Using Flask for Scientific Web Applications

Application of the MVC pattern

Before thinking of a web application, we first refactor our program such that it fits with the classical MVC pattern and a compute component. The refactoring does not change the functionality of the code, it just distributes the original statements in functions and modules. Here we create four modules: model, view, compute, and controller.

The compute module contains a function compute(r) that performs the mathematics and returns the value s, which equals sin(r).
The model module holds the input data, here r.
The view module has two functions, one for reading input data, get_input, and one for presenting the output, present_output. The latter takes the input, calls compute functionalty, and generates the output.
The controller module calls the view to initialize the model's data from the command line. Thereafter, the view is called to present the output.

The model.py file contains the r variable, which must be declared with a default value in order to create the data object:

r = 0.0    # input
s = None   # output

The view.py file is restricted to the communication with the user and reads

import sys
import compute

# Input: float r
# Output: "Hello, World! sin(r)=..."

def get_input():
    """Get input data from the command line."""
    r = float(sys.argv[1])
    return r

def present_output(r):
    """Write results to terminal window."""
    s = compute.compute(r)
    print 'Hello, World! sin(%g)=%g' % (r, s)

The mathematics is encapsulated in compute.py:

import math

def compute(r):
    return math.sin(r)

Finally, controller.py glues the model and the view:

import model, view

model.r = view.get_input()
view.present_output(model.r)

Let us try our refactored code:

Terminal> python controller.py 1.2
Hello, World! sin(1.2)=0.932039

The next step is to create a web interface to our scientific hello world program such that we can fill in the number r in a text field, click a Compute button and get back a new web page with the output text shown above: "Hello, World! sin(r)=s".

Flask4scientists

Application of the MVC pattern