How to handle non-existing path?

Question

I have a class that creates a "Test" object - an object based on (describes) an external test script.

The code can be found here: https://codeshare.io/5zlW0W

I use this class like this:

from test import Test

test = Test("/path/to/test")

This works perfectly well when the test file exists, but I hit the following error when it does not exist:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/repos/test.py", line 13, in __init__
    self.version = self.get_attribute("version")
  File "/home/user/repos/test.py", line 33, in get_attribute
    p = subprocess.Popen([self.path, '--' + attribute], stdout=subprocess.PIPE, universal_newlines=True)
  File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1551, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'bla'

So my question comes in two parts:

1. What is the best way to handle the case where the path does not exist?
2. Is it okay for me to define the initial variable using functions to grab that data as I have done in __init__?

Answer 1

Check the file exist in the get_attribute method by using os.path.exists(file_path)

def get_attribute(self, attribute):
        """Return a given attribute of the test.

        Runs a test subprocess with the --<attribute> argument.
        """
        if os.path.exists(self.path):
            p = subprocess.Popen([self.path, '--' + attribute], stdout=subprocess.PIPE, universal_newlines=True)
                attr = p.stdout.read().strip("\n")

            return attr

Answer 2

You could always use the standard OS library.

import os
from test import Test

if os.path.exists("/path/to/test"):
    test = Test("/path/to/test")

If you also wish to make sure the file is not empty, you can use

if os.stat("/path/to/test").st_size > 0:

Note this may cause race condition:

A good question regarding this matter may be found here: How does using the try statement avoid a race condition?