NameError: name '__file__' is not defined

The error

$ python -c "exec(open('tool.py').read())"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    exec(open('tool.py').read())
    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 2, in <module>
NameError: name '__file__' is not defined. Did you mean: '__name__'?

Here tool.py finds its data next to itself, as scripts often do:

from pathlib import Path
HERE = Path(__file__).parent

Why it happens

Python sets __file__ when it runs a file. Code that reaches it another way has no file: exec() of the source, python -c, the interactive prompt, or a Jupyter notebook. Agents hit it when they run part of a script through python -c or a heredoc to test it.

Piping it in with python - < tool.py is worse: __file__ is <stdin>, so there's no error, and HERE quietly becomes the current folder.

The fix

• Run the file: python tool.py. To try a piece of it, write that piece to its own .py file and run that.
• Run it from Python with runpy, which sets __file__ to the path you pass: runpy.run_path("tool.py").
• In a notebook or the prompt, there's no file to be next to. Use Path.cwd(), or pass the folder in.