If robot is run with command: robot -d {foo}_dir path/to/suite then Open Browser keyword will fail with Firefox on: KeyError: 'foo'. This is because Python format is used. Same problem also applies to Capture Page Screenshot.

Build a wrapper that handle formatting for Python 2 and Python 3. For Python 3: https://docs.python.org/3/library/stdtypes.html#str.format_map is nice. For Python 2 build a something like: https://stackoverflow.com/questions/17215400/python-format-string-unused-named-arguments