Thu Nov 11

Prelude : covid, zoom, and all that - today's protocol.

First : let's discuss the homework. Anything you'd like to go over about SQL - concepts or how it works?

Second : a look at how to use SQL from python.

Here are some of the concepts

• transactions - a collection of database changes which succeed or fail all together
• ORM : object-relational mapping - an object (or "class" in python) wrapper around the database tables

The overview in the python docs gives a nice summary.

It is often important to handle transactions correctly - the data exists both in memory and in the database, and if you're not careful the two can get out of sync.

As your project gets bigger, it can make sense to put in an abstraction layer between the database SQL code and your top-level program code - that is typically done with an "ORM" library. There are both advantages and disadvantages, as per this ORM for Python discussion.

Django, another python web back end, has its own built-in ORM.

Two other popular ones are

• sqlalchemy.org
• peewee-orm.com

I'm using peewee in my flask umber course content app - I'll show a bit of that code in class. For example, the peewee ORM object model is in model.py

flask

Our next (and last) technology is flask, which we'll discuss next week, and in the following weeks use to build a web project that incorporates as many of the ideas we've been discussing as we can cram in. ;)

Flask has much better debugging tools than the backend-only stuff we've been doing, at the cost of some complexity in managing both a local development and remote deployed environment.

concepts:

• virtual enviroment : it's typically a good idea to keep all your python dependencies in one place when doing this sort of work - that's usually managed with a "virtual environment".
• app : a collection of web pages will all be managed by one python program that ties everything together - database, logic, etc
• routes : the URLs that your correspond to your pages (including variable names) are set within the app
• templates : html files with an embedded language (which might or might not be quite python) are referenced from the app
• development environment : flask runs well on your local laptop, with great debugging built in
• deploy : the python environment needs to be copied to a remote server enviroment and run within a web server ... getting this right can be tricky, but there are a number of options with their own recipes

This digital ocean tutorial is a good walkthrough of a flask app - including the virtual environment.

For next week, please work through that example. I will start during class if there's time.

I've attached a few of the python files we used in class.

And you can find a link to a video recording of today's class on the videos page.

attachments

		last modified	size
	decorator.py	Thu Nov 11 2021 03:17 pm	320B
	demo.py	Thu Nov 11 2021 03:17 pm	140B