python 中的 -n 代表什么？

Question

import argparse

parser = argparse.ArgumentParser(description="Meow like a cat")
parser.add_argument("-n", default=1,help="number of times to meow", type=int)
args = parser.parse_args()

for _ in range(arg.n):
    print("meow")

I don't really understand the purpose of having -n in line 4, I know it's a value holder for command line argument, but why -n; is there a special meaning to it? Also, why in the for loop, the parameter takes arg.n instead of arg.-n since that's what it used before? Thank You for helping!

Answer 1

add_argument("-n", ...) defines an argument that takes '-n'` as a flag, and is used as with

$python your_script -n 1

argparse sets that value to the args object.

print(args)
Namespace(n=1)

which is then accessed as

args.n

All that should be explained in the argparse docs.

'n' is called the dest , and is derived from the add_argument option-string by stripping off the dash.

 add_argument('-n','--number', ...)

would set the dest to 'number', but accept either

 myscript -n 1
 myscript --number 3

Answer 2

It's a convention of passing arguments to programs, not a Python thing per se. So usually it's -{single-letter} or --{a word} . So -n would be the same as --number in that case

Answer 3

Command-line tools have two kinds of arguments: flags/options (denoted by either a single or double dash, in this case -n but could also be something like --version ) and positional arguments which are just data (for example a path). You can find one explanation of them here .

You can find documentation on argparse on Python docs that showcases what kind of options you have for different types of arguments and how they are translated into the resulting Python object.