check if file exists.

[DeaD_EyE]

Don't ask for permission ask for forgiveness.

Instead of checking every possible possibility, you should use try: except:

import sys


filename = 'not_existing_file'

try:
    fd = open(filename)
except OSError as e:
    print(e, file=sys.stderr)
    sys.exit(1)

Or you can be more concrete:

filename = '/dev/mem'

try:
    fd = open(filename)
except PermissionError as e:
    print('You don\'t have the permission to open the file {}'.format(filename), file=sys.stderr)
    sys.exit(1)
except OSError as e:
    # PermissionError is a subclass of OSError
    # First you should be very concrete, and later
    # you can catch all other Errors
    # NEVER USE except:
    # this will catch all Errors
    print(e, file=sys.stderr)
    sys.exit(2)