Include "%" in argparse help to die

Suppose you create a Python script like the one below.

import argparse

parser = argparse.ArgumentParser(description='')
parser.add_argument('--zoom',
    action='store',
    type=float,
    default=1.0,
    help="Expansion rate(%)")
args = parser.parse_args()

print("zoom is {}".format(args.zoom))

This can be done normally,

$ python hoge.py --zoom 2.0
zoom is 2.0

I get a Value Error when I try to display help.

$ python hoge.py -h
Traceback (most recent call last):
  File "hoge.py", line 9, in <module>
    args = parser.parse_args()
  File "C:\Python37\lib\argparse.py", line 1749, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "C:\Python37\lib\argparse.py", line 1781, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "C:\Python37\lib\argparse.py", line 1987, in _parse_known_args
    start_index = consume_optional(start_index)
  File "C:\Python37\lib\argparse.py", line 1927, in consume_optional
    take_action(action, args, option_string)
  File "C:\Python37\lib\argparse.py", line 1855, in take_action
    action(self, namespace, argument_values, option_string)
  File "C:\Python37\lib\argparse.py", line 1037, in __call__
    parser.print_help()
  File "C:\Python37\lib\argparse.py", line 2474, in print_help
    self._print_message(self.format_help(), file)
  File "C:\Python37\lib\argparse.py", line 2458, in format_help
    return formatter.format_help()
  File "C:\Python37\lib\argparse.py", line 284, in format_help
    help = self._root_section.format_help()
  File "C:\Python37\lib\argparse.py", line 215, in format_help
    item_help = join([func(*args) for func, args in self.items])
  File "C:\Python37\lib\argparse.py", line 215, in <listcomp>
    item_help = join([func(*args) for func, args in self.items])
  File "C:\Python37\lib\argparse.py", line 215, in format_help
    item_help = join([func(*args) for func, args in self.items])
  File "C:\Python37\lib\argparse.py", line 215, in <listcomp>
    item_help = join([func(*args) for func, args in self.items])
  File "C:\Python37\lib\argparse.py", line 525, in _format_action
    help_text = self._expand_help(action)
  File "C:\Python37\lib\argparse.py", line 614, in _expand_help
    return self._get_help_string(action) % params
ValueError: unsupported format character ')' (0x29) at index 5

As you can see at the end of the error message, the output string is formatted like return self._get_help_string (action)% params. It seems that unintended parsing is done when % is included.

The workaround is to avoid using % as shown below.

import argparse

parser = argparse.ArgumentParser(description='')
parser.add_argument('--zoom',
    action='store',
    type=float,
    default=1.0,
-    help="Expansion rate(%)")
+    help="Expansion rate(percent)")
args = parser.parse_args()

print("zoom is {}".format(args.zoom))

Alternatively, escape with %%.

-    help="Expansion rate(%)")
+    help="Expansion rate(%%)")

It came out.

$ python hoge.py -h
usage: hoge.py [-h] [--zoom ZOOM]

optional arguments:
  -h, --help   show this help message and exit
  --zoom ZOOM magnification(percent)

$ python hoge.py -h
usage: hoge.py [-h] [--zoom ZOOM]

optional arguments:
  -h, --help   show this help message and exit
  --zoom ZOOM magnification(%)