disallow no-prefix `ipython foo=bar` argument style. This style is in rc1, but will be removed in rc2. Since they don't match any flag pattern, rc1-style arguments will be interpreted by IPython as files to be run. So `ipython gui=foo -i` will exec gui=foo, and pass '-i' to gui=foo. Presumably this file won't exist, so there will be an error: Error in executing file in user namespace: gui=foo Assignments *must* have two leading '-', as in: ipython --foo=bar all flags (non-assignments) can be specified with one or two leading '-', as in: ipython -i --pylab -pdb --pprint script.py or ipython --i -pylab --pdb -pprint script.py but help only reports two-leading, as single-leading options will likely be removed on moving to argparse, where they will be replaced by single-letter aliases. The common remaining invalid option will be: ipython -foo=bar and a suggestion for 'did you mean --foo=bar'? will be presented in these cases.