How can I send a string from script_1 to script_2?

In your main_file.py:

def action(x):
    print(x)

In your file_1.py (must be in the same directory as main.py):

import main_file

x = "Carrot"
main_file.action(x)

Now let's say I had a directory with multiple sub-directories. What would be the best method to have file_1.py, file_2.py, file_3.py and so on (up to, say, several hundred files in perhaps a half a dozen sub-directories) all "find" main_file.py without hard coding a path to main_file.py?

You can order your project structure to something like that:

p
├───s1
│   └───file_1.py
├───s2
│   ├───s21
│   │   └───file_3.py
│   └───file_2.py
└───main_file.py

Then, in either file_1.py, file_2.py or file_3.py, you can import main_file at the top and use as the above snippet. To execute a particular file (let's say, file_3.py), you can change your current working directory to p using

cd p

and run

python -m s2.s21.file3