Importing a Flask-security instance into my views module breaks my webapp

Question

I'm writing the sign up/sign in system for a ecommerce site, and using flask-security (http://pythonhosted.org/Flask-Security/) to handle the signup feature. Part of the basic setup requires the following signup.py module: ``` from flask.ext.security import SQLAlchemyUserDatastore, Security from app.models import User, Role from app import app, db # Setup Flask Security user_datastore = SQLAlchemyUserDatastore(db, User, Role) security = Security(app, user_datastore) ``` I then have to import the user_datastore and security objects into my views.py module as follows: ``` from app.signup import user_datastore, security ``` The thing is, as soon as I include the above import statement into my views module, my whole app crashes, and I get the following traceback error when I try to run my unit or behavior tests (edited for readability) ``` ====================================================================== ERROR: Failure: AttributeError ('_FakeSignal' object has no attribute 'connect_v…

Accepted Answer

Short Answer: You are missing blinker library. EDIT: You confirmed that your virtual environment could not find blinker and you re-installed it.

Long Answer:

I think the error is coming from Flask Signals. Look at this code from signals:

```
signals_available = False
try:
    from blinker import Namespace
    signals_available = True
except ImportError:
    class Namespace(object):
        def signal(self, name, doc=None):
            return _FakeSignal(name, doc)
```

So I think that the code tries to find the `blinker` library and in your case, it is not able to import it and hence it tries to use the `_FakeSignal` class.

The `_FakeSignal` class does not have a `connect_via` attribute defined as you can see below

```
class _FakeSignal(object):
    """If blinker is unavailable, create a fake class with the same
    interface that allows sending of signals but will fail with an
    error on anything else.  Instead of doing anything on send, it
    will just ignore the arguments and do nothing instead.
    """

def __init__(self, name, doc=None):
        self.name = name
        self.__doc__ = doc
    def _fail(self, *args, **kwargs):
        raise RuntimeError('signalling support is unavailable '
                           'because the blinker library is '
                           'not installed.')
    send = lambda *a, **kw: None
    connect = disconnect = has_receivers_for = receivers_for = \
        temporarily_connected_to = connected_to = _fail
    del _fail
```

The `connect_via` attribute that Flask-Security is trying to load is actually provided by the `blinker` library and since no blinker, no connect_via. Hence it fails.

So you should install blinker first. However, I think that Flask-Security code should also check for `blinker` before trying to use `connect_via`.

Importing a Flask-security instance into my views module breaks my webapp

Solution

Problem