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.Zope uses the term acquisition to refer to this dynamic use of content and behavior.Now that you see how site structure fits into the way names are looked up, you can begin to understand thatwhere you place objects you are looking for is very important.An example of acquisition that you've already seen is how web pages use standard headers and footers.Toacquire the standard header just ask Zope to insert it with the var tag:It doesn't matter where your DTML Method or Document is located.Zope will search upwards until it findsthe standard_html_header that is defined in the root folder.You can take advantage of how Zope looks up variables to customize your header in different parts of yoursite.Just create a new standard_html_header in a folder and it will override global header for all web pagesin your folder and below it.Create a folder in the root folder with an id of Green.Enter the Green folder and create a DTML Documentwith an id of welcome.Edit the welcome document to have these contents:WelcomeNow view the welcome document.It should look like a simple web page with the word welcome, as shown inFigure 4-2.Dynamically Acquiring Content 53 IntroductionFigure 4-2 Welcome document.Now let's customize the header for the Green folder.Create a DTML Method in the Green folder with an idof standard_html_header.Then edit the contents of the header to the following:body {color: #00FF00;}p {font-family: sans-serif;}Notice that this is not a complete web page.This is just a fragment of HTML that will be used as a header.This header uses CSS (Cascading Style Sheets) to make some changes to the look and feel of web pages.Now go back to the welcome document and view it again, as shown in Figure 4-3.Dynamically Acquiring Content 54 IntroductionFigure 4-3 Welcome document with custom header.The document now looks quite different.This is because it is now using the new header we introduced in theGreen folder.This header will be used by all web pages in the Green folder and its sub-folders.You can continue this process of overriding default content by creating another folder inside the Green folderand creating a standard_html_header DTML Method there.Now web pages in the sub-folder will use theirlocal header rather than the Green folder's header.Using this pattern you can quickly change the look andfeel of different parts of your web site.If you later decide that an area of the site needs a different header, justcreate one.You don't have to change the DTML in any of the web pages; they'll automatically find the closestheader and use it.Using Python Expressions from DTMLSo far we've looked at simple DTML tags.Here's an example:This will insert the value of the variable named getHippo, whatever that may be.DTML will automaticallytake care of the details, like finding the variable and calling it.We call this basic tag syntax name syntax todifferentiate it from expression syntax.DTML expressions allow you to be more explicit about how to find and call variables.Expressions are tagattributes that contain snippets of code in the Python language.For example, instead of letting DTML findand call getHippo, we can use an expression to explicitly pass arguments:Here we've used a Python expression to explicitly call the getHippo method with the string argument, witha large net.To find out more about Python's syntax, see the Python Tutorial at the Python.org web site.Using Python Expressions from DTML 55 IntroductionMany DTML tags can use expression attributes.Expressions make DTML pretty powerful.For example, using Python expressions, you can easily testconditions:Foo is less than bar.Without expressions, this very simple task would have to be broken out into a separate method and wouldadd a lot of overhead for something this trivial.Before you get carried away with expressions, take care.Expressions can make your DTML hard tounderstand.Code that is hard to understand is more likely to contain errors and is harder to maintain.Expressions can also lead to mixing logic in your presentation.If you find yourself staring blankly at anexpression for more than five seconds, stop.Rewrite the DTML without the expression and use a Script to doyour logic.Just because you can do complex things with DTML doesn't mean you should.DTML Expression GotchasUsing Python expressions can be tricky.One common mistake is to confuse expressions with basic tagsyntax.For example:and:will end up giving you two completely different results.The first example of the DTML var tag willautomatically render variables.In other words it will try to do the right thing to insert your variable, no matterwhat that variable may be.In general this means that if the variable is a method it will be called withappropriate arguments.This process is covered more thoroughly in Chapter 7.In an expression, you have complete control over the variable rendering.In the case of our example,objectValues is a method.So:Will call the method.But:will not call the method, it will just try to insert it.The result will be not a list of objects but a string such as.If you ever see results like this, there is a good chance thatyou're returning a method, rather than calling it
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