Lflask

HWflask

In this lecture, we will use Python to process user comments obtained in the previous lecture.

• We will display information about individual users as a dynamic website written in Flask framework.
• We will use simple text processing utilities from ScikitLearn library to extract word use statistics from the comments.

Flask

Flask is a simple web server for Python. Using Flask you can write a simple dynamic website in Python.

Running Flask

You can find a sample Flask application at /tasks/flask/simple_flask. Beware, the database included in this folder is just an empty one. You need to either copy in your db from previous exercise or use one from directory above.

You can run the example Flask app using these commands:

cd <your directory>
export FLASK_APP=main.py
# this is optional, but recommended for debugging
# If you are running flask on your own machine you might want to use add `--debug` flag in the `flask run` command
# instead of the FLASK_ENV environment variable.
export FLASK_ENV=development 

# before running the following, change the port number
# so that no two users use the same number
flask run --port=PORT

PORT is a random number greater than 1024. This number should be different from other people running flask on the same machine (if you run into the problem where flask writes out lot of error messages complaining about permissions, select a different port number). Flask starts a webserver on port PORT and serves the pages created in your Flask application. Keep it running while you need to access these pages.

To view these pages, open a web browser on the same computer where the Flask is running, e.g. chromium-browser http://localhost:PORT/ (use the port number you have selected to run Flask).

However, if you are running flask on a server, you probably want to run the web browser on your local machine. In such case, you need to use ssh tunnel to channel the traffic through ssh connection:

• On your local machine, open another console window and create an ssh tunnel as follows: ssh -L PORT:localhost:PORT username@vyuka.compbio.fmph.uniba.sk (replace PORT with the port number you have selected to run Flask)
• For Windows machines, -L option works out of box in Ubuntu subsystem for Windows or Powershell ssh, see Connecting to server.
• If you use Putty on Windows, follow a tutorial how to create an ssh tunnel. Destination should be localhost:PORT, source port should be PORT. Do not forget to click add.
• Keep this ssh connection open while you need to access your Flask web pages; it makes port PORT available on your local machine
• In your browser, you can now access your Flask webpages, using e.g. chromium-browser http://localhost:PORT/

Structure of a Flask application

• The provided Flask application resides in the main.py script.
• Some functions in this script are annotated with decorators starting with @app.
• Decorator @app.before_request marks a function which will be executed before processing a particular request from a web browser. In this case we open a database connection and store it in a special variable g which can be used to store variables for a particular request. (Sidenote: Opening the connection before every request is quite bad practice. Also using sqlite3 for web applications is not ideal because it does not have advanced access control. If you want to build a serious web app you should use PostgreSQL and something like SQLAlchemy for handling connections. We are simplifying stuff here for educational purposes). If you open db connection using any other way than through g.db, e.g. as a normal global variable, you may get various unpleasant errors.
• Decorator @app.route('/') marks a function which will serve the main page of the application with URL http://localhost:4247/. Similarly decorator @app.route('/wat/<random_id>/') marks a function which will serve URLs of the form http://localhost:4247/wat/100 where the particular string which the user uses in the URL (here 100) will be stored in random_id variable accessible within the function.
• Functions serving a request return a string containing the requested webpage (typically a HTML document). For example, function wat returns a simple string without any HTML markup.
• To more easily construct a full HTML document, you can use jinja2 templating language, as is done in the home function. The template itself is in file templates/main.html. You may want to construct different templates for different webpages (e.g. main menu, user page).
• To fill in variables in the template we use {{ ... }} notation. There are also {% for x in something %} statemens and {% if ... %} statements.
• To get the url of some other page you can use url_for (see the provided template).

Processing text

The main tool we will use for processing text is CountVectorizer class from the Scikit-learn library. It transforms a text into a bag of words representation. In this representation we get the list of words and the count for each word. Example:

from sklearn.feature_extraction.text import CountVectorizer

vec = CountVectorizer(strip_accents='unicode')

texts = [
 "Ema ma mamu.",
 "Zirafa sa vo vani kupe a hneva sa."
]

t = vec.fit_transform(texts).toarray()

print(t)
# prints:
# [[1 0 0 1 1 0 0 0 0]
#  [0 1 1 0 0 2 1 1 1]]

print(vec.vocabulary_)
# prints:
# {'vani': 6, 'ema': 0, 'kupe': 2, 'mamu': 4, 
# 'hneva': 1, 'sa': 5, 'ma': 3, 'vo': 7, 'zirafa': 8}

NumPy arrays

Array t in the example above is a NumPy array provided by the NumPy library. This library has also lots of nice tricks. First let us create two matrices:

>>> import numpy as np
>>> a = np.array([[1,2,3],[4,5,6]])
>>> b = np.array([[7,8],[9,10],[11,12]])
>>> a
array([[1, 2, 3],
       [4, 5, 6]])
>>> b
array([[ 7,  8],
       [ 9, 10],
       [11, 12]])

We can sum these matrices or multiply them by some number:

>>> 3 * a
array([[ 3,  6,  9],
       [12, 15, 18]])
>>> a + 3 * a
array([[ 4,  8, 12],
       [16, 20, 24]])

We can calculate sum of elements in each matrix, or sum by some axis:

>>> np.sum(a)
21
>>> np.sum(a, axis=1)
array([ 6, 15])
>>> np.sum(a, axis=0)
array([5, 7, 9])

There are many other useful functions, check the documentation.

Other useful frameworks (for general interest, not for this lecture)

• FastAPI is sort of similar to Flask but more focused on making API (not webpages).
• Django is big web framework with all belts and whistles (e.g. database support, i18n, ...).
• Dash is another fully featured (read bloated) web framework for creating analytics pages (has extensive support, for graphs, tables, ...).