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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.

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".

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