I have been trying to load some monthly files containing air temperature data (from chess-met https://catalogue.ceh.ac.uk/documents/835a50df-e74f-4bfb-b593-804fd61d5eab; although the data themselves are not important for this error) using cf.load, with the intention of aggregating along the time axis. I get a crash from cf.aggregate when the files contain the actual_range attribute AND there are at least three files to aggregate.

This occurs because unlike some other attributes actual_range is not handled specially by cf.aggregate. The default behviour is to convert both to strings and append one to the other:

Since the valid_range attribute is defined in the CF-standard (https://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/build/ch02s05.html.) this should be handled. A simple correction would be to check if the property being updated is actual_range, and if so to set the parent's corresponding attribute to the minima and maxima of the two aggregated datasets. For instance:

        if prop in ("actual_range"):
            parent0.set_property(prop,[min(value0[0],value1[0]),max(value0[1],value1[1])])
            continue 

Alternatively it could be fixed by ignoring this attribute, as is currently done for valid_range & similar attributes.