And unless your application is extremely unusual, by far the biggest
expense is database queries. More than anything else, this is what
governs the speed of your application. So don't worry about where the
conditionals are being evaluated: worry about how much data you are
pulling from the database.
Keep in mind that Django's ORM has a lazy evaluation model, so your view
code could load a context variable with a list of objects from the
database, and if the template never uses the variable, then the database
query is never executed. This means that it can be difficult to
determine how many queries are really executing unless you actually
trace the database traffic.
--Ned.
John DeRosa wrote:
> Merric Mercer wrote:
>
>> I have view that returns a whole bunch of different variables and
>> boolean flags. The state of these flags determine what the user
>> actually sees on their screen.
>>
>> From a design perspective I seem to have two options:-
>>
>>
>> 1. Design a template with lots of {% if %} conditional statements to
>> check the status of various flags, in order to display the appropriate
>> information to the user.
>>
>> OR
>>
>> 2. Do the bulk of the work in the view, by doing something like:-
>>
> [snip]
>
>> 3. Do a combination of the two above
>>
>> Option number 2 lacks the flexibility of doing stuff in the template,
>> but would it make a big difference in speed? I am concerned that having
>> a lot of conditional statements in the template might be slower than
>> doing the work in the view.
>>
>> Can anybody shed some light on this and the trade off (if any) of speed
>> versus flexibility.
>>
>
> A view is Python code, which is compiled into bytecode and then executed
> by the Python interpreter. Whereas a template is interpreted by the
> Python template engine, which is Python code executed by the Python
> interpreter.
>
> A template will will always have that extra layer of interpretation.
> Think of it as a new language with a compiler written in Python. (Which
> it is...) So from the smallest perspective, an if-else in a template
> will always be slower than the equivalent {%if%}-{%else%}-{%endif%} in a
> view.
>
> But before you pull out a stopwatch and move all of your application's
> decisions into the views, think about the long-term maintainability,
> likelihood of bugs, the goodness of separating of "what" code from "how"
> code, etc. The history of software is littered with code that was
> extremely efficient but impossible to maintain or adapt to changing
> requirements. You can double your application's performance by buying
> or leasing a new box + more memory next year. What % improvement do you
> think code shifting from template to view will get you?
>
> John
>
>
> >
>
>
>
>
--
Ned Batchelder, http://nedbatchelder.com
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