Chicken Scratches: Developing ideas on developing.

Python templates – Django and Cheetah

May 28th, 2008 by Eddie Sullivan

When writing web applications, sooner or later (usually sooner),
everybody is going to need a template
language. String-interpolation just doesn’t cut it. We
need a way to write something that is almost all text
(or HTML, or XML, or whatever), but with some dynamic pieces
thrown in.

Since this need is so universal, and the basic requirements are
so easy to describe, many different groups of people have taken
it upon themselves to create Yet Another Template Language. As
developers, we can join the fray and roll our own, or we can
wade through the myriad options available to us to find the one
that meets our needs or philosophy. Those who use PHP or ASP
pretty much have the choice made for them, since the languages
themselves are glorified template processors. Python
programmers have a lot more options.

Here I’m just going to focus on the two Python templating
languages I have used in real applications: Cheetah and the
Django templating engine. (Django, of course, is more than
just templates, but the template subsystem can be used
independently.) I use and enjoy both of these, but there are
significant differences that are worth comparing and
contrasting, when deciding which to use for your particular
needs. There are other comparisons out there, including one by
the Benevolent Dictator for Life
himself (though that’s a bit
out of date and inaccurate). When choosing which to use, you
should read as many opinions as you can, then make the decision
yourself. Presented here are just my personal thoughts.


CSS: A one act play

April 16th, 2008 by Eddie Sullivan

Student: I just learned CSS and I’m very excited about it! It seems like a nice clean and logical way to do layout. Look, I can just say color:red and my text turns red.

Guru: Oh yes, it’s soo much better than using tables. Tables are bad, bad, bad! Never use tables!! You’ll ruin us all!!!

Student: Um, ok. No tables. But say, I have a question. I’m trying to make my links look like buttons, but when I set a width on them nothing happens. What’s that all about?

Guru: Duh! Don’t you know, you can’t set a width on an inline element? You have to set display:block .

Student: Ah, I see. So now I can set the width, but it just puts each link on a separate line by itself. What if I want three or four on the same line? They’re not very wide after all. Could I maybe just use a ta-

Guru: Nooooo! No tables, I said! Look, it’s very easy and logical. Just set a fixed width and set float:left on all the items you want on the same line. Why would you not think of that?

Student: Ah, of course, float:left . I guess that should have been obvious.

Guru: Very good.

Student: But wait.

Guru: Now what is it?

Student: Now my buttons are all squished up with the text that comes after them, even if I put a <br> tag after them. Why is it ignoring my <br> tag?

Guru: Don’t you know? You should never use the <br> tag! That’s mixing layout with content. It doesn’t work after floated elements, anyway. You have to make an empty <div>, put it after all the buttons, and set clear:both on it.

Student: Ah, of course. Why didn’t I think of that? We wouldn’t want to have extra markup who’s sole purpose is for layout, would we?

Guru: Of course not. Empty divs and wrapper divs don’t count. At least they’re not tables.

Student: Ok, now how do I center them?

Guru: What?

Student: I want to center my table of buttons. How do I do that?

Guru: Well, first of all, it’s not a “table” of buttons. Don’t ever let me hear you use that word again. It’s a “list” of buttons. Turn them into a list. Then it’s very easy to center them. Set list-style-type:none. Then set display:table, but only in Firefox. You need to use conditional HTML comments for IE. Then set margin-left:auto. That’s how we center things, of course. Oh yeah, and you have to enclose the whole thing in another wrapper <div>. Then set text-align:center on the wrapper. See, easy as pie. Aren’t you glad we’re not in the bad old days, when we used to have to write hacky HTML just to make the layout work?

Student : Maybe I’ll go into marketing. (exit stage left)

More on editing Django templates in XEmacs

March 27th, 2008 by Eddie Sullivan

Previously, I wrote about how I set up syntax-highlighting for
Django template files using MMM (multiple major modes) in
. This entry builds on the previous one, so if you haven’t
read that one, I suggest doing so now.

This post addresses the age old question, “If (X)Emacs is so
good, why can’t it do the typing for me?” Well, the answer, of
course, is it can. You just have to write a few lines of Lisp
first. Or you can copy and paste the Lisp code off of sites like
this one. ;)

Introducing dynamic abbreviations

Pressing M-/, (that is, holding down the Alt key and
pressing the forward slash key), runs the command
dabbrev-expand, which tries to finish whatever word you
are in the middle of typing. For example, if as I’m typing this
entry, I type the letter d, then press M-/,
“dabbrev-expand” shows up, because that was the last word I
typed that started with d. If I then type M-/
again, “dabbrev-expand” changes to “down”. If I keep pressing
it, the word cycles through different guesses for what I’m going
for. If you’ve ever used VisualStudio, you may see some
similarities to the Intellisense feature. I find I almost never
type a whole word anymore, I’ve become so dependent on dynamic

Templates and more abbreviations

dabbrev-expand is useful to avoid having to type long
words over and over, but there are also longer patterns that
seem to need typing frequently. Thats where the tempo
package comes in handy, especially when combined with the
abbrev library. Here is some documentation for
. Here is some documentation for abbrev.

Essentially, tempo allows you to specify “templates,” or
blocks of standard text that can be parameterized as they are
filled in: the same concept as Django templates, but meant for
interactive use. From now on, I will say “tempo template” or
“Django template” to avoid confusing the two types.

abbrev lets you define your own pre-set abbreviations,
which can be filled in automatically as you type or upon
request. This can be combined with tempo to do some
pretty powerful stuff very easily.

I have a few Django-specific tempo templates and abbrevs set up
in my XEmacs initialization file. For example, as soon as I type
“{% block” followed by a space, the entire framework of a Django
template block is filled in for me. I’ve also added a special
menu to the menu bar for tempo templates I use frequently.

HTML and templates can be very repetitive, so I’ve found this
saves me a lot of typing.

My initialization file

Here is the subset of my ~/.xemacs/init.el file. This includes
the mmm-mode stuff I discussed last time, as well as the
tempo templates and abbrevs. I haven’t put a tempo template for
every possible Django template tag, just the ones I use most
frequently. I’ll leave further extension as an exercise for the

Also, some people don’t like the expansion to happen
automatically as they are typing. To turn this off, comment out
the line that says “(setq abbrev-mode t)” by putting a semicolon
(;) in front of it. Then you can
manually expand tempo templates by pressing C-\

Once again, this is known to work in XEmacs version 21.4 on
Windows XP. It will most likely work in other versions of
XEmacs, and possibly in GNU Emacs.

;; CSS-Mode
(autoload 'css-mode "css-mode" "Mode for editing CSS files" t)
(add-to-list 'auto-mode-alist '("\\.css\\'" . css-mode))
(setq cssm-indent-function #'cssm-c-style-indenter)
(setq cssm-indent-level '2)

;; Use hm--html-mode for files that end in .tmpl (Django templates)
(add-to-list 'auto-mode-alist '("\\.tmpl\\'" . hm--html-mode))

;; Multiple Major Modes.
(require 'mmm-vars)
(require 'mmm-mode)
(require 'mmm-sample)
(setq mmm-global-mode 'maybe)

;; Custom MMM classes for Django templates
    :submode python-mode
    :face mmm-declaration-submode-face
    :front "{%"
    :back "%}"
    :include-front t
    :include-back t)))

    :submode python
    :face mmm-output-submode-face
    :front "{{"
    :back "}}"
    :include-front t
    :include-back t)))

(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'embedded-css)
(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'my-django-var)
(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'my-django-expr)
(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'html-js)

;; Use different colors for different sub-modes.
(setq mmm-submode-decoration-level 2)
;; Make the code submode a little more readable.
(set-face-background 'mmm-code-submode-face "#EEEEFF")

;; Tempo templates and abbrevs

;; Control-backslash to expand a tempo template manually.
(global-set-key "\C-\\" 'tempo-complete-tag)

(require 'adapt) ; Must be done before hm--html for some reason.
(require 'hm--html-mode) ; Needed for hm--html-mode-abbrev-table
(require 'tempo) ; Tempo templates.

;; Expands <span into an HTML span with a class defined.
(tempo-define-template "span"
                       '("<span class=\"" (p "Class? ") "\">" r "</span>" >)
                       "<span "
                       "Insert a template for a span with a class")
(define-abbrev hm--html-mode-abbrev-table
  "<span" "" 'tempo-template-span)

;; Expands <script into an HTML javascript block.
(tempo-define-template "script"
                       '(> "<script type=\"text/javascript\">" r "</script>" > %)
                       "<script "
(define-abbrev hm--html-mode-abbrev-table
  "<script" "" 'tempo-template-script)

;; Expands a Django template block.
(tempo-define-template "block"
                       '("{% block " (p "Block name? ") " %}" p "{% endblock %}")
                       "{% block "
(define-abbrev hm--html-mode-abbrev-table
  "{% block" "" 'tempo-template-block)

;; Expands a Django template if tag.
(tempo-define-template "django-if"
                       '("{% if " (p "Conditional? ") " %}" p "{% endif %}")
                       "{% if "
(define-abbrev hm--html-mode-abbrev-table
  "{% if" "" 'tempo-template-django-if)

;; Expands a Django template for tag.
(tempo-define-template "django-for"
                       '("{% for "
                         (p "Variable? ") " in "
                         (p "List? ") " %}" p
                         "{% endfor %}")
                       "{% for "
(define-abbrev hm--html-mode-abbrev-table
  "{% for" "" 'tempo-template-django-for)

;; Create a menu for inserting these templates.
(defun add-templates-menu ()
  (add-submenu nil '("Tem%_plates"
                     ["{%% %_block %%}" tempo-template-block]
                     ["{%% %_if %%}" tempo-template-django-if]
                     ["{%% %_for %%}" tempo-template-django-for]
                     ["<%_span>" tempo-template-span]
                     ["<s%_cript>" tempo-template-script])))

;; When entering hm--html-mode, turn on abbrevs and add the template menu.
(add-hook 'hm--html-mode-hook
          (lambda ()
            (setq abbrev-mode t)

How I edit Django templates

March 21st, 2008 by Eddie Sullivan

NOTE: This post is pretty old, and I no longer use mmm-mode for Django template editing.
This post and this post are still accurate, though.

Every programmer has their favorite editor and mode of
working. Some people have more than one. For example, I use
Microsoft Visual Studio when editing .NET code, DrScheme for editing
Scheme code, and XEmacs for pretty much everything else.

This post is about how I use XEmacs for editing Django template
files, in the hopes that others may find this useful.

The “Other” One True Editor

It’s rare that people will get passionate about something as
pedestrian as a way of editing plain text, but the brief history
of the internet is awash with flamewars and heated discussions
with titles like “Emacs vs. XEmacs,” “Emacs vs vi,” and so
on. I’m not about to go into the relative merits, but the fact
that certain editors pop up time and time again in these debates
must mean there is something special about them.

Early on in my college education I started using Emacs because
it was all that was available on the school’s servers. (Well,
that or vi, but vi was and still is black-magic to me.) I got
over the learning curve, and now I’m hooked.

At some point, I switched from Emacs to XEmacs, for reasons I
can’t remember. At the time, it had some feature which was to me
essential. That reason no longer applies, but neither have I had
a reason to switch back. These tips may apply even if you use
GNU Emacs, but they’ve only been tested in XEmacs.

My current setup is Xemacs 21.4 (patch 20), running on Windows
XP. *gasp!* Yes, I use Windows for Django development. Shocking,
I know. I have my reasons. In any case, these tips should work
equally well with XEmacs on other platforms.

Multiple Major Modes

I won’t include a full XEmacs tutorial here, since there is
already plenty of info on the web about it. The key point
is that there is a “major mode” for each programming
environment. There is a Python mode, a Java mode, and so
on. Django templates tend to combine more than one language in a
file, so that’s when the mmm library comes in handy. It
stands for “multiple major modes,” and it turns out to be just
exactly what we need. We can have html-mode for the HTML parts,
JavaScript-mode for the JavaScript parts, css-mode for embedded
CSS, and python-mode for the Django template filters and tags.

Which HTML mode?

As often happens in the world of Free Software, there are
several options to choose from when setting up HTML editing in

  • html-mode. This has the fullest support for HTML
    parsing and validation. The problem is, when dealing with
    templates, the HTMl will often not validate, so all kinds of
    error messages show up.
  • sgml-mode. This is a more general mode for SGML (of
    which XML and HTML 4 are subsets).
  • xml-mode. SGML mode specialized for XML.
  • hm–html-mode. I have no idea what the HM stands for,
    but this is a lightweight HTML mode, with basic syntax
    highlighting and indentation.

I use html-mode for full-fledged HTML documents, and
hm–html-mode for templates. So that XEMacs can tell the difference,
I use the suffix “.tmpl” for template files.

One problem

I did run into some problems getting mmm-mode to work with
XEmacs. It turns out the version of mmm-mode that is distributed
with the XEmacs package system is ancient – from 2001. I had to
download the newer version of mmm-mode and unzip it into my site-packages directory.

How it looks

Here’s a screenshot of me editing an example Django template
(borrowed from my beta-registration Django app). I’ve chosen
fairly bright colors to make the syntax highlighting more
obvious, but that’s all customizable. Notice how the Django tags
and variables are easy to find in the file. (Click on the image
for a larger size.)

My initialization file

Here is the subset of my ~/.xemacs/init.el file dealing with
setting up mmm-mode for XEmacs. I hope someone finds this
useful. Let me know if you do, or if you encounter problems.

;; CSS-Mode
(autoload 'css-mode "css-mode" "Mode for editing CSS files" t)
(add-to-list 'auto-mode-alist '("\\.css\\'" . css-mode))
(setq cssm-indent-function #'cssm-c-style-indenter)
(setq cssm-indent-level '2)

;; Use hm--html-mode for files that end in .tmpl (Django templates)
(add-to-list 'auto-mode-alist '("\\.tmpl\\'" . hm--html-mode))

;; Multiple Major Modes.
(require 'mmm-vars)
(require 'mmm-mode)
(require 'mmm-sample)
(setq mmm-global-mode 'maybe)

;; Custom MMM classes for Django templates
    :submode python-mode
    :face mmm-declaration-submode-face
    :front "{%"
    :back "%}"
    :include-front t
    :include-back t)))

    :submode python
    :face mmm-output-submode-face
    :front "{{"
    :back "}}"
    :include-front t
    :include-back t)))

(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'embedded-css)
(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'my-django-var)
(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'my-django-expr)
(mmm-add-mode-ext-class nil "\\.tmpl\\'" 'html-js)

;; Use different colors for different sub-modes.
(setq mmm-submode-decoration-level 2)
;; Make the code submode a little more readable.
(set-face-background 'mmm-code-submode-face "#EEEEFF")

When to use ToArray()

February 11th, 2008 by Eddie Sullivan

Some thoughts on when to return a List<> object and when to return
an array in C#.

The analogy: strings

Many times in C#, a function that needs to build a string from
constituent parts will use a StringBuilder internally, and then
on the last line of the function call ToString() on the
StringBuilder object. This is because strings in C# are
immutable. That is, once they are set then can never be
changed. Code that looks like it is changing a string in place
is usually actually allocating a new string to hold the new
value. Obviously, for a function that builds a string
incrementally by constantly appending to it, this can lead to
lots of reallocation, slowing down performance.

Here’s an example of how it’s normally done:

    public string GimmeString(int num)
        StringBuilder sb = new StringBuilder("Initiating countdown: ");

        for (int i = num; i > 0; i--)
            sb.Append(string.Format("{0}...", i));

        return sb.ToString();

This is a pretty obvious design pattern. It’s rare you would
want to return the StringBuilder object itself, so almost always
it is converted to a string before returning. The problem gets
trickier, however, when we try to use a similar design pattern
for arrays.


Arrays in C# are not immutable, at least not in the way strings
are. It could be said they are “partially immutable” (my own
made-up term). You can swap out elements in the array as much as
you like, but the array’s length is fixed. Therefore, if you
need to build up an array from constituent parts, it makes sense
to use a List<> object for doing
the work.

The analogy then becomes, in SAT terms:

StringBuilder : string :: List<> : array

The question then
becomes whether or not to call ToArray() on the List<> before returning
it. In this case, the answer’s not as obvious. Let’s examine the
pros and cons of each approach:

Reasons to call ToArray()

  • If the returned value is not meant to be modified, returning
    it as an array makes that fact a bit clearer.
  • If the caller is expected to perform many non-sequential
    accesses to the data, there can be a performance benefit to an
    array over a List<>.
  • If you know you will need to pass the returned value to a
    third-party function that expects an array.
  • Compatibility with calling functions that need to work with
    .NET version 1 or 1.1. These versions don’t have the List<> type (or any generic types, for
    that matter).

Reasons not to call ToArray()

  • If the caller ever does need to add or remove
    elements, a List<> is absolutely required.
  • The performance benefits are not necessarily guaranteed,
    especially if the caller is accessing the data in a sequential
    fashion. There is also the additional step of converting from
    List<> to array, which takes processing time.
  • The caller can always convert the list to an array themselves.

ToArray() or not ToArray()? That is the question.

Based on these points, it seems to make the most sense as a
general rule to simply return the List<> object directly, rather than
converting it to an array before returning. Let me know if you

Here’s an example:

    // A contrived example. Similar to Python's "range" function, but only
    // supports positive step.
    public List<int> GimmeInts(int start, int end, int step)
        List<int> ret = new List<int>();

        for (int i = start; i < end; i += step)
        // Here you could have:
        // return ret.ToArray();
        return ret;

What’s wrong with hotels

February 8th, 2008 by Eddie Sullivan

Anybody who needs to stay in hotels often knows there are certain areas where they could all do better. I tend to look at everything with an eye towards what could be improved. Here are some random thoughts on how hotels could be better. Some of these ideas are a bit far-fetched, but hey, that’s why I’m writing a weblog, not a manifesto.

More pricing logic

It sometimes seems like the cheaper a hotel is, the more services it provides. I’ve stayed in a broad range of hotels, from top of the line luxury suites to eight-cot-to-a-room hostels. Strangely, there are some areas where the lesser quality hotels actually do better. Why is it that when I pay $40 per night for a small family-run inn, there is personalized service and free wi-fi in every room, but in a hotel that charges $375 per night there is either no wi-fi available or it costs $20 extra? What are we actually getting for the higher price? Why do cheap hotels provide breakfast but expensive hotels don’t? It’s backwards.


After seeing watching a news report about hotel cleanliness issues, I’ve been a little bit paranoid about sanitation in hotels. A couple idea:

Get rid of the carpet

Carpeted floors are notoriously hard to clean. I don’t want to think about what kind of dirtiness people can leave on carpets that doesn’t get picked up by a vacuum cleaner. Get rid of the carpeted floor. Replace it with hardwoods, or if that’s too expensive, a synthetic material designed to not be too cold on our toes.

Auto-magically clean

Have you seen those coin-operated public restrooms in public cities that actually clean themselves after each use? Why can’t we refine this technology and use it for hotel bathrooms? When the maid comes in, he or she removes all the towels and paper products, closes the bathroom door, and presses a button. Instantly, the whole bathroom is sprayed with disinfecting detergent from a nozzle in the ceiling, rinsed, then dried with high-speed air jets.

Put the bed on hydraulic lifts to make it easy to clean underneath.

Instead of just layering a blanket on top of a sheet, use a large pillow-case-like wrapper for the whole blanket. Design the quilt with drawstrings to make changing the blanket case easy. There is nothing worse than tossing around at night and waking to realize the sheet has slid off the bed and you’re now face-to-face with someone else’s potential filth.


This should be obvious, but: free wi-fi. For everyone. Everywhere.

Provide an easy to use central control panel for things like the alarm clock, the thermostat, maybe even ordering room-service. Every hotel alarm clock has a different interface, leaving guests with no confidence they will actually wake up on time.

Digital thermostats for the showers. It takes me forever to get the temperature just right in an unfamiliar shower. I want to be able to just turn a dial or press a button for the exact temperature I need and let the electronics do the rest.

And more

The more I think, the more ideas I come up with. I know most of these will never happen, but it can’t hurt to dream, can it?

Feel free to add any ideas using the comment link below.

Programming for Not-so-dummies

January 30th, 2008 by Eddie Sullivan

Somebody posted a reply to my last post about Django’s autoescaping mechanism. (They were too cowardly to post on my site, so they posted it at The person said something like, "you shouldn’t trust yourself to remember to escape your own variables." Oh, heaven forbid I trust myself to be a good programmer! That really got me thinking about the recent trends towards designing frameworks, APIs, even languages for mediocre programmers. We are sacrificing speed, simplicity and efficiency to make common bugs less common, trying to design away the mistakes inexpensive and poorly-trained computer scientists make.

Now, of course when I say "recent trends," I should acknowledge that this type of thinking has been around for decades. It was first truly popularized with the introduction of Java. Some people forget to free memory, so add garbage collection. Some people forget to bounds-check arrays, so make that automatic. Ooh, pointers are scary! Let’s get rid of them. We can’t allow our outsourced foreign coders direct access to memory!

Good training, along with working for nearly a decade as an embedded software engineer, has taught me good programming habits. I’ve learned to be conscious of memory leaks, to always check return values, to program defensively, to bounds-check. I’ve created software for shipping products in such low-level and "unprotected" languages as C++, C, and even Assembly. I’ve written production code within less than the memory space required for a Java byte-code interpreter. And of course, I’m not alone in this. There is a large subset of software developers who had to learn to program carefully, due to constraints out of their control. These types of good programming habits carry over into whatever platform or language is used.

I feel a lot of the new safety-net style approaches are simply enabling poor programmers to work on increasingly sophisticated projects. To get back to the example from my last post, Django is a wonderful tool. You can program a sophisticated database-centered multi-user web application without even knowing how to spell SQL.
Django’s recent addition of autoescaping, and more importantly, the enabling of autoescaping globally by default, is yet another example of API-design for the lowest common denominator. (I should note that I love Django. It saves me writing a lot of redundant code and provides a lot of things for free that I would otherwise need to write from scratch, so I don’t mean to pick on Django here. It just happened to be the catalyst for this discussion.)

It’s not all bad

I know I’m starting to sound like an old curmudgeon. "In my day, we didn’t have variables, we just had to carry around rocks to count!" I’m not that old, really. And I’m certainly not advocating we go back to the days before garbage collection and bounds checking. Especially given the potential security ramifications of memory-management bugs, these things are especially important. I just want to urge caution before binding developers in a straitjacket. Rather than trying to design away all potential bugs at the level of the language or API, emphasize and facilitate good programming and testing practices. I’ve never once bought a For Dummies book, and I never will. Please don’t force me to use a For Dummies application framework.