Relative import doesn't work in debug mode

[benjamin-hodgson]

I get an exception when I attempt to use a relative import with Flask in debug mode.

Steps to reproduce

My package looks like this:

mypkg\
  __init__.py
  mymodule.py

Here's __init__.py:

xyz = 123

And in mymodule.py:

from flask import Flask
from . import xyz

app = Flask(__name__)
app.run(debug=True)

Run the application using python -m mypkg.mymodule. I'm using Python 3.4.3 on a Windows machine, with Flask 0.10.1 (the version from PyPI).

Actual results

The usual "you attempted to use a relative import but you're not in a package" exception:

Traceback (most recent call last):
  File "...\mymodule.py", line 2, in <module>
    from . import xyz
SystemError: Parent module '' not loaded, cannot perform relative import

If you remove debug=True, or use an absolute import (from mypkg import xyz), there is no error. The error persists if you name your module __main__.py (instead of mymodule.py) and use python -m mypkg. The error persists if you attempt to import a name from a submodule of the package (it's nothing to do with the import being from __init__.py).

I'm guessing it's something to do with Flask's magical reloading of edited files in debug mode.

Expected results

No error; xyz is imported as usual.

[ThiefMaster]

Are you sure that this is a Flask issue? Check if http://stackoverflow.com/questions/16981921/relative-imports-in-python-3 helps you.

[untitaker]

No, this is an unfixable bug in Werkzeug. See pallets/werkzeug#461

[benjamin-hodgson]

😞

[davidism]

@benjamin-hodgson while the bug is "unfixable", there is a solution: don't try to run a Flask app with python -m.