TypeError：无法连接“ str”

TypeError: cannot concatenate 'str' and 'builtin_function_or_method' objects当我尝试更改ftp中的目录时出现此错误。

ftp.py

from ftplib import FTP

ftp = FTP()

class FTPClient():

    connection_id = None
    login_ok = False
    message_array = []

    def __init__(self):
        pass

    def log_message(self, message, clear=True):
        if clear:
            self.message_array = message

    def get_message(self):
        return self.message_array

    def connect(self, server, ftp_user, ftp_password, is_passive = False):
        self.connection_id = ftp.connect(server)
        login_result = ftp.login(user=ftp_user, passwd=ftp_password)
        ftp.set_pasv(is_passive)

        if not self.connection_id or not login_result:
            self.log_message("FTP Connection has Failed")
            self.log_message("Attempted to connect to {0} for {1}".format(server, ftp_user))
            return False
        else:
            self.log_message("Connected to {0} for {1}".format(server, ftp_user))
            self.login_ok = True
            return True

    def make_dir(self, directory):
        if ftp.mkd(directory):
            self.log_message("Directory {0} created successfully".format(directory))
            return True
        else:
            self.log_message("Failed creating directory")
            return False

    def change_directory(self, directory):
        if ftp.cwd(directory):
            self.log_message("Current Directory is now {0}".format(ftp.pwd))
        else:
            self.log_message("Can't change Directory")

    def upload_file(self, file_from, file_to):
        if file_from.endswith(('.csv', '.txt')):
            with open(file_from, 'r') as f:
                self.connection_id.storelines("Uploaded from {0} to {1}".format(file_from, file_to), f)
        else:
            with open(file_from, 'rb') as f:
                self.connection_id.storebinary('Uploaded from {0} to {1}'.format(file_from, file_to), f)


ftp_obj = FTPClient()

FTP_HOST = "yyy"
FTP_USER = "xxx"
FTP_PASS = "zzz"

ftp_obj.connect(FTP_HOST, FTP_USER, FTP_PASS)
print(ftp_obj.get_message())
FILE_FROM = "config.txt"
FILE_TO = ftp_obj.change_directory(dir)
ftp_obj.upload_file(FILE_FROM, FILE_TO)
print(ftp_obj.get_message())

完整回溯：

Connected to yyy for xxx
Traceback (most recent call last):
  File "C:/Users/Ajay/PycharmProjects/database/test.py", line 67, in <module>
    FILE_TO = ftp_obj.change_directory(dir)
  File "C:/Users/Ajay/PycharmProjects/database/test.py", line 44, in change_directory
    if ftp.cwd(directory):
  File "C:\Python27\lib\ftplib.py", line 552, in cwd
    cmd = 'CWD ' + dirname
TypeError: cannot concatenate 'str' and 'builtin_function_or_method' objects

那里发生了什么事？以及为什么这个错误？