Django Middleware vs. Context Processors

This may be old news to many people, but it's something I just recently learned (after doing it wrong for about nine months), so I figured someone else may be able to benefit from my mistakes.

For a long time, when I needed to access the currently logged-in user in one of my Django views, they would follow this pattern:

[code lang="python"]from django.template import RequestContext

# some other view code

def my_view (request, obj_id):
context = RequestContext(request)
obj = get_object_or_404(SomeModel, pk=int(obj_id))

# do some stuff, including:
some_function(context["user"])

return render_to_response("some_url_name", {"some_var": some_val},
context_instance=context)[/code]

Now, seasoned Django vets (why are you reading this post, by the way?) are probably laughing, but this seemed perfectly fine to me. Luckily, my ignorance was revealed to me while asking on IRC about a problem which, while didn't seem so at the time, was completely tied to my misuse of RequestContext.

To put it simply, context processors are made to be used in templates. The only time they should ever be instantiated is at the very end of a view, like so:

[code lang="python"]return render_to_response("some_url_name", {"some_var": some_val},
context_instance=RequestContext(request))[/code]

I was using them all over views, and even in a few of my decorators. What's wrong with this? The main thing is, certain mutation functions are sometimes performed when a RequestContext is instantiated (such as user.get_and_delete_messages(), which gets all of a user's messages from the database and then deletes them), and performing them multiple times before loading the template can cause unexpected results. In my example, I was instantiating a RequestContext in a bunch of decorators, which meant that by the time my template was loaded, all of the user's messages were deleted (and stored in an earlier instance of RequestContext), making it look to me as if my auth messages were being thrown out.

What's the solution? Middleware. Middleware allows the programmer to attach variables to the request object before it reaches the view. With this (and the provided django.contrib.auth.middleware.AuthenticationMiddleware), I can still achieve my original goal (accessing the logged-in user from a view), but now I can do it without creating a RequestContext and potentially running mutation functions multiple times:

[code lang="python"]from django.template import RequestContext

# some other view code

def my_view (request, obj_id):
obj = get_object_or_404(SomeModel, pk=int(obj_id))

# do some stuff, including:
some_function(request.user)

return render_to_response("some_url_name", {"some_var": some_val},
context_instance=RequestContext(request))[/code]

I've removed all the references to RequestContext from my decorators, and made sure to only instantiate it at the very end of views. Now, messages work perfectly. I've even written my own middleware (which you can learn how to do here) to load instances of a few of my own models into the request.

To reiterate, I'm aware that this is common knowledge for many people, but it took me 9 months of moderate Django use and embarrassment on IRC to discover it for myself. Hopefully, this will help someone else in a similar position.

Are you a better programmer than you were two years ago?

Two years ago, I was working on a fairly complicated (for my level of experience) web app for posting news articles. Quite a few times, I ran into situations where I was about to create tight coupling between two somewhat unrelated parts of my app. Sometimes, I wouldn't even recognize this as a problem. Other times, I would, but not be able to think of an easy fix, so I'd continue on.

Today, I find myself fixing tightly-coupled situations almost instinctively. I use design patterns that I was barely aware of two years ago, and I do it without straying into "design pattern fever" territory. This isn't to say I'm an expert at software design; it still takes a lot of thought to actually think of the best fix, and I'm sure I still make plenty of mistakes. My point is, my growth as a programmer is very obvious to me in these situations.

How about you? If you've been programming for more than two years, you're probably a better programmer than you were two years ago, but can you tell? If so, how can you tell? Can you give any good examples of moments when you realized it?

Non-painful email on Django development servers

I've been actively learning and using Django since August 2008, and I've loved almost every bit of it. There are plenty of places to read all about the virtues of Django, so I'll leave that out for now.

One thing that's always bugged me about web development in general is the sending of emails. I do development on my local computer (with a badly set up Apache / MySQL / PHP / Python / whatever else stack), and I've never felt like dealing with the headache of setting up a mail server. This means, when I add something that's supposed to send an email (like an activation email after registration), I have to get very hacky to test and debug it (making sure the email text is being produced correctly, making sure it's being sent to and from the right people, etc.).

This was one of the few web development pains that I thought Django didn't solve. Whenever I'd test a bit of code that was supposed to send email, I'd get a "Connection refused" error page (meaning my computer has no mail server to send the email with). I would usually add in a bit of printf debugging to make sure the subject and body had the correct text, but beyond that, I'd usually wait to test the email portions until I uploaded to a server that could send email (usually the production server, unfortunately).

Yesterday, I bumped into a little section in the Django documentation that explains how to get around this. As usual, Python has all the solutions. First, set this code in your settings.py file:

[code lang="python"]EMAIL_HOST = 'localhost'
EMAIL_PORT = 1025 # replace this with some free port number on your machine[/code]

Then, assuming you're on a Unix system (I'm on a Mac), run the following on the command line to start a "dumb" Python mailserver:

[code lang="bash"]python -m smtpd -n -c DebuggingServer localhost:1025[/code]

Make sure to replace 1025 with whatever you filled in for EMAIL_PORT.

Now, try running the email-sending code in your Python application. Voila! No error pages (or at least, none related to email), and the full text of the email (headers and all) appears in whatever command line prompt you ran the dumb mailserver on. This allows you to the see senders, recipients, subject, and body of the email being sent out, all without getting hacky or sending to an email account you own.

Taking this a step further, I created a small bash script called "dumbmail" in /usr/local/bin that looks like the following:

[code lang="bash"]#!/usr/bin/env bash
if [ -z $1 ]
then port=1025
else port=$1
fi

echo "Starting dumb mail server on localhost:$port"
python -m smtpd -n -c DebuggingServer localhost:$port[/code]

Now, when I'm testing a Django application and I get to a section that is going to send an email, I just run "dumbmail" (or "dumbmail some_number" if I need to use a different port, for some reason I can't imagine), and I'm ready to go.

Hope this helps people. The documentation was always there - I just never noticed that part until yesterday.