Detecting for invalid file inputs, Python

I have an assignment to write a Python script which "detects whether the file is readable or not".

I am stuck as to which exceptions I should run. Let's say the input file is intended to be a text file, with extension *.txt

What is the exception I should raise? I suspect there should be multiple. At the moment, I have:

with open('example_file.txt") as textfile:
        if not textfile.lower().endswith('.txt'):
            raise argparse.ArgumentTypeError(
                'Not a text file! Argument filename must be of type *.txt')
        return textfile

However, that only checks the file extension. What else could I possibly check? What is the standard for file I/O in Python?

Solution

To check whether the file exists:

import os.path
if os.path.exists('example_file.txt'):
    print('it exists!')

Beyond this, successfully opening the file will demonstrate readability. The built-in open raises an IOError exception if it fails. Failure can occur for more than one reason, so we must check whether it failed due to readability:

import errno
try:
    textfile = open('example_file.txt', 'r')
    textfile.close()
    print("file is readable")
except IOError as e:
    if e.errno == errno.EACCES:
        print("file exists, but isn't readable")
    elif e.errno == errno.ENOENT:
        print("files isn't readable because it isn't there")

The relevant section of the docs on file permissions. Note that the use of os.access to check readability before calling open is discouraged.