Reloader fails to reload after SyntaxError when the app is in an __init__.py file

[segevfiner]

Expected Behavior

The reloader should work correctly even for an app in a package's __init__.py file.

In hello/__init__.py:

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'

set FLASK_APP=hello
set FLASK_DEBUG=1
flask run

Than edit __init__.py and introduce a syntax error.

Actual Behavior

I get the SyntaxError the first time I try to browse and afterwards get:

Traceback (most recent call last):
  File "flask\cli.py", line 48, in find_best_app
    'using a factory function.' % modules.__name__)
NoAppException: Failed to find application in module "hello". Are you sure it contains a Flask application? Maybe you wrapped it in WSGI middleware or you are using a factory function.

Cause

This is caused by weird CPython behavior which might be a bug:

>>> __import__('syntax_error')
...
SyntaxError: ...
>>> a = __import__('syntax_error')  # woot it works?!
>>> a.__file__
'<path to the directory containing __init__.py>'  # Instead of path to __init__.py

After the SyntaxError in the app, the Wekzeug reloader won't know about the __init__.py file any more since it iterates sys.modules and the entry there contains the path to the directory instead of the path to the __init__.py file, and Flask will call __import__ repeatedly but will get the weird empty module and will fail to find an app in it, leading to the described exception.

Environment

• Python version: 2.7.13
• Flask version: 0.12.2
• Werkzeug version: 0.12.2

Not sure if this happens in Python 3.

[davidism]

Does this happen with current master?

[segevfiner]

Does this happen with current master?

Yep.

[ghost]

I've been able to verify this behaviour with current master and Pythons 2.7 and 3.6. On first request, flask correctly identifies and returns the SyntaxError; on subsequent reloads, a NoAppException is returned instead. Presumably, as OP states, this is due to import module caching.

On Python3.6 no SyntaxError is ever returned; flask immediately replies with a NoAppException, but the error message is different:

Traceback (most recent call last):
# ... File this, File that, etc ...
    'Could not import "{name}"."'.format(name=module_name)
flask.cli.NoAppException: Could not import "hello"."

I'm assuming the desired behaviour is for Flask to forward errors that arise during importation rather than fail with NoAppException?

[segevfiner]

A fix was released for the reloader failing in Werkzeug. The NoAppException on subsequent requests remains though. But the reloader will continue reloading on changes to the init.py files.

[davidism]

I can't reproduce this on Python 2.7 or 3.6, Flask 0.12 or master. I tried starting with valid syntax then editing in an error, and vice versa.