Edit-time Error Handling

Here is a proposal for dealing with TAL errors in saved templates.


A Presentation Designer is creating or editing a template, not through the Zope Management Inferface, but through client software like GoLive or Dreamweaver.


The HTML file saved to Zope may have METAL/TAL syntax errors. Such a file is no longer a valid TAL template. However, the file is most likely okay HTML (as verified by the client software). What do we do when the saved file is broken?


  • Refusing to save the file (WebDAV or FTP PUT error) is very misleading. There is no consistent way to send back the intent of the rejection, so the user thinks that Zope is broken.
  • Silently ignoring the error means bad pages. Bad pages mean bad sites. Bad sites mean bad press.
  • The Presentation Designer has a workflow. TAL is already disruptive enough to the workflow. In addition, using ZPT is also a change in the way things are done. The potential for more errors will cause the designer to question how useful doing templates and Zope is to begin with.
  • The ZPT management interface can do fancy logging and presentation of the TAL syntax errors but a Presentation Designer may not want to leave his/her neat little GoLive world to wrangle with HTML forms (and the synchronization problem that editing in two places can introduce).
  • The workflow of the PresentationDesigner must be minimally impacted
    by the adoption of ZPT. In particular, TAL syntax errors will constantly remind the designer that they are working in a new paradigm. A paradigm not seamlessly supported by their HTML editing tool.


Mark the saved page as broken. When the page is rendered through a browser (from Zope), it should display detailed information (an error log) about what is wrong.

In addition, embed the error log in the original page. This way, the next time the page is loaded into the editor, the error will be evident. The designer will need to adopt a new process in which he reloads a page from Zope (through WebDAV or FTP) in order to make sure that a saved page has no errors.

A page with an error should contain a block of comments (after the DOCTYPE, XML declaration or the first place where a comment is legal) with text containing representing the error log, presented in such a manner that it is very clear that something is wrong. The log should be as verbose as necessary to point out exactly where the error occurs, the nature of the error and, perhaps, hints about how to fix it.


21march2001 - This has been reworked to address some of the concerns raised by Comments #1, #2 and #3*


Duncan (Mar 13, 2001 9:49 am; Comment #1) –

One option to consider would be to allow the client software to always succeed when it attempts to save some text, whether or not it is valid XML.

If the saved page is valid, then it is compiled and everything works as usual. If the saved text is invalid, then the actual text is saved and returned to the user on request, but any attempt to use the template will throw an exception with details of the error. Ideally the error message should also be displayed to the user when the invalid text is saved.

My reason for suggesting this is quite simple. If a user can get in a situation where they cannot save their work, and potentially cannot spot their mistake, this is a bad thing. But if the user is allowed to save at any point, then they or someone else can come back and fix the problem later.

evan (Mar 14, 2001 5:22 pm; Comment #2) –

I agree with Duncan. Saves should almost always succeed, except where the document is clearly nothing like HTML (eg. user accidentally tries to save a .jpeg to a template). If the page contains errors that prevent compilation, the list of errors is saved, and an indication is returned along with the success code (if possible). Now the template is in a broken state, and attempts to render it produce an error report instead, with complete descriptions of what needs to be fixed. Also, whenever the source is loaded, an HTML comment block is prepended. This would require scanning for and removing this block on each save.

Now, we have two strategies. After each save you reload the page into either your editor, or into a browser window open to the page on which you’re working. In the editor, a comment will appear right at the start, and in the browser a complete error report will appear.

gvanrossum (Mar 20, 2001 12:28 pm; Comment #3) –

The main text is inconsistent with Evan’s last comment. Is the error inserted in the source as an HTML comment or as an HTML ‘div’ section?

If we use a ‘div’, I question where to insert it. Right after ‘html’ would mean just before the ‘head’ element which might make the ‘head’ section placed invalid according to the DTD (?). Maybe a comment is safer? That could go almost right at the top (but after <?xml…?> and after <!DOCTYPE …>).

[Gosh I hate structured text.]

tcoram (Mar 21, 2001 9:46 am; Comment #4) –

Oops. Not after ‘html’. That was a mistake, I meant after ‘body’… But, anyway, it probably makes more since to include the errors in comments. For visual editors, like Dreamweaver, my concern was that comments are hidden in visual mode and the designer wouldn’t notice the erorr log. However, Dreamweaver (at least DW4) shows a yellow-highlighted question marks where comments should be (which can be expanded on demand).

Commments make a lot of sense. That way we aren’t mucking with the visual structure of the document: If they want to live with an error (to be caught later?) they can.