import json

data = {
       'var': {'var2': 'var3',
                'var4': int(1),
               'var7': int(100)}}

def updatedata():
    with open('data1.json', 'w', encoding='utf8') as outfile:
        var5 = data['var']['var4']
        var6 = var5 + 11
        data['var']['var4'] = var6
        str_ = json.dumps(data,
                          indent=4, sort_keys=True,
                          separators=(',', ': '), ensure_ascii=False)
        outfile.write(str_)

updatedata()