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Help:Toolforge/My first Flask OAuth tool

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Overview

Python webservices are used by many existing tools. Python is a high-level, interpreted programming language with many available libraries for making webservices and integrating with MediaWiki.

This stub webservice is designed to get a sample Python application installed onto Toolforge as quickly as possible. The application is written using the Flask framework.

The guide will teach you how to:

Getting started

Prerequisites

Skills

  • Basic knowledge of Python
  • Basic knowledge of SSH
  • Basic knowledge of the Unix command line
  • Familiarity with OAuth terminology would be nice but not required

Accounts

Steps to completion

  • Create a new tool account
  • Create a basic Flask WSGI webservice
  • Add a configuration file
  • Add support for OAuth authentication

Step-by-step guide

Step 1: Create a new tool account

  1. Create a new tool account.
    • For the examples in this tutorial, <TOOL NAME> is used to indicate places where your unique tool name is used in another command.
  2. SSH to login.tools.wmflabs.org.
    • If your local username is different from your shell account name on Toolforge, you will need to include your Toolforge shell account name in your ssh command (i.e. <shell_username>@login.tools.wmflabs.org). See Help:Access for more information on using SSH.
  3. Run become <TOOL NAME> to change to the tool user.

Step 2: Create a basic Flask WSGI webservice

What is Flask?

Flask is a popular web development framework for Python.

How to create a basic Flask WSGI webservice

Create the $HOME/www/python/src directory for your application

$ mkdir -p $HOME/www/python/src

Create a Python virtual environment for the application's external library dependencies

The virtual environment allows the tool to install Python libraries locally without needing a Toolforge administrator's help. The default webservice configuration will automatically load libraries from $HOME/www/python/venv.

The webservice will run on Kubernetes. A Kubernetes shell is required to create the virtual environment.

This will ensure that the version of Python the virtual environment uses matches the version of Python used by the Kubernetes runtime. 

$ webservice --backend=kubernetes python shell
If you don't see a command prompt, try pressing enter.
$ python3 -m venv $HOME/www/python/venv
$ source $HOME/www/python/venv/bin/activate
$ pip install --upgrade pip
Downloading/unpacking pip from [...]
[...]
Successfully installed pip
Cleaning up...

If the python3 -m venv $HOME/www/python/venv command doesn't work, try the following command instead: 

$ virtualenv $HOME/www/python/venv -p /usr/bin/python3

Add Flask to the virtual environment

Note: It is Python best practice to use a file named requirements.txt to keep track of the library dependencies of applications. 

$ cat > $HOME/www/python/src/requirements.txt << EOF
flask
EOF
$ pip install -r $HOME/www/python/src/requirements.txt
Collecting flask (from -r www/python/src/requirements.txt (line 1))
[...]
Successfully installed [...]

The initial virtual environment is now set-up. Exit out of the Kubernetes shell and return to the SSH session on the bastion. 

$ exit

Create a 'hello world' WSGI application

Place the following code in $HOME/www/python/src/app.py: 

# -*- coding: utf-8 -*-
#
# This file is part of the Toolforge flask WSGI tutorial
#
# Copyright (C) 2017 Bryan Davis and contributors
#
# This program is free software: you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the Free
# Software Foundation, either version 3 of the License, or (at your option)
# any later version.
#
# This program is distributed in the hope that it will be useful, but WITHOUT
# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
# more details.
#
# You should have received a copy of the GNU General Public License along
# with this program.  If not, see <http://www.gnu.org/licenses/>.

import flask


app = flask.Flask(__name__)


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

Note: The 'Hello World!' file above starts with a license header that places it under the GPLv3+ license.

Code on Toolforge should always be licensed under an Open Source Initiative (OSI) approved license. See the Right to fork policy for more information on this Toolforge policy.

Start the webservice

$ webservice --backend=kubernetes python start
Starting webservice.

Once the webservice is started, navigate to https://tools.wmflabs.org/<TOOL NAME>/ in your web browser, and see a 'Hello World!' message.

Notes

Toolforge uses a uWSGI configuration designed to make it easy to deploy a typical Python webservice. This configuration uses a 'convention over configuration' design with the following expectations:

  • The application will have a WSGI entry point in $HOME/www/python/src/app.py in a variable named app.
  • Python libraries will be loaded from a virtualenv located in $HOME/www/python/venv.
  • Logs will be written to $HOME/uwsgi.log

Expected file layout 

$HOME
 ├─ uwsgi.log
 └─ www
     └─ python
         ├─ src
         │   └─ app.py
         └─ venv

Troubleshooting

If there is an error when the webservice is started, look in $HOME/uwsgi.log and $HOME/error.log for an explanation.

One Unix utility to use for this is tail, which will display lines from the end of a file: 

$ tail -n 50 $HOME/uwsgi.log
$ tail -n 50 $HOME/error.log

Step 3: Add a configuration file

The application will eventually need some configuration data like OAuth secrets or passwords. These should not be hard coded into the Python files, because the secrets and passwords will be visible once the source code is made public.

There are many different ways to separate code from configuration, but the most straight forward when using Flask is to keep the configuration in a file that can be parsed easily, and then add it to the app.config object that Flask provides.

How to add a configuration file

Add PyYAML to the virtual environment

In this tutorial, a YAML file is used to hold secrets. YAML is a nice choice, because it has a simple syntax, is fairly easy for humans to read, and supports both comments and complex values like lists and dictionaries.

Python does not have built in support for parsing YAML files. Install a library to help out. 

$ webservice --backend=kubernetes python shell
If you don't see a command prompt, try pressing enter.
$ source $HOME/www/python/venv/bin/activate
$ cat >> $HOME/www/python/src/requirements.txt << EOF
pyyaml
EOF
$ pip install -r $HOME/www/python/src/requirements.txt
Requirement already satisfied: flask [...]
Collecting pyyaml (from -r req.txt (line 2))
[...]
Successfully installed pyyaml
$ exit

Read configuration from a file

Update the $HOME/www/python/src/app.py file to read configuration from a config.yaml file in the same directory and get the greeting from the configuration file: 

# -*- coding: utf-8 -*-
#
# This file is part of the Toolforge Flask + OAuth WSGI tutorial
#
# Copyright (C) 2017 Bryan Davis and contributors
#
# This program is free software: you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the Free
# Software Foundation, either version 3 of the License, or (at your option)
# any later version.
#
# This program is distributed in the hope that it will be useful, but WITHOUT
# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
# more details.
#
# You should have received a copy of the GNU General Public License along
# with this program.  If not, see <http://www.gnu.org/licenses/>.

import flask
import os
import yaml


app = flask.Flask(__name__)


# Load configuration from YAML file
__dir__ = os.path.dirname(__file__)
app.config.update(
    yaml.safe_load(open(os.path.join(__dir__, 'config.yaml'))))


@app.route('/')
def index():
    return app.config['GREETING']

A configuration file is now required or the application will produce an error. Eventually, secrets will be included in this file. The file's permissions should be changed so that only the tool user can read it. 

$ touch $HOME/www/python/src/config.yaml
$ chmod u=rw,go= $HOME/www/python/src/config.yaml
$ cat > $HOME/www/python/src/config.yaml << EOF
GREETING: Goodnight moon!
EOF

Now restart the webservice:

$ webservice restart
Restarting webservice...

Once the webservice has restarted, navigate to https://tools.wmflabs.org/<TOOL NAME>/ in your web browser and see the new 'Goodnight moon!' message.

Troubleshooting

If there is an error, look in $HOME/uwsgi.log and $HOME/error.log for an explanation.

Step 4: Add support for OAuth authentication

OAuth is a safe mechanism for authenticating a Wikimedia user in the application. For the basics, read more about OAuth on mediawiki.org.

How to add mwoauth to the virtual environment

mwoauth library is used to handle most of the complexity of making OAuth requests to MediaWiki. 

$ webservice --backend=kubernetes python shell
If you don't see a command prompt, try pressing enter.
$ source $HOME/www/python/venv/bin/activate
$ cat >> $HOME/www/python/src/requirements.txt << EOF
mwoauth
EOF
$ pip install -r $HOME/www/python/src/requirements.txt
Requirement already satisfied: flask [...]
Requirement already satisfied: pyyaml [...]
Collecting mwoauth (from -r req.txt (line 3))
[...]
Successfully installed [...]
$ exit

Update the application code

Here is our new $HOME/www/python/src/app.py file:

www/python/src/app.py
The following content has been placed in a collapsed box for improved usability. 
# -*- coding: utf-8 -*-
#
# This file is part of the Toolforge Flask + OAuth WSGI tutorial
#
# Copyright (C) 2017 Bryan Davis and contributors
#
# This program is free software: you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the Free
# Software Foundation, either version 3 of the License, or (at your option)
# any later version.
#
# This program is distributed in the hope that it will be useful, but WITHOUT
# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
# more details.
#
# You should have received a copy of the GNU General Public License along
# with this program.  If not, see <http://www.gnu.org/licenses/>.

import flask
import mwoauth
import os
import yaml


app = flask.Flask(__name__)


# Load configuration from YAML file
__dir__ = os.path.dirname(__file__)
app.config.update(
    yaml.safe_load(open(os.path.join(__dir__, 'config.yaml'))))


@app.route('/')
def index():
    greeting = app.config['GREETING']
    username = flask.session.get('username', None)
    return flask.render_template(
        'index.html', username=username, greeting=greeting)


@app.route('/login')
def login():
    """Initiate an OAuth login.
    
    Call the MediaWiki server to get request secrets and then redirect the
    user to the MediaWiki server to sign the request.
    """
    consumer_token = mwoauth.ConsumerToken(
        app.config['CONSUMER_KEY'], app.config['CONSUMER_SECRET'])
    try:
        redirect, request_token = mwoauth.initiate(
            app.config['OAUTH_MWURI'], consumer_token)
    except Exception:
        app.logger.exception('mwoauth.initiate failed')
        return flask.redirect(flask.url_for('index'))
    else:
        flask.session['request_token'] = dict(zip(
            request_token._fields, request_token))
        return flask.redirect(redirect)


@app.route('/oauth-callback')
def oauth_callback():
    """OAuth handshake callback."""
    if 'request_token' not in flask.session:
        flask.flash(u'OAuth callback failed. Are cookies disabled?')
        return flask.redirect(flask.url_for('index'))

    consumer_token = mwoauth.ConsumerToken(
        app.config['CONSUMER_KEY'], app.config['CONSUMER_SECRET'])

    try:
        access_token = mwoauth.complete(
            app.config['OAUTH_MWURI'],
            consumer_token,
            mwoauth.RequestToken(**flask.session['request_token']),
            flask.request.query_string)

        identity = mwoauth.identify(
            app.config['OAUTH_MWURI'], consumer_token, access_token)    
    except Exception:
        app.logger.exception('OAuth authentication failed')
    
    else:
        flask.session['access_token'] = dict(zip(
            access_token._fields, access_token))
        flask.session['username'] = identity['username']

    return flask.redirect(flask.url_for('index'))


@app.route('/logout')
def logout():
    """Log the user out by clearing their session."""
    flask.session.clear()
    return flask.redirect(flask.url_for('index'))
The above content has been placed in a collapsed box for improved usability.

The new app.py uses the Jinja template system that is built into Flask rather than the bare strings that were used in the 'hello world' version. One reason for this is that Jinja will automatically escape strings. This is important in any application that will be serving data gathered from a user or even a database to protect against security vulnerabilities like cross-site scripting.

By default Flask will look for templates in the $HOME/www/python/src/templates directory. 

$ mkdir $HOME/www/python/src/templates
$ edit $HOME/www/python/src/templates/index.html

<!DOCTYPE HTML>
<html>
    <head>
        <title>My first Flask OAuth tool</title>
    </head>
    <body>
        {% if username %}
        <p>Hello {{ username }}!</p>
        <p><a href="{{ url_for('logout') }}">logout</a></p>
        {% else %}
        <p>{{ greeting }}</p>
        <p><a href="{{ url_for('login') }}">login</a></p>
        {% endif %}
    </body>
</html>

Update the configuration to add OAuth secrets

Add new configuration values to $HOME/www/python/src/config.yaml file to go with the new code

  1. Register a new OAuth consumer.
  2. As callback URL, use: https://tools.wmflabs.org/<TOOL NAME>/oauth-callback
  3. As contact e-mail address, use the e-mail address linked to your Wikimedia unified account.
  4. Keep the default grant settings ('Request authorization for specific permissions.' with just 'Basic rights' selected)
    • You will be able to use your own account before the consumer has been approved.
  5. Copy the consumer token and secret token values that are generated. These are needed for the config.yaml file.
$ cat >> $HOME/www/python/src/config.yaml << EOF
SECRET_KEY: $(python -c "import os; print repr(os.urandom(24))")
OAUTH_MWURI: https://meta.wikimedia.org/w/index.php
CONSUMER_KEY: the 'consumer token' value from your OAuth consumer registration
CONSUMER_SECRET: the 'secret token' value from your OAuth consumer registration
EOF

Restart the webservice

 
$ webservice restart
Restarting webservice...

Once the webservice has restarted, navigate to https://tools.wmflabs.org/<TOOL NAME>/ in your web browser to see the new landing page.

Try using the login and logout links to test out the OAuth integration.

Additional troubleshooting

bash: webservice: command not found

  1. Check shell prompt.
  2. If it ends in @interactive $, you are inside a Kubernetes shell (webservice --backend=kubernetes python shell).
    • The webservice command is only available on the Toolforge bastions.
  3. Type exit to leave the Kubernetes shell and return to the bastion.

Error: An error occurred in the OAuth protocol: Invalid signature

  1. Double check the values set for CONSUMER_KEY and CONSUMER_SECRET

Get more debugging output from Flask

  1. Add Debug: True to config.yaml
  2. Check uwsgi.log for more information.

Note: This needs a webservice restart to take effect.

Next Steps

Now that your Flask OAuth tool is set-up here are some next steps to consider:

  • Use the OAuth token data stored in flask.session['access_token'] to make API calls as the authorized user.
  • Explore the mwclient library to make interacting with the MediaWiki Action API easier.
  • Publish your source code in a git repository.
  • Add a co-maintainer.
  • Create a description page for your tool.
  • Use the toolforge library to enforce HTTPS and set a nice user agent.
  • If possible, run the tool on your local system as well (clone the source code repository, install dependencies, run FLASK_APP=app.py FLASK_ENV=development flask run). In development mode (which you should never run on Toolforge!), Flask offers some additional conveniences, such as automatic reloading of source files on save or an in-browser debugger on errors.

See also

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