9 min read

The debug toolbar has a far more advanced way of displaying the information than simply embedding it in HTML comments. The capabilities are best shown by example, so we will immediately proceed with installing the toolbar.

Installing the Django Debug Toolbar

The toolbar can be found on the Python package index site: https://pypi.python.org/pypi/django-debug-toolbar. Once installed, activating the debug toolbar in a Django project is accomplished with the addition of just a couple of settings.

First, the debug toolbar middleware, debug_toolbar.middleware.DebugToolbarMiddleware, must be added to the MIDDLEWARE_CLASSES setting. The documentation for the toolbar notes that it should be placed after any other middleware that encodes the response content, so it is best to place it last in the middleware sequence.

Second, the debug_toolbar application needs to be added to INSTALLED_APPS. The debug_toolbar application uses Django templates to render its information, thus it needs to be listed in INSTALLED_APPS so that its templates will be found by the application template loader.

Third, the debug toolbar requires that the requesting IP address be listed in INTERNAL_IPS.

Finally, the debug toolbar is displayed only when DEBUG is True. We’ve been running with debug turned on, so again we don’t have to make any changes here. Note also that the debug toolbar allows you to customize under what conditions the debug toolbar is displayed. It’s possible, then, to set things up so that the toolbar will be displayed for requesting IP addresses not in INTERNAL_IPS or when debug is not turned on, but for our purposes the default configuration is fine so we will not change anything.

One thing that is not required is for the application itself to use a RequestContext in order for things such as the SQL query information to be available in the toolbar. The debug toolbar runs as middleware, and thus is not dependent on the application using a RequestContext in order for it to generate its information. Thus, the changes made to the survey views to specify RequestContexts on render_to_response calls would not have been needed if we started off first with the Django Debug Toolbar.

Debug toolbar appearance

Once the debug toolbar is added to the middleware and installed applications settings, we can see what it looks like by simply visiting any page in the survey application. Let’s start with the home page. The returned page should now look something like this:

The Django Debug Toolbar

Note this screenshot shows the appearance of the 0.8.0 version of the debug toolbar. Earlier versions looked considerably different, so if your results do not look like this you may be using a different version than 0.8.0. The version that you have will most likely be newer than what was available when this was written, and there may be additional toolbar panels or functions that are not covered here.

As you can see, the debug toolbar appears on the right-hand side of the browser window. It consists of a series of panels that can be individually enabled or disabled by changing the toolbar configuration. The ones shown here are the ones that are enabled by default.

Before taking a closer look at some of the individual panels, notice that the toolbar contains an option to hide it at the top. If Hide is selected, the toolbar reduces itself to a small tab-like indication to show that it is present:

The Django Debug Toolbar

This can be very useful for cases where the expanded version of the toolbar obscures application content on the page. All of the information provided by the toolbar is still accessible, after clicking again on the DjDT tab; it is just out of the way for the moment.

Most of the panels will provide detailed information when they are clicked. A few also provide summary information in the main toolbar display. As of debug toolbar version 0.8.0, the first panel listed, Django Version, only provides summary information. There is no more detailed information available by clicking on it. As you can see in the screenshot, Django 1.1.1 is the version in use here.

Note that the current latest source version of the debug toolbar already provides more information for this panel than the 0.8.0 release. Since 0.8.0, this panel has been renamed to Versions, and can be clicked to provide more details. These additional details include version information for the toolbar itself and for any other installed Django applications that provide version information.

The other three panels that show summary information are the Time, SQL, and Logging panels. Thus, we can see at a glance from the first appearance of the page that 60 milliseconds of CPU time were used to produce this page (111 milliseconds total elapsed time), that the page required four queries, which took 1.95 milliseconds, and that zero messages were logged during the request.

In the following sections, we will dig into exactly what information is provided by each of the panels when clicked. We’ll start first with the SQL panel, since it is one of the most interesting and provides the same information (in addition to a lot more).

The SQL panel

If we click on the SQL section of the debug toolbar, the page will change to:

The Django Debug Toolbar

At a glance, this is a much nicer display of the SQL queries for the page than what we came up with earlier. The queries themselves are highlighted so that SQL keywords stand out, making them easier to read. Also, since they are not embedded inside an HTML comment, their content does not need to be altered in any way—there was no need to change the content of the query containing the double dash in order to avoid it causing display problems. (Now would probably be a good time to remove that added query, before we forget why we added it.)

Notice also that the times listed for each query are more specific than what was available in Django’s default query history. The debug toolbar replaces Django’s query recording with its own, and provides timings in units of milliseconds instead of seconds.

The display also includes a graphical representation of how long each query took, in the form of horizontal bars that appear above each query. This representation makes it easy to see when there are one or more queries that are much more expensive than the others. In fact, if a query takes an excessive amount of time, its bar will be colored red. In this case, there is not a great deal of difference in the query times, and none took particularly long, so all the bars are of similar length, and are colored gray.

Digging deeper, some of the information we had to manually figure out earlier in this article is just a click away on this SQL query display. Specifically, the answer to the question of what line of our code triggered a particular SQL query to be issued. Each of the displayed queries has a Toggle Stacktrace option, which when clicked will show the stack trace associated with the query:

The Django Debug Toolbar

Here we can see that all queries are made by the home method in the survey views. py file. Note that the toolbar filters out levels in the stack trace that are within Django itself, which explains why each of these has only one level shown. The first query is triggered by Line 61, which contains the filter call added to test what will happen if a query containing two dashes in a row was logged. The remaining queries are all attributed to Line 66, which is the last line of the render_to_response call in the home view. These queries, as we figured out earlier, are all made during the rendering of the template. (Your line numbers may vary from those shown here, depending on where in the file various functions were placed.)

Finally, this SQL query display makes available information that we had not even gotten around to wanting yet. Under the Action column are links to SELECT, EXPLAIN, and PROFILE each query. Clicking on the SELECT link shows what the database returns when the query is actually executed. For example:

The Django Debug Toolbar

Similarly, clicking on EXPLAIN and PROFILE displays what the database reports when asked to explain or profile the selected query, respectively. The exact display, and how to interpret the results, will differ from database to database. (In fact, the PROFILE option is not available with all databases—it happens to be supported by the database in use here, MySQL.) Interpreting the results from EXPLAIN and PROFILE is beyond the scope of what’s covered here, but it is useful to know that if you ever need to dig deep into the performance characteristics of a query, the debug toolbar makes it easy to do so.

We’ve now gotten a couple of pages deep into the SQL query display. How do we get back to the actual application page? Clicking on the circled >> at the upper-right of the main page display will return to the previous SQL query page, and the circled >> will turn into a circled X. Clicking the circled X on any panel detail page closes the details and returns to displaying the application data. Alternatively, clicking again on the panel area on the toolbar for the currently displayed panel will have the same effect as clicking on the circled symbol in the display area. Finally, if you prefer using the keyboard to the mouse, pressing Esc has the same effect as clicking the circled symbol.

Now that we have completely explored the SQL panel, let’s take a brief look at each of the other panels provided by the debug toolbar.

The Time panel

Clicking on the Time panel brings up more detailed information on where time was spent during production of the page:

The Django Debug Toolbar

The total CPU time is split between user and system time, the total elapsed (wall clock) time is listed, and the number of voluntary and involuntary context switches are displayed. For a page that is taking too long to generate, these additional details about where the time is being spent can help point towards a cause.

Note that the detailed information provided by this panel comes from the Python resource module. This is a Unix-specific Python module that is not available on non-Unix-type systems. Thus on Windows, for example, the debug toolbar time panel will only show summary information, and no further details will be available.


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