some.py

#!/usr/bin/env python

# -1) Why wouldn't I use wc?

from pathlib import Path

# 0) Hopefully making things more maintainable...
import argparse

parser = argparse.ArgumentParser(
    description='Check text file length is below a limit.')
parser.add_argument(
    'file', type=Path, help='a file to check')
parser.add_argument(
    '--limit', type=int, help='file length limit (default is 100,000,000)',
    default=10**8)

args = parser.parse_args()

# 1) Using csv is just weird (and memory inefficient).
# 2) I'm fairly sure you want 10**8, not 10^8 (bit shift).
# 3) In any case, setting the 'limit' to a parameter.
# 4) Throwing an exception may be overkill here, but it helps maintainability
# 5) Fixing the function to return the actual line count
# 6) I think os is more maintainable than pathlib (debateable I
#    guess)... I'm not getting any benefit from pathlib the way I've
#    written it currently...
# 7) Stubbed out testing for pytest
# 8) Added a type check on the count function. Not sure if this is pythonic...
# 9) Added function descriptions

def count_lines(file=None, limit=100000000):
    """Return the number of lines in a given file. Return an exception if
    the length of the file is above a given limit.

    """
    if isinstance(file, Path):
        lines = 0
        with file.open() as file:
            for line in file:
                lines = lines + 1
                if lines > limit:
                    raise Exception('File is greater than ' + str(limit) + '.')
        return lines
    else:
        raise AttributeError



if __name__ == '__main__':

    file = args.file

    if file.is_file():
        lines = count_lines(file, args.limit)
        print(lines)
    else:
        print(0)
        exit(1)


def test_count_lines():
    """TODO: I could 'mock' a file here and do some tests. However, I
    should tidy away the argparse into a function first...

    """
    pass