The Nosetests -m argument is used to match directories, file names , classes, and functions. ( See the explanation of this document . In your case, the test file name (test_case_4.py) does not match -m (_size), so it never opens.
You may notice that if you force your nose to open a test file, it will only run the specified test:
nosetests -sv -m='_size' cases/test_case_4.py
In general, when I want to map specific tests or subsets of tests, I use the - attrib plugin , which is available by default to the install nose. You can also try to exclude tests that match some pattern.
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