process.py

from shapely.geometry import shape, mapping
from shapely.ops import unary_union
import fiona
import itertools

with fiona.open('Data/NonRegulatoryLayers_Subset.shp') as input:
    # preserve the schema of the original shapefile, including the crs
    meta = input.meta
    with fiona.open('Out/dissolved.shp', 'w', **meta) as output:
        # groupby clusters consecutive elements of an iterable which have the same key
        # so you must first sort the features by the target property
        prop = 'Ranked_Sol'
        e = sorted(input, key=lambda k: k['properties'][prop])

        # group by the prop field
        for key, group in itertools.groupby(e, key=lambda x:x['properties'][prop]):
            properties, geom = zip(*[(feature['properties'], shape(feature['geometry'])) for feature in group])

            # write the feature, computing the unary_union of the elements in the group with the properties of the first element in the group
            output.write({
              'geometry': mapping(unary_union(geom)),
              'properties': properties[0]
            })