How to fix FileNotFoundError?

I have encountered a problem while trying to open a .txt file from the same directory where my source code is located.
When I tried to open the file like this:

with open("pi_digits.txt") as file_object:
        contents = file_object.read()
print(contents)

I failed.
I also failed when I typed the whole path:

with open("Users\lukas\Documents\python_work\chapter_10") as file_object:
        contents = file_object.read()
print(contents)

But when I typed:

with open("\\Users\\lukas\\Documents\\python_work\\chapter_10\\pi_digits.txt") as file_object:
    contents = file_object.read()
print(contents)

I succeeded!

So my question is: Why can’t I run the code without error when I enter the following code:

with open('pi_digits.txt') as file_object:
    contents = file_object.read()
print(contents)

Thank you for your answers and sorry if my question was not well constructed.

>Solution :

@Sottvogel

In general, a python instance can be run from many different directories in your computer. One way to check where the running instance is:

import os 
os.getcwd()

If you confirm that you are in the same directory of your file, try and see
what the following returns:

os.listdir()

In case your file doesn’t appear in the returned string, then there has to be another problem. Make sure you checkout the docs as well.

Hope it helps!