I created an option with parser.addoption("--run")
Using this option worked find with the command line
py.test --run argument
However using the alternative syntax
py.test --run=argument
Caused the failure py.test: error: argument --runxfail: ignored explicit argument 'abc'. I assume that the option matching is using prefixes. I think the behaviour of runxfail being triggered by --run is fine, however the behaviour should be consistent between the two ways of specifying command line options.