Category Archives: SQLAlchemy

CL-App Postmortem

Today, sadly, I will be pulling down the CL-App site.  The site has been somewhat non-operational for a while now, as the IP address has been blocked by Craigslist.  I never meant for the site to be anything more than a demo project, so it was surprising that Craigslist was able to detect the activity.   Anyways, for posterity’s sake, I am going to do a quick overview of the site with some screenshots.

Disclaimer: Before Launching into the overview, i think it is worth discussing my thoughts on web scraping.  While I think scraping is a very handy tool to have, I also think it needs to be used responsibly.  If there is an API available, that should always be used instead.  I built the app for my own entertainment and education, it was a great way to learn how to stitch together a number of python libraries and frameworks into a fully functional site.  I had a long list of features and improvements that I though would be cool to implement, but in the end, because I knew Craigslist is hostile to scrapers, I felt it would be best too leave it as is and move on to other projects.

A Brief Overview

The purpose of the site was to alert users when new posts were created under a certain search criteria.  The alerts could be sent via e-mail or text message.  In order to prevent too many requests to Craigslist, the app would check for new updates once per hour, but the user could specify longer intervals between alerts if desired.  Below are some slides showing screenshots of the site.

  • Login Page

As you can see, the site was pretty bare bones.  Considering the main purpose of the app was to send alerts I didn’t feel the need to invest much time in the frontend.  I did add a feature that returned a xkcd style matplotlib plot that compared the number of new posts in the last 24 hours to the average number of new posts by hour.  Had I put more time into this project, adding some more analytics would have been fun.

If I had to rate the usefulness of this application on a scale of 1 (useless) to 10 (useful), I would give it about a 4 or 5.  I tested it out with a handful of different searches, and never found the alerts to be that useful.  Because I limited the alert interval to a minimum of an hour, this service wasn’t very useful for items that moved very quickly (i.e. free items).  In those cases, you would probably want a minimum of 15 minutes between scrapes.  I do think it would work well for someone looking for a very specific or rare item, but I never really bothered to test that hypothesis.  One feature that I found useful was that the alert status page would display the last 10 posts that fell under your search criteria.  I found that switching between my different alert status pages was a very convenient way to quickly check if there was anything interesting under any of the searches.  In essence, it was a page of bookmarks for various searches, and I found that to be pretty useful.

It should be noted that Craigslist allows you to save searches and to send e-mail alerts.  I just noticed this the other day.  You need to have an account with Craigslist and from what I can tell, you cannot control the e-mail alert too much.  I just started an alert on their site, so I can report back on that.

Anyways, that is all for now.  It was a fun project and a good learning experience, but I am glad to be putting this one away.  While I still have a few projects that involve web scraping, and I will continue to dabble with it from time to time, Craigslist is notoriously hostile to scrapers, so getting a cease and desist from them is one less thing I have to worry about.

CL Alerts

So my first completed web project is an app that notifies users when a new post appears on a craigslist search. The user can have as many searches as they want, and can receive notifications via email or text message. Here is the link to the app, CLAlerts. The app is pretty bare bones, with a very basic html/css front-end (taken from the flaskr tutorial) and a python/flask/sqlalchemy back-end. The github repo is here. Overtime, I may add a few more features or re-design the app, but at the moment I am just happy to have something that is functional that I can show off.

I was fairly familiar with running a simple flask app thanks to tutorials floating around the web, but this app involved regularly scraping craigslist to check for new posts, which as far as I know, is outside of the capabilities of flask. I have used the linux utility cron to run scraping scripts in the past, and a similar solution seemed appropriate for this app. I wanted to keep things as pyhtonic as possible, so I dug around and turns out there is a scheduler python library called APScheduler. I used the library to run the scraping and message sending tasks for the app. You can check out the final product on github, but below is a sample code snippet.

from apscheduler.scheduler import Scheduler
import time

sched = Scheduler()

@sched.interval_schedule(hours=1)
def run_tasks():
    task1()
    task2()

sched.start()

while True:
    time.sleep(1)
    pass

It’s pretty straight forward. You use the ‘@sched’ decorator to tell APScheduler what functions you want to run. You can define the interval a couple of ways, in this case I am using the interval scheduler, but you can also use a date based or cron style scheduler. The while loop at the end keeps the process running. I read somewhere that using the ‘time.sleep(1)’ line in the while loop stops the process from hogging up CPU resources.

So far this seems to be working. I have launched the app using a micro aws instance. I’ve setup two tmux sessions and have the flask app running in one session and the scheduler running in another. The scheduler runs once an hour and scrapes craigslist and sends out the alerts using the Twilio API for text messages and the python SMTP library for email. So far it seems to be running fairly reliably. At one point, the scheduler appeared to freeze and I had to kill the process and restart the script. I still haven’t figured out what caused it, my only guess is that the script froze up on a database connection error. I am currently using sqlite which can’t handle simultaneous writing, but at the moment, I don’t have a way to test this theory. If I find the time, I might try and set up a test for this. I’ll make sure to write a post about it if I do.

Flask-SqlAlchemy Login

As I mentioned in my previous post, I was struggling to get the Flask login manager and the sqlalchemy ORM to work together, so once I got it working, I felt it would be a good idea to create a separate github repository with template code so that I don’t have to repeat the process again. It turns out that flask has it’s own sqlalchemy extension that abstracts away a lot of the sqlalchemy setup and integration. Unfortunately, I was not made aware of this until after I finished the template. Regardless, I got the login manager working without the flask-sqlalchemy extension, and it turns out I prefer the syntax of this method anyway, so I am sticking with it.

The github repo for the login template is here. The readme contains the installation and setup instructions for linux users. Once the flask server is up and running you should be able to view the app through a local web browser.

One of the big benefits of using the flask login manager (as opposed to dealing with authentication yourself) is that it is now very simple to require authentication to your various views in your app. Here is some example code on setting up a view that requires authentication:

@app.route("/example_page")
@login_required
def example_page():
    message1 = "Hello, " + current_user.name
    message2 = "Here is your example page"
    return render_template("example_page.html", message1=message1, message2=message2)

Simply adding the ‘@login_rquired’ decorator is all that is required to ensure that the page can only be viewed by authenticated users. Additionally, you can access the logged in user through the global current_user object.

Some additional benefits of the flask login manager is it will handle cookie management and “remember me” functionality. It should be noted that this template is pretty bare bones. Outside of getting the flask login manager setup with a sqlalchemy database and some simple html pages, there really isn’t much else there. So there you go, let me know if you have any questions.

Web Development

So I finally caved. After almost a year of resisting, I inevitably jumped into the world of web-dev (well, more accurately, I have timidly stepped into it). I think what finally pushed me over the edge was that websites and apps are both useful and convenient methods of showcasing one’s work to the world (even if the main focus of the project is not web related), and the ability to create a demonstrable final product provides the necessary motivation to actually see some of these projects through to completion.

For the time being, I am taking a fairly minimalist approach to web-dev, focusing primarily on Python based libraries (Flask and SQLAlchemy), and only doing limited amounts of non-python front-end work (html, css). Because web development isn’t my preferred domain, I feel that taking this minimalist approach will allow me to continue to increase my python experience, while keeping the ‘web-only’ stuff to a minimum. And who knows, if I ever get a change of heart, I can always pick up JavaScript and css later (so far it’s been fun, who knows).

I decided that my first project was going to be a web app that monitors craigslist and alerts users about new posts that fall under particular search criteria. In the past year, I have written a number of web scrappers, and have also played around with the Twilio API to send text messages, so I figured this would be a good project to demonstrate some of that experience. Additionally, I had gone through some Flask tutorials (a python based web framework) as well as done some work with SQLAlchemy (a database ORM), so hooking everything up seemed like it would be straight forward enough. But of course, there are always hiccups….

One of the aspects of data analysis projects that I find appealing is that it is very easy (for me at least) to conceptualize the process that takes place when writing an analysis program. Even when using large, powerful libraries and tools to work with the data, I find it relatively easy to understand the transformations and operations that are taking place. This makes writing and debugging code fairly straightforward as I know what I want to do, and I can anticipate where problems might arise. One of the issues I have with modern web development is how much of the work is actually black-boxed. Over the last 10 years, web development libraries and tools have advanced tremendously, allowing developers to greatly increase their productivity, but the downside to this is that until you have significant amount of experience under your belt, it can be very difficult to develop insight into the inner workings of the libraries you are using. So for someone like myself who has limited experience with these frameworks, running into bugs can be very frustrating, as I do not posses the intuition or insight to solve the problem quickly.

This brings me to the subject matter of my next post. In the process of setting up the web code for the craigslist project, I ran into a frustrating issue that seemed to fall between the cracks of the available tutorials and stackoverflow posts. The issue arose when I attempted to setup the flask login manager to take care of authentication for the different web pages (essentially controls what users can see what pages). The available example and tutorials demonstrated the use of the flask login manager with a generic, non-specific ORM. For those well versed in how Flask interacts with ORM’s this probably was the best way to present the information, but for me it resulted in some long, very frustrating hours trying to get the login manager to hook up to SQLAlchemy. I’ll save the details for the next post, but in the end, I decided that it would be a good idea to create a Flask-SQLAlchemy-Login template so that the next time I needed a login system for a project, I wouldn’t have to go through the painful process again.