Lecture 34

Planning our web app

MCS 275 Spring 2022
David Dumas

Lecture 34: Planning our web app

Course bulletins:

  • Please install Flask, e.g. with
    python3 -m pip install Flask
    in preparation for using it in upcoming assignments.
  • Project 4 will be posted today.


Create the rough design for a web application we'll write in Python in the next few lectures.

Continue study of HTML and CSS by making some mockups of the user interfaces.

Toy models

I will prioritize getting a working product that uses things we've discussed in the course.

That will mean skipping some natural features (e.g. authentication) and avoiding some technologies because they'd take too long to introduce (e.g. JS), or because they'd hide the use of things we're learning (e.g. SQLAlchemy).

Work order system

A work order (WO) is a request that a certain job be done (e.g. "swap mainboard of laptop to fix bad HDMI port" or "replace damaged lock on tiger cage").

After a WO is created, it needs to be assigned to a person who will do the job.

Later, the person who the WO was assigned to may mark it complete.

A system to track WOs would have several user types:

  • Submitters
    • Create new WOs
  • Workers
    • Take on WO as assignment, do the work
    • Mark an assigned WO complete
  • Administrators
    • Modify/delete WOs in other ways


As we won't build any user account system, we'll just create separate pages in our web application that serve the needs of these different user types.

App name?

Let's try something like businessnamegenerator.com

UI Mockups

Let's make some HTML+CSS documents that look like the applications we're planning to build.

Later, we'll use these as the starting point for the HTML the app will generate.


A language that styles HTML. Basic syntax is

        SELECTOR {
            PROPERTY1: VALUE1;
            PROPERTY2: VALUE2;

Elements that match the SELECTOR will have their styling changed by adjusting values of the given properties (things like color, font size, border).

CSS has a complex system of units for values like distances and sizes.

        body {
            font-family: "Verdana", sans-serif;
            background: #C0C0C0;
        p {
            border: 1px solid #000000;

The body element of the document (hence everything) will use the font called "Verdana", or if that isn't available, some sans serif font.

The body will be displayed with a light gray background.

Each paragraph will be surrounded by a 1-pixel black border.

Hex colors

The color code



  • Amount of red = 0x78 = 120 out of 255
  • Amount of green = 0x9A = 154 out of 255
  • Amount of blue = 0xBC = 188 out of 255

Google has a color picker that can be helpful.

CSS properties

There are lots, e.g. color, background, font-family, font-size, width, height, border, margin, margin-top, margin-left, padding, padding-left, ...

CSS properties reference at w3schools

Applying a stylesheet

You can embed a stylesheet (block of CSS) directly in HTML by placing it inside <style> within <head>.

Or you can use the HTML <link> tag to specify the CSS can be found at another URL (e.g. is in another file).

        <link rel="stylesheet" href="mysite.css">

Like <style>, <link> should go in the header.

The latter approach makes stylesheets reusable.


An HTML element in a document can be given a unique identifier with the id attribute.

        <p id="selector-intro">Let's talk about CSS selectors...</p>

If two elements have same id, your HTML is invalid.

You can link to an element within a document by id e.g.

            We discuss <a href="#selector-intro">selectors</a> below.


You can create classes (categories) for elements in a HTML document. Mainly used so items in a category (e.g. important, outdated, footnote) can be styled differently (e.g. red, faded, small text).

Specify the category of an element using the class attribute.

        <p class="urgent-warning">Space wasps approaching.</p>

No need to declare classes in advance, nor to refer to every class in your CSS.

class and id selectors

Apply class- or id-specific styles in CSS:

        /* Text in paragraphs is orange */
        p { color: #FFA500; }

        /* Paragraphs of class "urgent-warning" have big, red
           text. This overrides the previous line, as the more
           specific selector gets precedence.  */
        p.urgent-warning { color: #FF0000; font-size: 120%; }

        /* The paragraph with id "selector-intro" is bold */
        p#selector-intro { font-weight: bold; }

child and descendant selectors

        div ul {
            /* style for any ul that has
               a div ancestor */
        div > ul {
            /* style for any ul that has
               a div as its parent */
        ul ul {
            /* style for ul inside another ul */

Many other selectors

first-child, only-child, selection by adjacency, values of attributes, ...

w3schools CSS selectors reference

Python's built-in HTTP server

python3 -m http.server

Opens a web server that serves files in the current directory and its subdirectories.

Visit http://localhost:8000/ in a browser (or substitute other port number shown in startup message) to see index.html.

Firewall rules typically prevent incoming connections from the internet (and maybe the local network too). That's good! Or

python3 -m http.server --bind

will make sure it only listens for connections from the same machine.


Revision history

  • 2022-04-06 Initial publication