def _get_value(quantitystr, fitparams, fixedparams):
<EOL>fitparamskeys, fixedparamskeys = fitparams.keys(), fixedparams.keys()<EOL>if quantitystr in fitparamskeys:<EOL><INDENT>quantity = fitparams[quantitystr]<EOL><DEDENT>elif quantitystr in fixedparamskeys:<EOL><INDENT>quantity = fixedparams[quantitystr]<EOL><DEDENT>return quantity<EOL>
This decides if a value is to be fit for or is fixed in a model fit.

    When you want to get the value of some parameter, but you're not sure if
    it's being fit or if it is fixed. then, e.g. for `period`::

        period_value = _get_value('period', fitparams, fixedparams)