带有可检测开关的python argparse可选位置参数 [英] python argparse optional positional argument with detectable switch

问题描述

我想像 program -s 一样调用我的程序.我想分配一个默认值，但也希望能够检测是否给出了 -s 开关.我有什么:

I would like to invoce my programm like program -s <optional value>. I would like to assign a default value, but would also like to be able to detect if the -s switch was given. What I have:

max_entries_shown = 10
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-s",
    nargs = '?',
    default = max_entries_shown)
args = parser.parse_args()

如果我不在命令行上给出 -s 和 None，这给我一个 args.s 的值如果我指定 -s 没有值.我想要的是 args.s 等于 None 如果没有给出开关，并且 args.s 设置为带有 的默认值-s 给定，并且 args.s 等于 custom_value 如果作为 program -s custom_value 运行.我怎样才能做到这一点?

This gives me a value of 10 for args.s if I don't give -s on the command line, and None if I specify -s without a value. What I want is args.s equal to None if no switch is given, and args.s set to the default value with -s given, and args.s equal to custom_value if run as program -s custom_value. How can I achive this?

推荐答案

你必须使用 const 而不是 default.引用自 argparse Python Docs 关于何时使用 const:

You have to use const instead of default. Quote from argparse Python Docs about when to use const:

当使用选项字符串(如 -f 或 --foo)和 nargs='?' 调用 add_argument() 时.这将创建一个可选参数，后面可以跟零个或一个命令行参数.解析命令行时，如果遇到选项字符串后面没有命令行参数，则将假定为 const 的值.有关示例，请参阅 nargs 说明.

When add_argument() is called with option strings (like -f or --foo) and nargs='?'. This creates an optional argument that can be followed by zero or one command-line arguments. When parsing the command line, if the option string is encountered with no command-line argument following it, the value of const will be assumed instead. See the nargs description for examples.

此外，我添加了一个 type=int 因为我假设您想将输入视为 int.

Additionally, I added a type=int because I assumed that you want to treat the input as an int.

因此，您的代码应该是:

So, your code should be:

max_entries_shown = 10
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-s",
    nargs = '?',
    const = max_entries_shown,
    type=int)
args = parser.parse_args()

此代码返回(带有打印参数)

This code returns (with print args)

$ python parse.py
Namespace(s=None)
$ python parse.py -s
Namespace(s=10)
$ python parse.py -s 12
Namespace(s=12)

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