import json, sys
# Read 2 filenames from command line
# Output to stdout features in file2 not in file1

if len(sys.argv) == 1:
    print "Must provide 2 command line arguments"
    print 'Number of arguments:', len(sys.argv)-1
    exit
    
with open(sys.argv[1], "r") as read_file:
    j = json.load(read_file)

features1 = j["features"]

# Create a set of ID's from first file of features
alpha = set()
for f in features1:
    alpha.add(f["id"]);

with open(sys.argv[2], "r") as read_file:
    j = json.load(read_file)

features2 = j["features"]

# Lookup each feature ID from file2 in set     
bravo = []
for f in features2:
    target = f["id"]
    if target not in alpha:
        bravo.append(target);

# bravo contains items in dat2 not in dat1
for b in bravo:
    print b
    
# Note features dicts are saved for future use