python - argparse 参数的可选值

Question

我想区分这三种情况:

• 标志根本不存在 python example.py;
• 标志存在但没有值 python example.py -t;和
• 标志存在并具有值 python example.py -t ~/some/path。

如何使用 Python argparse 执行此操作？ action='store_true' 将涵盖前两种情况，但随后第三种情况将无效。

Answer 1

您可以使用 nargs='?' 执行此操作:

One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from default will be produced. Note that for optional arguments, there is an additional case - the option string is present but not followed by a command-line argument. In this case the value from const will be produced.

你的三个案例会给出:

1. 默认值；
2. const 值；和
3. '~/some/path'

分别。例如，给定以下简单实现:

from argparse import ArgumentParser

parser = ArgumentParser()
parser.add_argument('-t', nargs='?', default='not present', const='present without value')

print(parser.parse_args().t)

你会得到这样的输出:

$ python test.py
not present

$ python test.py -t
present without value

$ python test.py -t 'passed a value'
passed a value