The Django Book

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Chapter 3: The Basics of Dynamic Web Pages

In the previous chapter, we explained how to set up a Django project and run the Django development server. Of course, that site doesnt actually do anything useful yetall it does is display the It worked! message. Lets change that. This chapter introduces how to create dynamic Web pages with Django.

Your First View: Dynamic Content

As our first goal, lets create a Web page that displays the current date and time. This is a good example of a dynamic Web page, because the contents of the page are not staticrather, the contents change according to the result of a computation (in this case, a calculation of the current time). This simple example doesnt involve a database or any sort of user inputjust the output of your servers internal clock.

To create this page, well write a view function . A view function, or view for short, is simply a Python function that takes a Web request and returns a Web response. This response can be the HTML contents of a Web page, or a redirect, or a 404 error, or an XML document, or an image or anything, really. The view itself contains whatever arbitrary logic is necessary to return that response. This code can live anywhere you want, as long as its on your Python path. Theres no other requirementno magic, so to speak. For the sake of putting the code somewhere , lets create a file called views.py in the mysite directory, which you created in the previous chapter.

Heres a view that returns the current date and time, as an HTML document:

from django.http import HttpResponse
import datetime

def current_datetime(request):
    now = datetime.datetime.now()
    html = "<html><body>It is now %s.</body></html>" % now
    return HttpResponse(html)

Lets step through this code one line at a time:

First, we import the class HttpResponse , which lives in the django.http module. See Appendix H for further details on the HttpRequest and HttpResponse objects.

Then we import the datetime module from Pythons standard library, the set of useful modules that comes with Python. The datetime module contains several functions and classes for dealing with dates and times, including a function that returns the current time.

Next, we define a function called current_datetime . This is the view function. Each view function takes an HttpRequest object as its first parameter, which is typically named request .

Note that the name of the view function doesnt matter; it doesnt have to be named in a certain way in order for Django to recognize it. Were calling it current_datetime here, because that name clearly indicates what it does, but it could just as well be named super_duper_awesome_current_time , or something equally revolting. Django doesnt care. The next section explains how Django finds this function.

The first line of code within the function calculates the current date/time, as a datetime.datetime object, and stores that as the local variable now .

The second line of code within the function constructs an HTML response using Pythons format-string capability. The %s within the string is a placeholder, and the percent sign after the string means Replace the %s with the value of the variable now . (Yes, the HTML is invalid, but were trying to keep the example simple and short.)

Finally, the view returns an HttpResponse object that contains the generated response. Each view function is responsible for returning an HttpResponse object. (There are exceptions, but well get to those later.)

Djangos Time Zone

Django includes a TIME_ZONE setting that defaults to America/Chicago . This probably isnt where you live, so you might want to change it in your settings.py . See Appendix E for details.

Mapping URLs to Views

So, to recap, this view function returns an HTML page that includes the current date and time. But how do we tell Django to use this code? Thats where URLconfs come in.

A URLconf is like a table of contents for your Django-powered Web site. Basically, its a mapping between URL patterns and the view functions that should be called for those URL patterns. Its how you tell Django, For this URL, call this code, and for that URL, call that code. Remember that the view functions need to be on the Python path.

Your Python Path

Your Python path is the list of directories on your system where Python looks when you use the Python import statement.

For example, lets say your Python path is set to ['', '/usr/lib/python2.4/site-packages', '/home/username/djcode/'] . If you execute the Python code from foo import bar , Python will first check for a module called foo.py in the current directory. (The first entry in the Python path, an empty string, means the current directory.) If that file doesnt exist, Python will look for the file /usr/lib/python2.4/site-packages/foo.py . If that file doesnt exist, it will try /home/username/djcode/foo.py . Finally, if that file doesnt exist, it will raise ImportError .

If youre interested in seeing the value of your Python path, start the Python interactive interpreter and type import sys , followed by print sys.path .

Generally you dont have to worry about setting your Python pathPython and Django will take care of things for you automatically behind the scenes. (If youre curious, setting the Python path is one of the things that the manage.py file does.)

When you executed django-admin.py startproject in the previous chapter, the script created a URLconf for you automatically: the file urls.py . Lets edit that file. By default, it looks something like this:

from django.conf.urls.defaults import *

urlpatterns = patterns('',
    # Example:
    # (r'^mysite/', include('mysite.apps.foo.urls.foo')),

    # Uncomment this for admin:
#     (r'^admin/', include('django.contrib.admin.urls')),
)

Lets step through this code one line at a time:

The first line imports all objects from the django.conf.urls.defaults module, including a function called patterns .

The second line calls the function patterns() and saves the result into a variable called urlpatterns . The patterns() function gets passed only a single argumentthe empty string. The rest of the lines are commented out. (The string can be used to supply a common prefix for view functions, but well skip this advanced usage for now.)

The main thing to note here is the variable urlpatterns , which Django expects to find in your ROOT_URLCONF module. This variable defines the mapping between URLs and the code that handles those URLs.

By default, everything in the URLconf is commented outyour Django application is a blank slate. (As a side note, thats how Django knew to show you the It worked! page in the last chapter. If your URLconf is empty, Django assumes you just started a new project and, hence, displays that message.)

Lets edit this file to expose our current_datetime view:

from django.conf.urls.defaults import *
from mysite.views import current_datetime

urlpatterns = patterns('',
    (r'^time/$', current_datetime),
)

We made two changes here. First, we imported the current_datetime view from its module (mysite/views.py , which translates into mysite.views in Python import syntax). Next, we added the line (r'^time/$', current_datetime), . This line is referred to as a URLpattern its a Python tuple in which the first element is a simple regular expression and the second element is the view function to use for that pattern.

In a nutshell, we just told Django that any request to the URL /time/ should be handled by the current_datetime view function.

A few things are worth pointing out:

Note that, in this example, we passed the current_datetime view function as an object without calling the function. This is a key feature of Python (and other dynamic languages): functions are first-class objects, which means you can pass them around just like any other variables. Cool stuff, eh?

The r in r'^time/$' means that '^time/$ is a Python raw string. This allows regular expressions to be written without overly verbose escaping.

You should exclude the expected slash at the beginning of the '^time/$' expression in order to match /time/ . Django automatically puts a slash before every expression. At first glance, this may seem odd, but URLconfs can be included in other URLconfs, and leaving off the leading slash simplifies matters. This is further covered in Chapter 8.

The caret character (^ ) and dollar sign character ($ ) are important. The caret means require that the pattern matches the start of the string, and the dollar sign means require that the pattern matches the end of the string.

This concept is best explained by example. If we had instead used the pattern '^time/' (without a dollar sign at the end), then any URL that starts with time/ would match, such as /time/foo and /time/bar , not just /time/ . Similarly, if we had left off the initial caret character ('time/$' ), Django would match any URL that ends with time/ , such as /foo/bar/time/ . Thus, we use both the caret and dollar sign to ensure that only the URL /time/ matches. Nothing more, nothing less.

You may be wondering what happens if someone requests /time . This is handled as youd hope (via a redirect) as long as the APPEND_SLASH setting is True . (See Appendix E for some good bedtime reading on this topic.)

To test our changes to the URLconf, start the Django development server, as you did in Chapter 2, by running the command python manage.py runserver . (If you left it running, thats fine, too. The development server automatically detects changes to your Python code and reloads as necessary, so you dont have to restart the server between changes.) The server is running at the address http://127.0.0.1:8000/ , so open up a Web browser and go to http://127.0.0.1:8000/time/ . You should see the output of your Django view.

Hooray! Youve made your first Django-powered Web page.

Regular Expressions

Regular expressions (or regexes ) are a compact way of specifying patterns in text. While Django URLconfs allow arbitrary regexes for powerful URL-matching capability, youll probably use only a few regex patterns in practice. Heres a small selection of common patterns:

Symbol	Matches
`.` (dot)	Any character
`\d`	Any digit
`[A-Z]`	Any character, A-Z (uppercase)
`[a-z]`	Any character, a-z (lowercase)
`[A-Za-z]`	Any character, a-z (case insensitive)
`+`	One or more of the previous expression (e.g., `\d+` matches one or more digit)
`[^/]+`	All characters except forward slash
`?`	Zero or more of the previous expression (e.g., `\d*` matches zero or more digits)
`{1,3}`	Between one and three (inclusive) of the previous expression

For more on regular expressions, see http://www.djangoproject.com/r/python/re-module/.

How Django Processes a Request

We should point out several things about what just happened. Heres the nitty-gritty of what goes on when you run the Django development server and make requests to Web pages:

The command python manage.py runserver imports a file called settings.py from the same directory. This file contains all sorts of optional configuration for this particular Django instance, but one of the most important settings is ROOT_URLCONF . The ROOT_URLCONF setting tells Django which Python module should be used as the URLconf for this Web site.

Remember when django-admin.py startproject created the files settings.py and urls.py ? Well, the autogenerated settings.py has a ROOT_URLCONF that points to the autogenerated urls.py . Convenient.

When a request comes insay, a request to the URL /time/ Django loads the URLconf pointed to by the ROOT_URLCONF setting. Then it checks each of the URLpatterns in that URLconf in order, comparing the requested URL with the patterns one at a time, until it finds one that matches. When it finds one that matches, it calls the view function associated with that pattern, passing an HttpRequest object as the first parameter to the function. (More on HttpRequest later.)

The view function is responsible for returning an HttpResponse object.

You now know the basics of how to make Django-powered pages. Its quite simple, reallyjust write view functions and map them to URLs via URLconfs. You might think it would be slow to map URLs to functions using a series of regular expressions, but youd be surprised.

How Django Processes a Request: Complete Details

In addition to the straightforward URL-to-view mapping just described, Django provides quite a bit of flexibility in processing requests.

The typical flowURLconf resolution to a view function which returns an HttpResponse can be short-circuited or augmented via middleware. The deep secrets of middleware will be fully covered in Chapter 15, but a quick sketch (see Figure 3-1) should aid you in conceptually fitting the pieces together.

Figure 3-1: The complete flow of a Django request and response.

When an HTTP request comes in from the browser, a server-specific handler constructs the HttpRequest passed to later components and handles the flow of the response processing.

The handler then calls any available Request or View middleware. These types of middleware are useful for augmenting incoming HttpRequest objects as well as providing special handling for specific types of requests. If either returns an HttpResponse , processing bypasses the view.

Bugs slip by even the best programmers, but exception middleware can help squash them. If a view function raises an exception, control passes to the Exception middleware. If this middleware does not return an HttpResponse , the exception is re-raised.

Even then, all is not lost. Django includes default views that create a friendly 404 and 500 response.

Finally, response middleware is good for post-processing an HttpResponse just before its sent to the browser or doing cleanup of request-specific resources.

URLconfs and Loose Coupling

Nows a good time to highlight a key philosophy behind URLconfs and behind Django in general: the principle of loose coupling . Simply put, loose coupling is a software-development approach that values the importance of making pieces interchangeable. If two pieces of code are loosely coupled, then changes made to one of the pieces will have little or no effect on the other.

Djangos URLconfs are a good example of this principle in practice. In a Django Web application, the URL definitions and the view functions they call are loosely coupled; that is, the decision of what the URL should be for a given function, and the implementation of the function itself, reside in two separate places. This lets a developer switch out one piece without affecting the other.

In contrast, other Web development platforms couple the URL to the program. In typical PHP (http://www.php.net/) applications, for example, the URL of your application is designated by where you place the code on your filesystem. In early versions of the CherryPy Python Web framework (http://www.cherrypy.org/), the URL of your application corresponded to the name of the method in which your code lived. This may seem like a convenient shortcut in the short term, but it can get unmanageable in the long run.

For example, consider the view function we wrote earlier, which displays the current date and time. If we wanted to change the URL for the application say, move it from /time/ to /currenttime/ we could make a quick change to the URLconf, without having to worry about the underlying implementation of the function. Similarly, if we wanted to change the view functionaltering its logic somehowwe could do that without affecting the URL to which the function is bound. Furthermore, if we wanted to expose the current-date functionality at several URLs, we could easily take care of that by editing the URLconf, without having to touch the view code.

Thats loose coupling in action. Well continue to point out examples of this important philosophy throughout this book.

404 Errors

In our URLconf thus far, weve defined only a single URLpattern: the one that handles requests to the URL /time/ . What happens when a different URL is requested?

To find out, try running the Django development server and hitting a page such as http://127.0.0.1:8000/hello/ or http://127.0.0.1:8000/does-not-exist/ , or even http://127.0.0.1:8000/ (the site root). You should see a Page not found message (see Figure 3-2). (Pretty, isnt it? We Django people sure do like our pastel colors.) Django displays this message because you requested a URL thats not defined in your URLconf.

Figure 3-2. Djangos 404 page

The utility of this page goes beyond the basic 404 error message; it also tells you precisely which URLconf Django used and every pattern in that URLconf. From that information, you should be able to tell why the requested URL threw a 404.

Naturally, this is sensitive information intended only for you, the Web developer. If this were a production site deployed live on the Internet, we wouldnt want to expose that information to the public. For that reason, this Page not found page is only displayed if your Django project is in debug mode . Well explain how to deactivate debug mode later. For now, just know that every Django project is in debug mode when you first create it, and if the project is not in debug mode, a different response is given.

Your Second View: Dynamic URLs

In our first view example, the contents of the pagethe current date/time were dynamic, but the URL (/time/ ) was static. In most dynamic Web applications, though, a URL contains parameters that influence the output of the page.

Lets create a second view that displays the current date and time offset by a certain number of hours. The goal is to craft a site in such a way that the page /time/plus/1/ displays the date/time one hour into the future, the page /time/plus/2/ displays the date/time two hours into the future, the page /time/plus/3/ displays the date/time three hours into the future, and so on.

A novice might think to code a separate view function for each hour offset, which might result in a URLconf like this:

urlpatterns = patterns('',
    (r'^time/$', current_datetime),
    (r'^time/plus/1/$', one_hour_ahead),
    (r'^time/plus/2/$', two_hours_ahead),
    (r'^time/plus/3/$', three_hours_ahead),
    (r'^time/plus/4//$', four_hours_ahead),
)

Clearly, this line of thought is flawed. Not only would this result in redundant view functions, but also the application is fundamentally limited to supporting only the predefined hour rangesone, two, three, or four hours. If, all of a sudden, we wanted to create a page that displayed the time five hours into the future, wed have to create a separate view and URLconf line for that, furthering the duplication and insanity. We need to do some abstraction here.

A Word About Pretty URLs

If youre experienced in another Web development platform, such as PHP or Java, you may be thinking, Hey, lets use a query string parameter!, something like /time/plus?hours=3 , in which the hours would be designated by the hours parameter in the URLs query string (the part after the ? ).

You can do that with Django (and well tell you how later, if you really must know), but one of Djangos core philosophies is that URLs should be beautiful. The URL /time/plus/3/ is far cleaner, simpler, more readable, easier to recite to somebody aloud and just plain prettier than its query string counterpart. Pretty URLs are a sign of a quality Web application.

Djangos URLconf system encourages pretty URLs by making it easier to use pretty URLs than not to.

Wildcard URLpatterns

Continuing with our hours_ahead example, lets put a wildcard in the URLpattern. As we mentioned previously, a URLpattern is a regular expression; hence, we can use the regular expression pattern \d+ to match one or more digits:

from django.conf.urls.defaults import *
from mysite.views import current_datetime, hours_ahead

urlpatterns = patterns('',
    (r'^time/$', current_datetime),
    (r'^time/plus/\d+/$', hours_ahead),
)

This URLpattern will match any URL such as /time/plus/2/ , /time/plus/25/ , or even /time/plus/100000000000/ . Come to think of it, lets limit it so that the maximum allowed offset is 99 hours. That means we want to allow either one- or two-digit numbersin regular expression syntax, that translates into \d{1,2} :

(r'^time/plus/\d{1,2}/$', hours_ahead),

Note

When building Web applications, its always important to consider the most outlandish data input possible, and decide whether or not the application should support that input. Weve curtailed the outlandishness here by limiting the offset to 99 hours. And, by the way, The Outlandishness Curtailers would be a fantastic, if verbose, band name.

Now that weve designated a wildcard for the URL, we need a way of passing that data to the view function, so that we can use a single view function for any arbitrary hour offset. We do this by placing parentheses around the data in the URLpattern that we want to save. In the case of our example, we want to save whatever number was entered in the URL, so lets put parentheses around the \d{1,2} :

(r'^time/plus/(\d{1,2})/$', hours_ahead),

If youre familiar with regular expressions, youll be right at home here; were using parentheses to capture data from the matched text.

The final URLconf, including our previous current_datetime view, looks like this:

from django.conf.urls.defaults import *
from mysite.views import current_datetime, hours_ahead

urlpatterns = patterns('',
    (r'^time/$', current_datetime),
    (r'^time/plus/(\d{1,2})/$', hours_ahead),
)

With that taken care of, lets write the hours_ahead view.

Coding Order

In this example, we wrote the URLpattern first and the view second, but in the previous example, we wrote the view first, then the URLpattern. Which technique is better? Well, every developer is different.

If youre a big-picture type of person, it may make the most sense to you to write all of the URLpatterns for your application at the same time, at the start of your project, and then code up the views. This has the advantage of giving you a clear to-do list, and it essentially defines the parameter requirements for the view functions youll need to write.

If youre more of a bottom-up developer, you might prefer to write the views first, and then anchor them to URLs afterward. Thats OK, too.

In the end, it comes down to which technique fits your brain the best. Both approaches are valid.

hours_ahead is very similar to the current_datetime view we wrote earlier, with a key difference: it takes an extra argument, the number of hours of offset. Add this to views.py :

def hours_ahead(request, offset):
    offset = int(offset)
    dt = datetime.datetime.now() + datetime.timedelta(hours=offset)
    html = "<html><body>In %s hour(s), it will be %s.</body></html>" % (offset, dt)
    return HttpResponse(html)

Lets step through this code one line at a time:

Just as we did for our current_datetime view, we import the class django.http.HttpResponse and the datetime module.

The view function, hours_ahead , takes two parameters: request and offset .

request is an HttpRequest object, just as in current_datetime . Well say it again: each view always takes an HttpRequest object as its first parameter.

offset is the string captured by the parentheses in the URLpattern. For example, if the requested URL were /time/plus/3/ , then offset would be the string '3' . If the requested URL were /time/plus/21/ , then offset would be the string '21' . Note that captured strings will always be strings , not integers, even if the string is composed of only digits, such as '21' .

We decided to call the variable offset , but you can call it whatever youd like, as long as its a valid Python identifier. The variable name doesnt matter; all that matters is that its the second argument to the function (after request ). Its also possible to use keyword, rather than positional, arguments in an URLconf. We cover that in Chapter 8.

The first thing we do within the function is call int() on offset . This converts the string value to an integer.

Note that Python will raise a ValueError exception if you call int() on a value that cannot be converted to an integer, such as the string 'foo' . However, in this example we dont have to worry about catching that exception, because we can be certain offset will be a string containing only digits. We know that because the regular-expression pattern in our URLconf (\d{1,2}) captures only digits. This illustrates another nicety of URLconfs: they provide a fair level of input validation.

The next line of the function shows why we called int() on offset . On this line, we calculate the current time plus a time offset of offset hours, storing the result in dt . The datetime.timedelta function requires the hours parameter to be an integer.

Next, we construct the HTML output of this view function, just as we did in current_datetime . A small difference in this line from the previous line is that it uses Pythons format-string capability with two values, not just one. Hence, there are two %s symbols in the string and a tuple of values to insert: (offset, dt) .

Finally, we return an HttpResponse of the HTMLagain, just as we did in current_datetime .

With that view function and URLconf written, start the Django development server (if its not already running), and visit http://127.0.0.1:8000/time/plus/3/ to verify it works. Then try http://127.0.0.1:8000/time/plus/5/ . Then http://127.0.0.1:8000/time/plus/24/ . Finally, visit http://127.0.0.1:8000/time/plus/100/ to verify that the pattern in your URLconf only accepts one- or two-digit numbers; Django should display a Page not found error in this case, just as we saw in the 404 Errors section earlier. The URL http://127.0.0.1:8000/time/plus/ (with no hour designation) should also throw a 404.

If youre following along while coding at the same time, youll notice that the views.py file now contains two views. (We omitted the current_datetime view from the last set of examples for clarity.) Put together, views.py should look like this:

from django.http import HttpResponse
import datetime

def current_datetime(request):
    now = datetime.datetime.now()
    html = "<html><body>It is now %s.</body></html>" % now
    return HttpResponse(html)

def hours_ahead(request, offset):
    offset = int(offset)
    dt = datetime.datetime.now() + datetime.timedelta(hours=offset)
    html = "<html><body>In %s hour(s), it will be %s.</body></html>" % (offset, dt)
    return HttpResponse(html)

Djangos Pretty Error Pages

Take a moment to admire the fine Web application weve made so far now lets break it! Well deliberately introduce a Python error into our views.py file by commenting out the offset = int(offset) line in the hours_ahead view:

def hours_ahead(request, offset):
    #offset = int(offset)
    dt = datetime.datetime.now() + datetime.timedelta(hours=offset)
    html = "<html><body>In %s hour(s), it will be %s.</body></html>" % (offset, dt)
    return HttpResponse(html)

Load up the development server and navigate to /time/plus/3/ . Youll see an error page with a significant amount of information, including a TypeError message displayed at the very top: "unsupported type for timedelta hours component: str" .

What happened? Well, the datetime.timedelta function expects the hours parameter to be an integer, and we commented out the bit of code that converted offset to an integer. That caused datetime.timedelta to raise the TypeError . Its the typical kind of small bug that every programmer runs into at some point.

The point of this example was to demonstrate Djangos error pages. Take some time to explore the error page and get to know the various bits of information it gives you.

Here are some things to notice:

At the top of the page, you get the key information about the exception: the type of exception, any parameters to the exception (the "unsupported type" message in this case), the file in which the exception was raised, and the offending line number.

Under the key exception information, the page displays the full Python traceback for this exception. This is similar to the standard traceback you get in Pythons command-line interpreter, except its more interactive. For each frame in the stack, Django displays the name of the file, the function/method name, the line number, and the source code of that line.

Click the line of source code (in dark gray), and youll see several lines from before and after the erroneous line, to give you context.

Click Local vars under any frame in the stack to view a table of all local variables and their values, in that frame, at the exact point in the code at which the exception was raised. This debugging information is invaluable.

Note the Switch to copy-and-paste view text under the Traceback header. Click those words, and the traceback will switch to a alternate version that can be easily copied and pasted. Use this when you want to share your exception traceback with others to get technical support such as the kind folks in the Django IRC chat room or on the Django users mailing list.

Next, the Request information section includes a wealth of information about the incoming Web request that spawned the error: GET and POST information, cookie values, and meta information, such as CGI headers. Appendix H has a complete reference of all the information a request object contains.

Below the Request information section, the Settings section lists all of the settings for this particular Django installation. All the available settings are covered in detail in Appendix E. For now, take a look at the settings to get an idea of the information available.

The Django error page is capable of displaying more information in certain special cases, such as the case of template syntax errors. Well get to those later, when we discuss the Django template system. For now, uncomment the offset = int(offset) line to get the view function working properly again.

Are you the type of programmer who likes to debug with the help of carefully placed print statements? You can use the Django error page to do sojust without the print statements. At any point in your view, temporarily insert an assert False to trigger the error page. Then, you can view the local variables and state of the program. (Theres a more advanced way to debug Django views, which well explain later, but this is the quickest and easiest.)

Finally, its obvious that much of this information is sensitiveit exposes the innards of your Python code and Django configurationand it would be foolish to show this information on the public Internet. A malicious person could use it to attempt to reverse-engineer your Web application and do nasty things. For that reason, the Django error page is only displayed when your Django project is in debug mode. Well explain how to deactivate debug mode later. For now, just know that every Django project is in debug mode automatically when you start it. (Sound familiar? The Page not found errors, described in the 404 Errors section, work the same way.)

Whats next?

Weve so far been producing views by hard-coding HTML into the Python code. Unfortunately, this is nearly always a bad idea. Luckily, Django ships with a simple yet powerful template engine that allows you to separate the design of the page from the underlying code. Well dive into Djangos template engine in the next chapter.

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