Argparse-如何指定默认子命令 [英] Argparse - How to Specify a Default Subcommand

问题描述

我正在使用Python 2.7的argparse软件包为命令行工具编写一些选项解析逻辑.该工具应接受以下参数之一:

I am using the argparse package of Python 2.7 to write some option-parsing logic for a command-line tool. The tool should accept one of the following arguments:

打开":打开功能.
"OFF":关闭功能.
[未提供任何参数]:回显函数的当前状态.

"ON": Turn a function on.
"OFF": Turn a function off.
[No arguments provided]: Echo the current state of the function.

查看argparse文档使我相信，我希望定义两个(可能是三个)子命令，因为这三个状态是互斥的，代表着不同的概念活动.这是我目前对代码的尝试:

Looking at the argparse documentation led me to believe that I wanted two--possibly three--subcommands to be defined, since these three states are mutually exclusive and represent different conceptual activities. This is my current attempt at the code:

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers()
parser.set_defaults(func=print_state) # I think this line is wrong.

parser_on = subparsers.add_parser('ON')
parser_on.set_defaults(func=set_state, newstate='ON')

parser_off = subparsers.add_parser('OFF')
parser_off.set_defaults(func=set_state, newstate='OFF')

args = parser.parse_args()

if(args.func == set_state):
    set_state(args.newstate)
elif(args.func == print_state):
    print_state()
else:
    args.func() # Catchall in case I add more functions later

给我的印象是，如果提供0个参数，则主解析器将设置func=print_state；如果提供1个参数，则主解析器将使用适当的子命令的默认值并调用func=set_state.取而代之的是，我得到了以下带有0个参数的错误:

I was under the impression that if I provided 0 arguments, the main parser would set func=print_state, and if I provided 1 argument, the main parser would use the appropriate subcommand's defaults and call func=set_state. Instead, I get the following error with 0 arguments:

usage: cvsSecure.py [-h] {ON,OFF} ...
cvsSecure.py: error: too few arguments

如果我提供"OFF"或"ON"，则会调用print_state而不是set_state.如果我注释掉parser.set_defaults行，则会正确调用set_state.

And if I provide "OFF" or "ON", print_state gets called instead of set_state. If I comment out the parser.set_defaults line, set_state is called correctly.

我是一名熟练的程序员，但是是Python的入门级新手.关于如何使它正常工作的任何建议?

I'm a journeyman-level programmer, but a rank beginner to Python. Any suggestions about how I can get this working?

编辑:我正在查看子命令的另一个原因是我正在考虑的将来的第四个功能:

Edit: Another reason I was looking at subcommands was a potential fourth function that I am considering for the future:

"FORCE txtval":将函数的状态设置为txtval.

"FORCE txtval": Set the function's state to txtval.

推荐答案

顶级解析器的默认设置将覆盖子解析器的默认设置，因此忽略子解析器的默认值func. ，但子解析器默认设置中的newstate的值是正确的.

The defaults of the top-level parser override the defaults on the sub-parsers, so setting the default value of func on the sub-parsers is ignored, but the value of newstate from the sub-parser defaults is correct.

我认为您不想使用子命令.当可用选项和位置参数根据所选择的子命令而变化时，将使用子命令.但是，您没有其他选择或位置参数.

I don't think you want to use subcommands. Subcommands are used when the available options and positional arguments change depending on which subcommand is chosen. However, you have no other options or positional arguments.

以下代码似乎可以满足您的要求:

The following code seems to do what you require:

import argparse

def print_state():
    print "Print state"

def set_state(s):
    print "Setting state to " + s

parser = argparse.ArgumentParser()
parser.add_argument('state', choices = ['ON', 'OFF'], nargs='?')

args = parser.parse_args()

if args.state is None:
    print_state()
elif args.state in ('ON', 'OFF'):
    set_state(args.state)

请注意parser.add_argument的可选参数. "choices"参数指定允许的选项，同时将"nargs"设置为?"指定应使用1个参数(如果可用)，否则不使用.

Note the optional parameters to parser.add_argument. The "choices" parameter specifies the allowable options, while setting "nargs" to "?" specifies that 1 argument should be consumed if available, otherwise none should be consumed.

编辑:如果要添加带有参数的FORCE命令，并为ON和OFF命令提供单独的帮助文本，则需要使用子命令.不幸的是，似乎没有一种指定默认子命令的方法.但是，您可以通过检查空的参数列表并提供自己的参数来解决此问题.这是一些示例代码，说明了我的意思:

If you want to add a FORCE command with an argument and have separate help text for the ON and OFF command then you do need to use subcommands. Unfortunately there doesn't seem to be a way of specifying a default subcommand. However, you can work around the problem by checking for an empty argument list and supplying your own. Here's some sample code illustrating what I mean:

import argparse
import sys

def print_state(ignored):
    print "Print state"

def set_state(s):
    print "Setting state to " + s

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers()
on = subparsers.add_parser('ON', help = 'On help here.')
on.set_defaults(func = set_state, newstate = 'ON')
off = subparsers.add_parser('OFF', help = 'Off help here.')
off.set_defaults(func = set_state, newstate = 'OFF')
prt = subparsers.add_parser('PRINT')
prt.set_defaults(func = print_state, newstate = 'N/A')
force = subparsers.add_parser('FORCE' , help = 'Force help here.')
force.add_argument('newstate', choices = [ 'ON', 'OFF' ])
force.set_defaults(func = set_state)

if (len(sys.argv) < 2):
    args = parser.parse_args(['PRINT'])
else:
    args = parser.parse_args(sys.argv[1:])

args.func(args.newstate)

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