check if file exists.

[snippsat]

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

I agree with this.
There is a couple of thing that can be done different.
No need to import sys and use file=sys.stderr,just print error give the same.
and use with open().

>>> try:
...     with open('ll.txt') as f:
...         f.read()
... except OSError as error:
...     print(error)
[Errno 2] No such file or directory: 'll.txt'

>>> import sys
>>> try:
...     with open('ll.txt') as f:
...         f.read()
... except OSError as e:
...     print(e, file=sys.stderr)
[Errno 2] No such file or directory: 'll.txt'