如果没有传递命令行参数，则为Python默认参数

Question

I'l like to build a program with this behaviour:

usage: sage 4ct.py [-h] (-r R | -i I | -p P) [-o O]

But if you don't give any parameter, I'd like to have "-r 100" as the default.

Is it possible?

parser = argparse.ArgumentParser(description = '4ct args')

group_input = parser.add_mutually_exclusive_group(required = True)
group_input.add_argument("-r", "-random", help = "Random graph: dual of a triangulation of N vertices", nargs = 1, type = int, default = 100)
group_input.add_argument("-i", "-input", help = "Input edgelist filename (networkx)", nargs = 1)
group_input.add_argument("-p", "-planar", help = "Load a planar embedding of the graph G.faces() - Automatically saved at each run: input_planar_file.serialized", nargs = 1)
parser.add_argument("-o", "-output", help="Output edgelist filename (networkx)", nargs = 1, required = False)

args = parser.parse_args()

Answer 1

Just remove the required argument of the add_mutually_exclusive_group function call (or set it to False) and you're done:

import argparse

parser = argparse.ArgumentParser(description = '4ct args')

group_input = parser.add_mutually_exclusive_group(required = False)
group_input.add_argument("-r", "--random", help = "Random graph: dual of a triangulation of N vertices", type = int, default = 100)
group_input.add_argument("-i", "--input", help = "Input edgelist filename (networkx)")
group_input.add_argument("-p", "--planar", help = "Load a planar embedding of the graph G.faces() - Automatically saved at each run: input_planar_file.serialized")
parser.add_argument("-o", "--output", help="Output edgelist filename (networkx)", required = False)

print(parser.parse_args())
# Namespace(input=None, output=None, planar=None, random=100)
print(parser.parse_args("-r 77".split()))
# Namespace(input=None, output=None, planar=None, random=77)
print(parser.parse_args("-o some/path".split()))
# Namespace(input=None, output='some/path', planar=None, random=100)
print(parser.parse_args("-i some/path".split()))
# Namespace(input='some/path', output=None, planar=None, random=100)
print(parser.parse_args("-i some/path -o some/other/path".split()))
# Namespace(input='some/path', output='some/other/path', planar=None, random=100)
print(parser.parse_args("-r 42 -o some/other/path".split()))
# Namespace(input=None, output='some/other/path', planar=None, random=42)

As you can see, the random option is defaulted to 100 even if:

• the output option is provided, which seems normal
• an option from the mutual exclusive group other than random is provided, which can be problematic. you will have to check in your code if random is the only exclusive option which has a value before taking it in account.

---

This example also includes some tiny improvement to your option parser:

• use long option names with two dashes (it is a convention but it also allows argparse to correctly recognise option name).
• remove the nargs=1 in your options definitions which makes you retrieve a list of one value. By removing it, you could retrieve directly the value.

Answer 2

Give the following a try:

import argparse
import sys

parser = argparse.ArgumentParser(description='4ct args')

group_input = parser.add_mutually_exclusive_group(required=True)
group_input.add_argument("-r", "-random", help="Random graph: dual of a triangulation of N vertices", nargs=1, type=int,             default=100)
group_input.add_argument("-i", "-input", help="Input edgelist filename (networkx)", nargs=1)
group_input.add_argument("-p", "-planar", help="Load a planar embedding of the graph G.faces() - Automatically saved at each run: input_planar_file.serialized",nargs=1)
parser.add_argument("-o", "-output", help="Output edgelist filename (networkx)", nargs=1, required=False)

if not sys.argv[1:]:
    sys.argv.extend(['-r', '100'])

args = parser.parse_args(sys.argv[1:])

Essentially you are checking if any commandline parameters are given at all, and if not, you append the desired -r 100