Reading a file using a relative path in a Python project

Say I have a Python project that is structured as follows:

project
    /data
        test.csv
    /package
        __init__.py
        module.py
    main.py

__init__.py:

from .module import test

module.py:

import csv

with open("..data/test.csv") as f:
    test = [line for line in csv.reader(f)]

main.py:

import package

print(package.test)

When I run main.py I get the following error:

 C:\Users\Patrick\Desktop\project>python main.py
Traceback (most recent call last):
  File "main.py", line 1, in <module>
    import package
  File "C:\Users\Patrick\Desktop\project\package\__init__.py", line 1, in <module>
    from .module import test
  File "C:\Users\Patrick\Desktop\project\package\module.py", line 3, in <module>
    with open("../data/test.csv") as f:
FileNotFoundError: [Errno 2] No such file or directory: '../data/test.csv'

However, if I run module.py from the package directory, I don’t get any errors. So it seems that the relative path used in open(...) is only relative to where the originating file is being run from (i.e __name__ == "__main__")? How can deal with this, using relative paths only?

Relative paths are relative to current working directory. If you do not want your path to be relative, it must be absolute.

But there is an often used trick to build an absolute path from current script: use its __file__ special attribute:

from pathlib import Path

path = Path(__file__).parent / "../data/test.csv"
with path.open() as f:
    test = list(csv.reader(f))

This requires python 3.4+ (for the pathlib module).

If you still need to support older versions, you can get the same result with:

import csv
import os.path

my_path = os.path.abspath(os.path.dirname(__file__))
path = os.path.join(my_path, "../data/test.csv")
with open(path) as f:
    test = list(csv.reader(f))

[2020 edit: python3.4+ should now be the norm, so I moved the pathlib version inspired by jpyams' comment first]