python - 参数解析 : ValueError when using --help or -h option with nargs

Question

当使用 argparse 时，我得到了这个奇怪的 ValueError 帮助选项。所有其他选项都可以正常工作。只有帮助选项不起作用，这很奇怪。

这是错误信息:

chaudhary@localhost:~/code/python/logmein$ ./logmein.py --bla
Usage:  ./logmein.py  [filename <Default: .login.txt>] || [password] || [username password]
chaudhary@localhost:~/code/python/logmein$ ./logmein.py --boo
Usage:  ./logmein.py  [filename <Default: .login.txt>] || [password] || [username password]
chaudhary@localhost:~/code/python/logmein$ ./logmein.py -b
Usage:  ./logmein.py  [filename <Default: .login.txt>] || [password] || [username password]
chaudhary@localhost:~/code/python/logmein$ ./logmein.py -h
Unexpected Error: <class 'ValueError'>
Details: unsupported format character 'p' (0x70) at index 1
Traceback (most recent call last):
  File "./logmein.py", line 222, in <module>
    main(sys.argv)
  File "./logmein.py", line 158, in main
    args, otherthings = parser.parse_known_args()
  File "/usr/lib64/python3.3/argparse.py", line 1737, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib64/python3.3/argparse.py", line 1943, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/usr/lib64/python3.3/argparse.py", line 1883, in consume_optional
    take_action(action, args, option_string)


 File "/usr/lib64/python3.3/argparse.py", line 1811, in take_action
    action(self, namespace, argument_values, option_string)
  File "/usr/lib64/python3.3/argparse.py", line 1015, in __call__
    parser.print_help()
  File "/usr/lib64/python3.3/argparse.py", line 2339, in print_help
    self._print_message(self.format_help(), file)
  File "/usr/lib64/python3.3/argparse.py", line 2323, in format_help
    return formatter.format_help()
  File "/usr/lib64/python3.3/argparse.py", line 276, in format_help
    help = self._root_section.format_help()
  File "/usr/lib64/python3.3/argparse.py", line 206, in format_help
    func(*args)
  File "/usr/lib64/python3.3/argparse.py", line 293, in _format_usage
    usage = usage % dict(prog=self._prog)
ValueError: unsupported format character 'p' (0x70) at index 1

这是我的代码

# Parse command line arguments
from argparse import ArgumentParser
usage = "%prog [-f credential_file]"
parser = ArgumentParser(usage=usage)
parser.add_argument("-f", "--file", type=str, dest="file",
                    help="Use the specified file")
parser.add_argument("-o", "--logout", action='store_true', dest="logout",
                    help="Logout")
#parser.add_argument('otherthings', nargs='*')
#args = parser.parse_args()
args, otherthings = parser.parse_known_args()
argc = len(otherthings)

if args.file != None:
    username, password = parse_file_for_credential(args.file) 

现在我找不到为什么会出现这个 ValueError，而且也只是针对 --help 选项。

Answer 1

问题是，当 argparse 尝试插入您的 usage 字符串时，Python 会看到以下字符序列:%p。因为 p 不是 a valid conversion flag , Python 引发了 ValueError。

您可以自己在 REPL 中生成此错误:

>>> "This is valid: %s" % "magic"
"This is valid: magic"

>>> "This will raise a ValueError: %p" % "magic"
ValueError: unsupported format character 'p' (0x70) 

发生这种情况的原因是因为您使用了 %prog 而不是正确的 %(prog)s (请注意 %s 中键周围的括号)。按照 Rob 的建议将 usage = "%prog [-f credential_file]" 更改为 usage = "%(prog)s [-f credential_file]" 一切都会正常进行。

为什么 argparse 使用这种格式化语法？

argparse 通过named 格式参数提供程序名称等元数据，以允许参数以任何顺序出现，并允许重复。位置参数不能重复使用，必须按照提供的顺序出现:

>>> "If I want another %s then I need to add another %s" % ("param", "placeholder")
"If I want another param then I need to add another placeholder"
# There is no way to change the order around to be
# "If I want another placeholder then I need to add another param"
# without changing the order of the parameters passed to `%`

虽然命名参数可以重复并以任何顺序使用:

>>> s = "Any order, %(two)s / %(one)s. Repeats %(one)s / %(two)s are possible"
>>> s % {"one": "first", "two": "last"}
"Any order, last / first. Repeats first / last are possible"