ModuleNotFoundError when running imported Flask app - python-3.x

Question

I have a python module with the following layout:
foo
| __init__.py
| __main__.py
| bar.py
__init__.py is empty.
Content of foo/bar.py:
from flask import Flask
app = Flask(__name__)
def baz(): pass
When running python3 -m foo i get confusing results.
Contents of foo/__main__.py
# Results in a ModuleNotFoundError: No module named 'foo'
from foo.bar import app
app.run()
# Raises no error and correctly prints the type
from foo.bar import app
print(type(app))
# Also runs without an error
from foo.bar import baz
baz()
Why is it possible to import and execute a function from this module, but when trying to do the same with a flask app it results in a ModuleNotFoundError?
I just can't see any way this makes any sense.
Edit:
The error is persistent even with this code:
from foo.bar import app
print(type(app))
app.run()
Output:
<class 'flask.app.Flask'>
* Serving Flask app "foo.bar" (lazy loading)
* Environment: production
WARNING: Do not use the development server in a production environment.
Use a production WSGI server instead.
* Debug mode: on
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
* Restarting with stat
Traceback (most recent call last):
File "/home/user/projects/ftest/foo/__main__.py", line 1, in <module>
from foo.bar import app
ModuleNotFoundError: No module named 'foo'
So, obviously the module can be imported, because type(app) works just fine and flask does start. It seems like flask does a reload and is messing around with imports somehow.
Edit 2:
I turned debug mode off and it works just fine.
This error only occurs if you set export FLASK_DEBUG=True or explicitly enable debug via app.config["DEBUG"] = True

Answer 1

It turns out it's a bug in werkzeug.
The code works as expected if werkzeug's reloader is disabled.
How to reproduce the behaviour
Directory structure:
foo
| __init__.py
| __main__.py
Content of __init__.py:
from flask import Flask
app = Flask(__name__)
app.config["DEBUG"] = True
Content of __main__.py:
from foo import app
app.run()
If we run it:
$python3 -m foo
* Serving Flask app "foo" (lazy loading)
* Environment: development
* Debug mode: on
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
* Restarting with stat
Traceback (most recent call last):
File "/home/user/projects/ftest/foo/__main__.py", line 1, in <module>
from foo import app
ModuleNotFoundError: No module named 'foo'
If we change __main__.py:
from foo import app
app.run(use_reloader=False)
Everything works just fine.
What's going on
The problem is in werkzeug._reloader.ReloaderLoop.restart_with_reloader. It calls a subprocess with the arguments provided by werkzeug._reloader._get_args_for_reloading but this function does not behave as expected when executing a package via the -m switch.
def _get_args_for_reloading():
"""Returns the executable. This contains a workaround for windows
if the executable is incorrectly reported to not have the .exe
extension which can cause bugs on reloading.
"""
rv = [sys.executable]
py_script = sys.argv[0]
if os.name == 'nt' and not os.path.exists(py_script) and \
os.path.exists(py_script + '.exe'):
py_script += '.exe'
if os.path.splitext(rv[0])[1] == '.exe' and os.path.splitext(py_script)[1] == '.exe':
rv.pop(0)
rv.append(py_script)
rv.extend(sys.argv[1:])
return rv
In our case it returns ['/usr/local/bin/python3.7', '/home/user/projects/ftest/foo/__main__.py']. This is because sys.argv[0] is set to the full path of the module file but it should return ['/usr/local/bin/python3.7', '-m', 'foo']` (At least from my understanding it should and it works this way).
I have no good idea on how to fix this behaviour, or if it is something that need to be fixed. It just seems weird to me that I'm the only one that has encountered this problem, since it doesn't seem too much of a corner case to me.

Answer 2

Adding the following line before app.run() works around the werkzeug reloader bug:
os.environ['PYTHONPATH'] = os.getcwd()
Thanks to #bootc for the tip! https://github.com/pallets/flask/issues/1246

Answer 3

Have you tried from foo import app in your main.py file?