Getting Flask to use Python3 (Apache/mod_wsgi)

Question

I've got a basic "hello world" Flask app running.

I'm on Ubuntu 14.04, using Apache 2.4. I've installed mod_wsgi.

I've created a ~/web/piFlask/venv/ to hold a virtualenv-created Python2 with flask installed.

However, I wish to have my flaskapp import a Python3.x module I have written.

What do I need to do to make this happen?

I tried creating a ~/web/piFlask/venv3/ and modifying ~/web/piFlask/piFlask.wsgi:

import os, sys

PROJECT_DIR = '/home/pi/web/piFlask'

activate_this = os.path.join(PROJECT_DIR, 'venv3/bin', 'activate_this.py')
execfile(activate_this, dict(__file__=activate_this))
sys.path.insert(0, PROJECT_DIR)

from piFlask import app as application

application.debug = True

But I don't think this is sufficient. This .wsgi is in fact a Python file that will get executed by mod_wsgi, which I'm sure will use a Py2.x interpreter to do the execution.

So if I'm understanding correctly, mod_wsgi fires up the system Python in order to execute this .wsgi, which will in turn fire up my ~/web/piFlask/venv/ interpreter to actually process the request.

I think I could persuade mod_wsgi to use either a system Python3 or my own venv3/... by setting WSGIPythonPath /home/pi/web/piFlask/venv3/lib/python3.4/site-packages in /etc/apache2/mods-available/wsgi.conf

But I found an instruction somewhere saying you have to compile mod_wsgi for Py3, and the bottom quickly falls out to this problem.

Answer 1

Took me awhile, although it was really simple in the end. Unfortunately I've lost my bash history so if you find a mistake, please leave a comment or edit in the correction.

Note that I had already got mod_wsgi working, so I had to remove it:

$ a2dismod wsgi
$ sudo apt-get remove libapache2-mod-wsgi

EDIT: Due to apt-get installing an outdated package (which results in a minor error creeping into Apache's error log) I don't at time of writing recommend this:

$ sudo apt-get install libapache2-mod-wsgi-py3
$ a2enmod wsgi

Instead, first set up your py3 virtual environment, (and might as well in install Flask into it while you're at it)

$ pip install virtualenv

$ cd ~/web/flaskapp
$ virtualenv -p python3 ./venv3
$ source ./venv3/bin/activate     #  enter virtual environment
$ pip install Flask

Now follow Installing mod_wsgi for Python3 on Ubuntu to install mod_wsgi into this venv, and ...

$ deactivate                      #  ... & exit

This was from the original apt-get approach, now it is not relevant:

Now I have to make mod_wsgi use this new Py3:

$ sudo nano /etc/apache2/mods-enabled/wsgi.conf

And do:

#WSGIPythonPath directory|directory-1:directory-2:...
WSGIPythonPath /home/pi/web/flaskapp/venv3/lib/python3.4/site-packages

Finally I need to modify my ~/web/flaskapp/flaskapp.wsgi so that it now supports Python3:

$ cat web/piFlask/piFlask.wsgi

# http://flask.pocoo.org/docs/0.10/deploying/mod_wsgi/#creating-a-wsgi-file
# http://www.enigmeta.com/2012/08/16/starting-flask/

# for Py3:
# http://askubuntu.com/questions/488529/pyvenv-3-4-error-returned-non-zero-exit-status-1

import os, sys

PROJECT_DIR = '/home/pi/web/flaskapp'
sys.path.insert(0, PROJECT_DIR)


def execfile(filename):
    globals = dict( __file__ = filename )
    exec( open(filename).read(), globals )

activate_this = os.path.join( PROJECT_DIR, 'venv3/bin', 'activate_this.py' )
execfile( activate_this )


from piFlask import app as application
application.debug = True

Just a reminder of how Apache uses mod_wsgi to execute this file when the user points their browser to http://myfoo.org/flask:

$ cat /etc/apache2/sites-available/000-default.conf
:
WSGIDaemonProcess flaskProcess user=pi group=pi threads=5
WSGIScriptAlias /flask /home/pi/web/flaskapp/flaskapp.wsgi

<Location /flask>
    WSGIProcessGroup flaskProcess
    WSGIApplicationGroup %{GLOBAL}
    Require all granted
</Location>
:

Discussion: It seems a bit messy that I am installing mod_wsgi module into Apache, but it resides in a venv3 which is inside my flaskapp folder. But this is at least ensuring Apache -> mod_wsgi -> (uses the correct i.e. venv3 Python) -> {to launch my FlaskApp}.

The alternative would be to update the system Python, and use that instead of venv.

Answer 2

Correct, mod_wsgi needs to be compiled for a specific Python version as it never actually executes 'python' executable. Instead the Python library is linked into mod_wsgi.

The end result is you cannot mix Python 3 code within an application running using the Python 2 interpreter.

You would have to consider making your code runnable under both Python 2 and 3 and then you could choose which of those you want to use and use a version of mod_wsgi compiled for the version you choose to use.

Answer 3

Taken from the P i's answer. The following now works and fixes the problem:

$ sudo apt-get install libapache2-mod-wsgi-py3
$ a2enmod wsgi

Answer 4

I started running python3.5.2 with flask 0.12 in a venv on a Ubuntu 16.04 (virtual machine).

@Graham pointed out activating the the venv from the WSGI script file is not the preferred way. More info here: http://modwsgi.readthedocs.io/en/develop/user-guides/virtual-environments.html

Just for anyone else looking for info on this I'm putting all your steps together:
Note: my app name is blazing, path ~/jan/blazing. Replace them with your own.

sudo apt install python3-pip

sudo apt-get install python3-venv
python3 -m venv ~/blazing/venv
source /home/jan/blazing/venv/bin/activate

pip3 install --upgrade pip    
pip3 install Flask

sudo apt-get install apache2-dev
(venv) pip3 install mod_wsgi
(venv) sudo venv3/bin/mod_wsgi-express install-module

OUTPUT:
LoadModule wsgi_module "/usr/lib/apache2/modules/mod_wsgi-py35.cpython-35m-x86_64-linux-gnu.so"
WSGIPythonHome "/home/jan/blazing/venv"

sudo vim /etc/apache2/mods-available/wsgi_express.load

ADD: LoadModule wsgi_module "/usr/lib/apache2/modules/mod_wsgi-py35.cpython-35m-x86_64-linux-gnu.so"

sudo vim /etc/apache2/mods-available/wsgi_express.conf

ADD:

WSGIPythonHome "/home/jan/blazing/venv"
WSGIPythonPath /home/jan/blazing/venv/lib/python3.x/site-packages

then

sudo a2enmod wsgi_express

vim /home/jan/blazing/project/flaskapp.wsgi

ADD:

import os, sys

PROJECT_DIR = '/home/jan/blazing' sys.path.insert(0, PROJECT_DIR)

activate_this = os.path.join( PROJECT_DIR, 'venv/bin', 'activate_this.py' )
with open(activate_this) as file_:
    exec(file_.read(), dict(__file__=activate_this)


from piFlask import app as application application.debug = True

and

vim /etc/apache2/sites-available/flask.conf

ADD:

WSGIDaemonProcess flaskProcess user=www-data group=www-data threads=5
WSGIScriptAlias /flask /home/jan/blazing/project/flaskapp.wsgi

ErrorLog ${APACHE_LOG_DIR}/error_flask.log #you can see the errors here..
CustomLog ${APACHE_LOG_DIR}/access_flask.log combined

<Location /flask>
    WSGIProcessGroup flaskProcess
    WSGIApplicationGroup %{GLOBAL}
    Require all granted
</Location>

Or if you have other sites running and would like to have flask as one of the Virtual hosts, you can use this conf file:

<VirtualHost *>
    ServerName flask.ubuntuws
    ErrorLog ${APACHE_LOG_DIR}/error_flask.log
    CustomLog ${APACHE_LOG_DIR}/access_flask.log combined

    WSGIDaemonProcess flaskProcess user=www-data group=www-data threads=5
    WSGIScriptAlias / /home/jan/blazing/project/flaskapp.wsgi

    <Location />
        WSGIProcessGroup flaskProcess
        WSGIApplicationGroup %{GLOBAL}
        Require all granted
    </Location>
</VirtualHost>

and make sure your PROJECT_DIR in flaskapp.wsgi points to your PROJECT_NAME.py.
enable the virtual site and reload apache:

lastly:

sudo a2ensite flask.conf
sudo service apache2 reload