here, assertRaises() is not called with an exception type but with an
exception instance. I'd implement it something like this:
def assertRaises(self, exception, ...):
# divide input parameter into type and instance
if isinstance(exception, Exception):
exception_type = type(exception)
exception_type = exception
exception = None
# call testee and verify results
...call function here...
except exception_type as e:
if not exception is None:
This of course requires the exception to be equality-comparable.
1. Does this sound like a useful extension or am I missing another
obvious solution to my problem?
2. The assertRaises() sketch above tries to auto-detect whether the
given parameter is the type or an instance. Given the highly dynamic
nature of Python, an object can be both instance and type, is the above
detection mechanism reliable?
Of course I'm open for other suggestions to solve my problem. One that I
thought of but which I haven't really looked into was to modify __init__
or __new__ of my exception class to return an instance of a derived
class that uniquely identifies the error. I.e.
MyException(SOME_FOO_ERROR) would not create a MyException instance but
a MyFooErrorException instance (which has MyException as a baseclass).
In that case, the existing code that checks for the right exception type
would suffice for my needs.
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