Querying for variables by frequency does not support no frequency

[aidanheerdegen]

Currently the get_variables API call has an optional frequency argument to disambiguate variables.

It is not possible to filter for frequency==None. If this is specified no filtering is done as this is the default case.

This only affects data which has a time coordinate of non-zero length or a time coordinate without proper time units. A case in point is restart variables, which have a time coordinate defined like so:

    double Time(Time) ;
        Time:long_name = "Time" ;
        Time:units = "time level" ;
        Time:cartesian_axis = "T" ;