DTML Basics (part 2) - Comparing Apples And Oranges
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The example you've just seen was very rudimentary. To really add some punch, you need to know how to construct what the geeks call a conditional statement. And the very basis of a conditional statement is comparison - for example, "if this is equal to that, do thus and such".

Since Zope is built around Python, and Python comes with a bunch of useful operators designed specifically for use in conditional statements, you can use Python operators to build conditional statements in your DTML. Here's a list:

Assume delta = 12 and omega = 9



Operator  What It Means           	Expression

Evaluates To

------------------------------------------------------------------------

-



==        is equal to                   delta == omega          False



!=        is not equal to               delta != omega          True



        is greater than               delta > omega           True



<         is less than                  delta < omega           False



=        is greater than or equal to   delta >= omega          True



<=        is less than or equal to      delta <= omega          False

These comparison operators can be used for both strings and numbers. A positive result returns true (1), while a negative result returns false (0).

Here's an example, a DTML Method named "access", which illustrates how these can be used:

<dtml-if expr="name == 'neo'">

<font face="Arial" size="-1">

Welcome to the Matrix, Neo. Access granted.

</font>

</dtml-if>



<dtml-if expr="name != 'neo'">

<font face="Arial" size="-1">

I wonder if you've heard of Shakespeare, <dtml-var name>.

<p>He postulated that a rose by any other name would smell just as

sweet.</p> <p>Unfortunately for you, I disagree. Access denied.</p>

</font> </dtml-if>

Now, if you view this example in your browser, you'll see an ugly Zope error - this is because Zope cannot locate the variable "name", despite its best attempts to acquire it from the global namespace. Therefore, in order to see this script work properly, you need to first make sure that a variable named "name" actually exists. You can do this by adding this variable to the REQUEST object, via the URL GET method; just call the DTML script using a URL similar to this one:

http://localhost:8080/DTML%20Basics/access?name=Joe

Here's the output:.

I wonder if you've heard of Shakespeare, Joe. 



He postulated that a rose by any other name would smell just as sweet.



Unfortunately for you, I disagree. Access denied.

Flip it around to see what it looks like when the variable named "name" holds the value "neo". Here's the URL to use,

http://localhost:8080/DTML%20Basics/access?name=neo

and here's the output:

Welcome to the Matrix, Neo. 

In this case, since the conditional expression in the "if" statement evaluates to true, the output displayed by the script will be different.

There's one more thing that needs to be explained in the code above - the "expr" keyword. Normally, you can directly test whether or not a variable exists using the <dtml-var> or the <dtml-if> tags (as I've done in my very first example). However, if you want to do anything more complicated - for example, evaluate a DTML expression such as

"name == 'neo'" 

or

"temperature > 98.6"

- you need to first tell the DTML interpreter that you're talking about an expression and not a variable. And that's where "expr" comes in - it lets Zope know that it's dealing with a conditional expression, and not just a regular variable.

>>> More Zope Articles          >>> More By Harish Kamath, (c) Melonfire