Web frameworks
The MVC pattern
A very simple application
Application of the MVC pattern
Making a Flask application
Programming the Flask application
Equipping the input page with output results
Splitting the app into model, view, and controller files
Troubleshooting
Handling multiple input variables in Flask
Programming the Flask application
Implementing error checking in the template
Using style sheets
Using LaTeX mathematics
Rearranging the elements in the HTML template
Bootstrap HTML style
Custom validation
Avoiding plot files
Plotting with the Bokeh library
Autogenerating the code
User login and storage of computed results
Uploading of files
Exercises
Exercise 1: Add two numbers
Exercise 2: Upload data file and visualize curves
Exercise 3: Plot a user-specified formula
Exercise 4: Visualize Taylor polynomial approximations
Exercise 5: Extend the gen app
Exercise 6: Make a web app with multiple apps
Exercise 7: Equip the gen app with more data types
Exercise 8: Auto-generate code from function signature
Project 9: Interactive function exploration
Flask resources
Computational scientists may want to offer their applications through a web interface, thereby making a web application. Basically, this means that users can set input data to the application on a web page, then click on some Compute button, and back comes a new web page with the results of the computations. The web interface can either be used as a GUI locally on the scientist's computer, or the interface can be depolyed to a server and made available to the whole world.
Web applications of the mentioned type can be created from scratch using CGI scripts in (e.g.) Python, but the code quickly gets longer and more involved as the complexity of the web interface grows. Nowadays, most web applications are created with the aid of web frameworks, which are software packages that simplify the programming tasks of offering services through the Internet. The downside of web frameworks is that there is a significant amount of steps and details to learn before your first simple demo application works. The upside is that advanced applications are within reach, without an overwhelming amount of programming, as soon as you have understood the basic demos.
We shall in the forthcoming text explore the very easy-to-use Flask framework. At the time of this writing, the primary Flask documentation is the official web site and the WTForms Documentation. That is why we have made specific examples for the application of Flask in computational science.
The problem for a computational scientist who wants to enable mathematical calculations through the web is that most of the introductory examples on utilizing a particular web framework address web applications of very different nature, e.g., blogs and polls. Therefore, we have made an alternative introduction which explains, in the simplest possible way, how web frameworks can be used to
All the files associated with this document are available in a GitHub repository. The relevant files for the web applications are located in a subtree doc/src/web4sa/src-web4sa/apps of this repository.
Our introductory examples were also implemented in the web2py framework, but according to our experience, Flask and Django are easier to explain to scientists. A framework quite similar to Flask is Bottle. An even simpler framework is CherryPy, which has an interesting extension Spyre for easy visualization of data. Once you know the basics of Flask, CherryPy is easy to pick up by reading its tutorial. (There are some comments on the Internet about increased stability of Flask apps if they are run on a CherryPy server.)