MCS 275 Spring 2024
Emily Dumas
Reminders and announcements:
python3 -m pip install FlaskFirst, let's check in on page mockups for the task tracking app.
Reminder: You can always get these from the sample code repository. You can also browse history of previous versions there.
div { ... })p.intro { ... })So far, I've been opening files in the web browser, using URLs with the file protocol.
There's no network communication here. The browser just opens the file using the OS interface.
To make an actual web site or application, we need an HTTP server.
python3 -m http.serverOpens 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 127.0.0.1will make sure it only listens for connections from the same machine.
Most HTTP servers that deliver resources from a filesystem will look for a file called
index.html and send it in response to a request that ends in a /.
(i.e. if no filename is given, index.html is used.)
Flask is a Python web framework. It makes it easy to write Python programs that respond to HTTP requests (e.g. web applications, APIs).
Competitors include:
from flask import Flask
app = Flask(__name__)
@app.route("/positivity/") # URL (public name for this action)
def message(): # Logical name for this action
return """<!doctype html>
<html>
<body>
You can do it!
</body>
</html>
"""
app.run()
A flask app may not generate all the content it serves; some things (like CSS files or images) may be prepared in advance and used unchanged.
If you create a subdirectory static of the directory where your Flask application runs,
you can put such "static" files there.
Refer to files there using URLs like static/image.png.
You can make an HTML file in which some parts are to be replaced by values of variables.
Surround the parts to be replaced by {{ and }}.
Put these in subdirectory templates and then call flask.render_template to
generate the HTML as a return value.
Pass the variables to be given to the template as kwargs.
The full template language is very rich; we'll see more of it later.
@app.route("/foo")
def bar():
# Find "templates/david.html"
# In it, replace {{ x }} with 12 and {{ y }} with 15
# then serve the result as the HTML for this route
return render_template("david.html",x=12,y=15)