Restrict input string length in argparse add_argument - python-3.x

Question

I want to add an argument of type string but restrict the length of the string to 1.
group.add_argument(
"--test_param",
dest="test_param",
type=str,
required=True,
default=None,
)
Is there any way I can do it here without having to raise an error by checking len(test_param) later?

Answer 1

You can try using a lambda function in type
Ex:
import argparse
group = argparse.ArgumentParser(description='Data.')
group.add_argument(
"--test_param",
dest="test_param",
type=lambda x: x if x.isalpha() and len(x) <=1 else False, #Sample Function
required=True,
default=None,
)
args = group.parse_args()
value = getattr(args, 'test_param')
if value:
print(value)
else:
print("error!!!")
Output:
python sample.py --test_param fddd #->>error!!
python sample.py --test_param f #->>f

Answer 2

You can do this with the type parameter of add_argument. The recommended way is to raise an exception during the type check:
import argparse
import sys
parser = argparse.ArgumentParser()
def len_gt_0(s):
if len(s) > 0:
return s
raise argparse.ArgumentTypeError("Value must have length greater than 0")
parser.add_argument("--foo", type=len_gt_0, required=True)
opts = parser.parse_args(sys.argv[1:])
print(f"{ opts.foo = }")
Output:
$ python3.8 /tmp/foo.py --foo=abc
opts.foo = 'abc'
$ python3.8 /tmp/foo.py --foo=
usage: foo.py [-h] --foo FOO
foo.py: error: argument --foo: Value must have length greater than 0
$ python3.8 /tmp/foo.py
usage: foo.py [-h] --foo FOO
foo.py: error: the following arguments are required: --foo

Answer 3

You can use the choices argument available while adding a argument for example in my code i just want to select from 1 or 2. I used the following snippet as follows:
parser.add_argument(
"report",
type=int,
metavar="[report #]",
help="1. for Annual Max/Min Temperature 2. for Hottest day of each year",
choices=[1, 2]
)
it will restrict the input from these choices only and will show the error result if you have given out of these choices like this:
usage: [report#] [data_dir]
[report #]
1 for Annual Max/Min Temperature
2 for Hottest day of each year
[data_dir]
Directory containing weather data files
main.py: error: argument [report #]: invalid choice: 3 (choose from 1, 2)
If you don't want to add the choices you can use lamda or a function for validating the data. You can get help from my example below:
parser.add_argument(
"data_dir",
type=check_directory_availability,
metavar="[data_directory]",
help="Directory containing weather data files",
)
def check_directory_availability(dir_name):
if not os.path.isdir(dir_name):
raise NotADirectoryError("Provided directory name doesn't exists")
return dir_name
Hope so it may help you.