testr run doesn't complain when filtering results in no selected tests

I agree with Thomas Herve suggestion. I ran the below non existing test case and got the success message, however I didn't get test count(since there was no test run) .

It would be nice if it could inform user about non-existent tests.

I ran two set of tests. With non existing tests I got following message
PASSED (id=5)

With existing tests I got
Ran 8 tests in 7.346s (+2.018s)
PASSED (id=4)

Please find below tests I ran
-----------------------------------------------------------------
# 1 . testr run tempest.api.compute.flavors.test_flavors_negative1
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-500} \
OS_TEST_LOCK_PATH=${OS_TEST_LOCK_PATH:-${TMPDIR:-'/tmp'}} \
${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./tempest/test_discover} --list
-------------
PASSED (id=5)
-----------

Ran the following test , run ran successfully along with number of tests it ran .
# testr run tempest.api.compute.flavors.test_flavors_negative

running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-500} \
OS_TEST_LOCK_PATH=${OS_TEST_LOCK_PATH:-${TMPDIR:-'/tmp'}} \
${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./tempest/test_discover} --list
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-500} \
OS_TEST_LOCK_PATH=${OS_TEST_LOCK_PATH:-${TMPDIR:-'/tmp'}} \
${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./tempest/test_discover} --load-list /tmp/tmpKZMyg_
Ran 8 tests in 7.346s (+2.018s)
PASSED (id=4)

I'm not sure what to do about this - its certainly a valid complaint, but I worry about the difference between None and 0 here - basically filtering down to nothing to do is a legitimate thing. Note that the arguments are regexes, so a) you can be a lot more pithy with your selectors and b) we don't know (and currently can't) whether something is a module to import or whatnot - testr is multi-language.