def concatFiles(outName, inFileNames):

    def listGenerator():
        for inName in inFileNames:
            with open(inName, 'r') as f:
                for item in json.load(f):
                    yield item
with open(outName, 'w') as f:
    f.write('[')
for item in listGenerator():
    f.write(json.dumps(item, f) + ',')
f.write(']')
Python: make a list generator JSON serializable