Difference between python 2.7 and 3.3+ when importing in __init__.py and module from same directory

Question

Recently I had an issue where a signal I was using from flask-security was not behaving as expected in python 3.3. In looking at the source code for flask-security I noticed that the signal I was importing from a module in the flask-security package was also imported in __init__.py. By importing the signal from the top level of the package I was able to solve my problem (since the signal gets imported when package is initialized).

If I run the following code:

from flask.ext.security import user_registered
from flask.ext.security.signals import user_registered as user_reg_sig
user_registered==user_reg_sig

I'll get True in python 2.7 and I'll get False for python 3.3+.

What is different in python 3.3+ that causes this difference in import behaviour?

EDIT: I'm still stumped by the python 2.7 vs 3.3+ issue, but have managed to narrow down that the issue is occurring when the __init__.py of flask.ext is called and uses the ExtensionImporter class from exthook.py to import flask-security.

Running the following under python 3.4 returns True when flask-security is directly imported avoiding the extension hook:

from flask_security.signals import user_registered as user_reg_sig
from flask_security import user_registered
user_registered==user_reg_sig

Here is repr() for signals for flask.ext.security and flask_security examples:

from flask_security.signals import user_registered as user_reg_sig
from flask_security import user_registered

repr(user_registered)
>>> "<blinker.base.NamedSignal object at 0x7fb38e258400; 'user-registered'>"

repr(user_reg_sig)
>>> "<blinker.base.NamedSignal object at 0x7fb38e258400; 'user-registered'>"

from flask.ext.security import user_registered
from flask.ext.security.signals import user_registered as user_reg_sig

repr(user_registered)
>>> "<blinker.base.NamedSignal object at 0x7fb38e258400; 'user-registered'>"

repr(user_reg_sig)
>>> "<blinker.base.NamedSignal object at 0x7fb38dd030b8; 'user-registered'>"

Leesaleese

And if you print out the repr of each? – Estranged

Added the repr within the question. So definitely a different object when accessed through flask.ext. – Leesaleese

I guess it has something to do with [#4799089 and the changes in Python3.3 in that area. Also read this [python-notes.curiousefficiency.org/en/latest/python_concepts/…. – Ingeminate

Answer 1

There are multiple cases where Python can decide to create a new instance of an object when you re-import the module.

In fact that's not even Python 3 specific and it can happen in various scenarios.

Mainly you have to assume that the two objects may be different and work around this.

Uriel

Thanks. Seems like there is a bit of an issue with the use of signals by Flask and what happens when imports happen through flask.ext.* – Leesaleese