破坏if语句，无法弄清楚为什么这不起作用

Question

即使目录确实存在于列表中，代码也会卡住。我不确定这里发生了什么。当我注释掉我编写的代码时，如果目录不存在则重新询问用户目录，代码完美无缺。

def path_sel():
path = raw_input("Select desired working directory: ")
b = []
ftp.retrlines('LIST', b.append)
if path not in b:
    print "ERROR- Directory does not exist.\n"
    path_sel()
else:
    print '\nChanging to '+ path, '\n'
    ftp.cwd(path)
    print path
    ftp.retrlines('LIST')
    c = []
    ftp.retrlines('LIST', c.append)
    if 'd' in str(c[0]):
            path_sel()
file_dl()

Answer 1

这似乎对我有用（我选择了一个随机的FTP服务器）：

from ftplib import FTP 

def path_sel():
   path = raw_input("Select desired working directory: ")
   b = []
   ftp.retrlines('LIST', b.append)

   found = False
   for entry in b:
      if path in entry:
         found = True

   if not found:
      print "ERROR- Directory does not exist.\n"
      path_sel()
   else:
      print '\nChanging to '+ path, '\n'
      ftp.cwd(path)
      print path
      ftp.retrlines('LIST')
      # After printing the directory, you probably
      # want to ask the user to enter a new file/path
      # rather than just assuming that they are interested
      # in the first entry in the ftp.retrlines() printout
      c = []
      ftp.retrlines('LIST', c.append)
      if 'd' in str(c[0]):
         path_sel()
   # ---------------------------------------------------
   # Once you've extracted a filename, run your function
   # ---------------------------------------------------
   print 'Running file_dl()'
   #file_dl(filename)

ftp = FTP('ftp.debian.org')
ftp.login()
path_sel()

它会不断返回“ERROR-目录不存在”，并在输入有效路径之前请求新输入。一旦你找到一个文件（不是以'drwxrwxrwx ....'开头），你想要执行你的file_dl（）函数。

以下是我的控制台中的内容：