一种可选的参数，不需要位置参数 [英] One optional argument which does not require positional arguments

问题描述

我对python的argparse有疑问:是否可以有一个不需要位置参数的可选参数?

I have a question regarding python's argparse: Is it possible to have a optional argument, which does not require positional arguments?

示例:

parser.add_argument('lat', help="latitude")
parser.add_argument('lon', help="longitude")
parser.add_argument('--method', help="calculation method (default: add)", default="add")
parser.add_argument('--list-methods', help="list available methods", action="store_true")

正常命令行为test.py 47.249 -33.282或test.py 47.249 -33.282 --method sub.但是，一旦我用test.py --list-methods调用脚本以列出所有可用的方法，我就会得到error: to few arguments.如何使用argparse来具有此可选参数(--list-methods)而没有位置参数(lat，lon)?

The normal command line would be test.py 47.249 -33.282 or test.py 47.249 -33.282 --method sub. But as soon as I call the script with test.py --list-methods to list all available methods, I get error: to few arguments. How can I use argparse to have this optional argument (--list-methods) without having positional arguments (lat, lon)?

推荐答案

• 设置默认值，并为位置参数设置nargs='?'

• 在代码中手动检查未处于 list-methods 模式时是否已设置了纬度和经度

  • set a default value and nargs='?' for your positional arguments

  • check manually in your code that both latitude and longitude have been set when you're not in list-methods mode

    parser = argparse.ArgumentParser()
    
    parser.add_argument('lat', help="latitude",default=None, nargs='?')
    parser.add_argument('lon', help="longitude",default=None, nargs='?')
    parser.add_argument('--method', help="calculation method (default: add)", default="add")
    parser.add_argument('--list-methods', help="list available methods", action="store_true")
    
    args = vars(parser.parse_args())
    
    if not args['list_methods'] and (args['lat'] == None or args['lon'] == None):
        print '%s: error: too few arguments' % sys.argv[0]
        exit(0)
    
    if args['list_methods']:
        print 'list methods here'
    else :
        print 'normal script execution'
    

  给出:

  $ test.py --list-methods
  在此处列出方法

  $ test.py --list-methods
  list methods here

  $ test.py 4
  test.py:错误:参数太少

  $ test.py 4
  test.py: error: too few arguments

  test.py 4 5
  正常脚本执行

  test.py 4 5
  normal script execution

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