While providing feedback to participants of Online Seminar of Informatics I’ve encountered a code, which used sys.path[0] to get the current working directory.

sys.path is a list of folders used by Python interpreter during import resolving.

For simple cases sys.path[0] is indeed the current working directory, but beware that there are a lot of things, which can change the sys.path variable - automatic reload of a Flask server is one of them.

The following is my minimal reproducer to show the behavior.

from flask import Flask
import sys

print("sys.path[0]:", sys.path[0])
print("sys.argv[0]:", sys.argv[0])

app = Flask(__name__)
sys.path[0]: c:\tmp\flask_sys
sys.argv[0]: c:\tmp\flask_sys\app.py
 * Serving Flask app "app" (lazy loading)
# ... Change the python file and save it, it triggers automatic reload.
 * Restarting with stat
sys.argv[0]: C:\tmp\flask_sys\app.py

More on sys.path for example in this great tutorial.