def legal_check(obj):
        if isinstance(obj, list):
            for i in obj:
                if i in keyword.kwlist:
                    raise SyntaxError("%s is an invalid name (Found in Keyword List)".format(i))
                elif not re.match("^[a-zA-Z][a-zA-Z0-9_=]*$", str(i)):
                    raise SyntaxError("%s is an invalid name (Invalid characters used)".format(i))
        elif isinstance(obj, str):
            obj = obj.replace(',', ' ')
            obj = obj.split()
            for i in obj:
                if i in keyword.kwlist:
                    raise SyntaxError("%s is an invalid name (Found in Keyword List)".format(i))
                elif not re.match("^[a-zA-Z][a-zA-Z0-9_=]*$", str(i)):
                    raise SyntaxError("%s is an invalid name (Invalid characters used)".format(i))

    legal_check(type_name)
    legal_check(field_names)


    for i in defaults:
        if i not in field_names:
            raise SyntaxError('Dictionary Key not in field_names')

    def gen_init(f_names):

        init_string = """
    def __init__(self, """

        if isinstance(f_names, str):
            f_names = f_names.replace(', ', ' ')
            f_names = f_names.replace(',', ' ')
            f_names = f_names.split(' ')

        if defaults:
            for k,v in defaults:
                init_string += "%s=%s" % (k,v)
        else:
            for idx, i in enumerate(f_names):
                if idx != len(f_names) - 1:
                    init_string += "%s, " % i
                else:
                    init_string += "%s):" % i


        for i in f_names:
            init_string += "\n        self.%s = %s" % (i, i)
        return init_string



    class_header = """
class {type_name}:
    _fields = {field_names}
    _mutable = {mutable}
    """.format(type_name = str(type_name), field_names = str(field_names), mutable = str(mutable))



    class_definition = class_header + gen_init(field_names)