argparse validation in a python class

I'm trying an OOP approach to my Python code which eventually will be converted to an .EXE file created with PyInstaller. The idea is to pass a series of arguments from the user input (n to a program that eventually will go something like (myprogram.exe -secureFolder C:/Users -thisisacsvfile.csv -countyCode 01069 -utmZone 15).

I can initialize a class definition and pass the arguments like:

import argparse
import sys

class myprogram():
    def __init__(self, secureFolder, inputCsvFile, countyCode, utmZone):
        self.secureFolder = secureFolder
        self.inputCsvFile = inputCsvFile
        self.countyCode = countyCode
        self.utmZone = utmZone
        
        
if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument("secureFolder", help = "A directory where the files are located", type=str)
    parser.add_argument("inputCsvFile",  help="A CSV file containing the results for a particular county (e.g. 36107.csv)", type=str)
    parser.add_argument("countyCode", help = "The FIPS county code", type = str)
    parser.add_argument("utmZone", help = "The UTM zone code for that specific county (e.g. 18)", type = int)

However, I need to validate every user argument and that's the part where I'm getting confused. In other words, I need to check if the secureFolder exists, if the inputCsvFile is indeed a CSV and contains some specific columns and other operations for the rest of the arguments. What I don't know exactly, where do I perform these operations? After the class definition?
Before the OOP approach, I was doing something like:

# Check if all the arguments were passed
undefined_arguments = [attr for attr in vars(args) if getattr(args, attr) is None]
if undefined_arguments:
    print("The following arguments were not defined:", undefined_arguments)
else:
    print("All arguments were defined.")

# 1a. Check inputCsvFile
if args.inputCsvFile is None:
    sys.exit("Please select an input CSV file to process (e.g. inputCsvFile.../myfile.csv) ")
else:
    if not os.path.isfile(args.inputCsvFile):
        sys.exit (f"File {args.inputCsvFile} doesn't appear to exists...please check if the file exists or if you have privileges to access it")
    else:
        grid_file_csv = args.inputCsvFile 
        print (f"{args.inputCsvFile} found...")

# 1b. Check if inputCsvFile is a CSV:
if not args.inputCsvFile.endswith('.csv'):
    raise ValueError("Invalid input file. Expected a CSV file.")
    sys.exit('No propper CSV file has been passed...')

# 2. Check if the FIPS code
if args.countyCode is None:
   sys.exit("Please specify a valid county code (e.g. -countyCode3607)")
             
# Check the UTM area code
if args.utmzone is None:
   sys.exit("Please specify a valid UTM zone area (e.g. -utmZone 16): ")

if args.utmZone is not None:
    val = args.utmZone 
    if val < 1 and val > 20:
        raise Exception('UTM zone area should be between 1 and 20')
        sys.exit()

This is largely a question of style and preference. In my view, wherever possible, values needed to construct any class should be validated before the class is constructed - especially when it relies on user input.

So, get the command line arguments, validate them, then construct your class. Something like this:

import argparse
from sys import stderr
import os
class myprogram():
def __init__(self, secureFolder, inputCsvFile, countyCode, utmZone):
self.secureFolder = secureFolder
self.inputCsvFile = inputCsvFile
self.countyCode = countyCode
self.utmZone = utmZone
def __str__(self):
return f'{self.secureFolder=}, {self.inputCsvFile=}, {self.countyCode=}, {self.utmZone=}'
@staticmethod
def validate(ns):
if not os.path.isdir(ns.secureFolder):
print(f'{ns.secureFolder} is not a valid folder', file=stderr)
return None
try:
with open(ns.inputCsvFile) as _:
...
except Exception as e:
print(f'Unable to open {ns.inputCsvFile} due to {e}', file=stderr)
return None
if ns.countyCode is None:
print(f'{ns.countyCode} is an invalid county code', file=stderr)
return None
if (z := ns.utmZone) is None or z < 1 or z > 60:
print(f'{z} is not a valid utmZone', file=stderr)
return None
return myprogram(ns.secureFolder, ns.inputCsvFile, ns.countyCode, ns.utmZone)
if __name__ == '__main__':
parser = argparse.ArgumentParser()
cli = [
('-secureFolder', 'A directory where the files are located', str),
('-inputCsvFile', 'A CSV file containing the results for a particular county (e.g., 36107.csv)', str),
('-countyCode', 'The FIPS county code', str),
('-utmZone', 'The UTM zone code for that specific county (e.g., 18)', int)
]
for c, h, t in cli:
parser.add_argument(c, help=h, type=t)
args = parser.parse_args()
if (mp := myprogram.validate(args)) is None:
print('Unable to construct class instance')
else:
# at this point we have a valid myprogram class instance (mp)
print(mp)

The argparse library can do some validations for you, such as ensuring that the argument is required. For the rest, you can write small, concise validation functions:

import argparse
import pathlib


class MyProgram:
    def __init__(self, secureFolder, inputCsvFile, countyCode, utmZone):
        self.secureFolder = secureFolder
        self.inputCsvFile = inputCsvFile
        self.countyCode = countyCode
        self.utmZone = utmZone


def validate_dir(value):
    path = pathlib.Path(value)
    if not path.is_dir():
        raise ValueError()
    return value


def validate_csv(value):
    path = pathlib.Path(value)
    if not path.exists():
        raise ValueError()
    if path.suffix != ".csv":
        raise ValueError()
    return value


def validate_utm_zone(value):
    value = int(value)
    if not (1 <= value <= 20):
        raise ValueError()
    return value


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "-secureFolder",
        help="A directory where the files are located",
        type=validate_dir,
        required=True,
    )
    parser.add_argument(
        "-inputCsvFile",
        help="A CSV file containing the results for a particular county",
        type=validate_csv,
        required=True,
    )
    parser.add_argument("-countyCode", help="The FIPS county code", required=True)
    parser.add_argument(
        "-utmZone",
        help="The UTM zone code for that specific county (e.g. 18)",
        type=validate_utm_zone,
        required=True,
    )

    options = parser.parse_args()
    print(options)
    my_program = MyProgram(
        secureFolder=options.secureFolder,
        inputCsvFile=options.inputCsvFile,
        countyCode=options.countyCode,
        utmZone=options.utmZone,
    )

Here are some sample runs

# Happy path
$ python3  main.py -secureFolder ./myfolder -inputCsvFile input.csv -countyCode code-blah -utmZone 15 
Namespace(secureFolder='./myfolder', inputCsvFile='input.csv', countyCode='code-blah', utmZone=15)

# None of the arguments supplied
$ python3  main.py                                                                                   
usage: main.py [-h] -secureFolder SECUREFOLDER -inputCsvFile INPUTCSVFILE -countyCode COUNTYCODE -utmZone UTMZONE
main.py: error: the following arguments are required: -secureFolder, -inputCsvFile, -countyCode, -utmZone

# Directory /foobar does not exist
$ python3  main.py -secureFolder /foobar -inputCsvFile input.csv -countyCode code-blah -utmZone 15
usage: main.py [-h] -secureFolder SECUREFOLDER -inputCsvFile INPUTCSVFILE -countyCode COUNTYCODE -utmZone UTMZONE
main.py: error: argument -secureFolder: invalid validate_dir value: '/foobar'

# Invalid utmZone
$ python3  main.py -secureFolder ./myfolder -inputCsvFile input.csv -countyCode code-blah -utmZone foo
usage: main.py [-h] -secureFolder SECUREFOLDER -inputCsvFile INPUTCSVFILE -countyCode COUNTYCODE -utmZone UTMZONE
main.py: error: argument -utmZone: invalid validate_utm_zone value: 'foo'

# utmZone not in range
$ python3  main.py -secureFolder ./myfolder -inputCsvFile input.csv -countyCode code-blah -utmZone 21 
usage: main.py [-h] -secureFolder SECUREFOLDER -inputCsvFile INPUTCSVFILE -countyCode COUNTYCODE -utmZone UTMZONE
main.py: error: argument -utmZone: invalid validate_utm_zone value: '21'

# CSV file does not end with .csv
$ python3  main.py -secureFolder ./myfolder -inputCsvFile main.py -countyCode code-blah -utmZone 15
usage: main.py [-h] -secureFolder SECUREFOLDER -inputCsvFile INPUTCSVFILE -countyCode COUNTYCODE -utmZone UTMZONE
main.py: error: argument -inputCsvFile: invalid validate_csv value: 'main.py'

# csv does not exist
$ python3  main.py -secureFolder ./myfolder -inputCsvFile input.csv -countyCode code-blah -utmZone 15
usage: main.py [-h] -secureFolder SECUREFOLDER -inputCsvFile INPUTCSVFILE -countyCode COUNTYCODE -utmZone UTMZONE
main.py: error: argument -inputCsvFile: invalid validate_csv value: 'input.csv'

# Get help
$ python3 main.py -h                                                                                 
usage: main.py [-h] -secureFolder SECUREFOLDER -inputCsvFile INPUTCSVFILE -countyCode COUNTYCODE -utmZone UTMZONE

options:
  -h, --help            show this help message and exit
  -secureFolder SECUREFOLDER
                        A directory where the files are located
  -inputCsvFile INPUTCSVFILE
                        A CSV file containing the results for a particular county
  -countyCode COUNTYCODE
                        The FIPS county code
  -utmZone UTMZONE      The UTM zone code for that specific county (e.g. 18)