Python unit test that uses an external data file

Usually what I do is define

THIS_DIR = os.path.dirname(os.path.abspath(__file__))

at the top of each test module. Then it doesn’t matter what working directory you’re in – the file path is always the same relative to the where the test module sits.

Then I use something like this is in my test (or test setup):

my_data_path = os.path.join(THIS_DIR, os.pardir, 'data_folder/data.csv')

Or in your case, since the data source is in the test directory:

my_data_path = os.path.join(THIS_DIR, 'testdata.csv')

Edit: for modern python

from pathlib import Path

THIS_DIR = Path(__file__).parent

my_data_path = THIS_DIR.parent / 'data_folder/data.csv'

# or if it's in the same directory
my_data_path = THIS_DIR / 'testdata.csv'