argparse可选的位置参数和子解析器参数 [英] argparse optional positional argument and subparsers arguments

问题描述

我有一个python脚本，它带有一个可选的位置参数，并且有一些子命令.其中一些子命令需要位置参数，而某些则不需要.当我尝试使用不需要位置参数的子命令时，出现了我遇到的问题.考虑以下测试文件:

I have a python script that takes in an optional positional argument and has a few subcommands. Some of these subcommands require the positional argument, some don't. The problem I have appears when I try to use a subcommand that does not require the positional argument. Consider the following test file:

import argparse

argp = argparse.ArgumentParser()
argp.add_argument('inputfile', type=str, nargs='?',
                  help='input file to process')
argp.add_argument('--main_opt1', type=str,
                  help='global option')

subp = argp.add_subparsers(title='subcommands',
                           dest='parser_name',
                           help='additional help',
                           metavar="<command>")

tmpp = subp.add_parser('command1', help='command1 help')
tmpp.add_argument('pos_arg1', type=str,
                  help='positional argument')

print repr(argp.parse_args())

当我尝试将子命令command1与第一个参数一起使用时，一切都会顺利进行.

When I try to use the subcommand command1 with the first argument everything goes well.

macbook-pro:~ jmlopez$ python pytest.py filename command1 otherarg
Namespace(inputfile='filename', main_opt1=None, parser_name='command1', pos_arg1='otherarg')

但是现在让我们假设command1不需要第一个位置参数.

But now let us assume that command1 doesn't need the first positional argument.

macbook-pro:~ jmlopez$ python pytest.py command1 otherarg
usage: pytest.py [-h] [--main_opt1 MAIN_OPT1] [inputfile] <command> ...
pytest.py: error: argument <command>: invalid choice: 'otherarg' (choose from 'command1')

我以某种方式期望将inputfile设置为None. argparse有什么方法可以预测command1实际上是一个子命令，因此inputfile应该设置为None?

I was somehow expecting inputfile to be set to None. Is there any way that argparse can predict that command1 is actually a subcommand and thus inputfile should be set to None?

推荐答案

对于argp，subparser参数看起来就像另一个位置，它接受选择(子解析器的名称).同样，argp对pos_arg1一无所知.在tmpp的参数列表中.

To argp the subparser argument looks just like another positional, one that takes choices (the names of the subparsers). Also argp knows nothing about pos_arg1. That's in tmpp's list of arguments.

当argp看到filename command1 otherarg时，filename和command1满足其2个位置.然后otherarg传递到tmpp.

When argp sees filename command1 otherarg, filename and command1 satisfy its 2 positionals. otherarg is then passed on the tmpp.

再次使用command1 otherarg，2个字符串，2个argp位置. command被分配给inputfile.没有逻辑可以回溯并说command1更适合subcommands，或者说'tmpp'需要这些字符串之一.

With command1 otherarg, again 2 strings, 2 argp positionals. command is assigned to inputfile. There's no logic to backtrack and say command1 fits subcommands better, or that `tmpp' needs one of those strings.

您可以将1st位置更改为可选的--inputfile.

You could change the 1st positional to an optional, --inputfile.

或者您可以inputfile另一个位置tmpp.如果许多子解析器需要它，请考虑使用parents.

Or you could inputfile another positional of tmpp. If a number of the subparsers need it it, consider using parents.

argparse并不像您一样聪明，也不能想先"或后退".如果它看起来做得很聪明，那是因为它使用re模式匹配来处理nargs值(例如?，*，+).

argparse isn't a smart as you, and can't 'think ahead' or 'backtrack'. If it appears to do something smart it's because it uses re pattern matching to handle nargs values (e.g. ?, *, +).

编辑

诱骗" argparse将其识别为子解析器的第一个位置的一种方法是在其后插入一个可选内容.使用command1 -b xxx otherarg时，-b xxx分解位置字符串的列表，因此只有command1与inputfile和subcommands相匹配.

One way to 'trick' argparse into recognizing the first positional as the subparser is to insert an optional after it. With command1 -b xxx otherarg, -b xxx breaks up the list of positional strings, so only command1 is matched against inputfile and subcommands.

p=argparse.ArgumentParser()
p.add_argument('file',nargs='?',default='foo')
sp = p.add_subparsers(dest='cmd')
spp = sp.add_parser('cmd1')
spp.add_argument('subfile')
spp.add_argument('-b')

p.parse_args('cmd1 -b x three'.split())
# Namespace(b='x', cmd='cmd1', file='foo', subfile='three')

这里的问题是argparse如何处理具有变量nargs的位置.第二个位置是次解析器这一事实并不重要.尽管argparse允许以任何顺序可变长度的位置，但它如何处理它们却可能造成混淆.如果只有一个这样的位置，并且在末尾出现，则更容易预测argparse将要执行的操作.

The issue here is how argparse handles postionals with variable nargs. The fact that the 2nd positional is a subparser is not important. While argparse allows variable length positionals in any order, how it handles them can be confusing. It's easier to predict what argparse will do if there is only one such positional, and it occurs at the end.

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