Variable Number of narg Arguments Python Argparse

Question

This is likely beyond the scope of the argparse module, but I'll try to describe my issue using an example.

I have some fruits and some files attributed to each fruit. Specifically Apple, Banana, and Orange. Apple has 10 files associated with it, Banana has 7, and Oranges has 9. I can hardcode -a, -b, -o each taking nargs='+' to handle this toy example in a Python command-line script. But say I have a variable number of fruits, or a very large number (maybe 50). It would be crazy to hardcode a flag for each type, what is the best solution here?

maybe you can write a configuration file (json format ?) and use it as an arg in your python command line — iFlo
– iFlo, Commented Dec 14, 2016 at 17:14
Something like python script.py -x1 a b c -x2 d e f -x3 g h i ? Except that I could keep going until x100, x500, x10000000, etc. — E.Cross
– E.Cross, Commented Dec 14, 2016 at 17:23
You can't actually give a program ten million command-line arguments. I don't remember where the limitation arises, but command-line arguments are limited to a lot fewer than that. Consider reading this stuff from a file instead of the command line arguments, or at least providing an option to do so. — user2357112
– user2357112, Commented Dec 14, 2016 at 17:46
Who or what determines the number and names of the valid fruits/flags? And who wants to type in, via commandline, 50 fruits with 10 file names each? — hpaulj
– hpaulj, Commented Dec 14, 2016 at 17:51

Robᵩ · Accepted Answer · 2016-12-14 17:42:57Z

I would create an argument for each of the fruits, but I would do it in the DRYest way I could:

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("-i", "--insecticide")
for fruit, nargs in (
    ('Apple', 10),
    ('Banana', 7),
    ('Orange', 9),
    ):
    parser.add_argument(
        "--" + fruit.lower(), nargs=nargs, metavar='FLIES', 
        help="specify {} species of {} pests".format(nargs, fruit))
args = parser.parse_args()
print(args)

Here is the resulting help message:

$ python x.py -h
usage: x.py [-h] [-i INSECTICIDE]
            [--apple FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES]
            [--banana FLIES FLIES FLIES FLIES FLIES FLIES FLIES]
            [--orange FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES]

optional arguments:
  -h, --help            show this help message and exit
  -i INSECTICIDE, --insecticide INSECTICIDE
  --apple FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES
                        specify 10 species of Apple pests
  --banana FLIES FLIES FLIES FLIES FLIES FLIES FLIES
                        specify 7 species of Banana pests
  --orange FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES FLIES
                        specify 9 species of Orange pests

If there were a variable number of fruits (controlled by an environment variable, or the presence of configuration files, for example), then my loop wouldn't be hardcoded, but instead it might be:

for config_section in config_data():
    parser.add_argument("--" + config_section.name, ...)

If I wanted to name the parameters sequentially, as OP's comment suggests, I might code the loop:

for i, fruit in enumerate(fruit_names, 1):
    parser.add_argument("--x{}".format(i), 
                        nargs="+", 
                        help="{} files".format(fruit))

And here is the resulting help message:

$ python x2.py -h
usage: x2.py [-h] [-i INSECTICIDE] [--x1 X1 [X1 ...]] [--x2 X2 [X2 ...]]
             [--x3 X3 [X3 ...]]

optional arguments:
  -h, --help            show this help message and exit
  -i INSECTICIDE, --insecticide INSECTICIDE
  --x1 X1 [X1 ...]      Apple files
  --x2 X2 [X2 ...]      Banana files
  --x3 X3 [X3 ...]      Orange files

I suppose reading from a config file for the names and then generating the arguments as you suggest would solve the problem, nice solution

Cade Brown · Accepted Answer · 2016-12-14 17:16:46Z

0

You can use nargs=7 for example, and it will only accept exactly 7 (or return None if the flag is not entered)

For example, add

parser.add_argument('-x', nargs=7, help='testing')

to your argument list and print args.x

Say your file was a.py

$ python a.py

> None

$ python a.py -x 1

> a.py: error: argument -x: expected 7 argument(s)

$ python a.py -x 1 2 3 4 5 6 7

> ['1', '2', '3', '4', '5', '6', '7'] `

answered Dec 14, 2016 at 17:16

Cade Brown

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1 Comment

E.Cross Over a year ago

Sorry you've misunderstood my question. I am aware of how to specify the number of arguments I want to nargs. I'm asking if there is a way to specify a variable number of a variable number of arguments

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