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?

Answer 1

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

Answer 2

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'] `