tal:content="<expr>" where <expr> evaluates to None omits tag

[Sidnei da Silva]

If you have an expression that evaluates to None, the tag will be omitted from the output. This is problematic, especially when used with tables, which is pretty common. Consider this for example:

<tr>
  <td>Attribute 1</td>
  <td>Attribute 2</td>
</tr>
<tr tal:repeat="item view/foo">
  <td tal:content="item/attribute1">Attribute 1</td>
  <td tal:content="item/attribute2">Attribute 2</td>
</tr>

In this case, if any of (attribute1, attribute2) is None, the <td> will be omitted, causing a broken table to be generated. There are easy workarounds, but the behaviour is surprising and not backwards compatible, and we should opt for the path of least surprise.

[Malthe Borch (mborch)]

One compromise, which makes more sense to me is to render this as a self-closing tag, <td />. This enables developers to realize what happened and guard against it.

However, it may be that for z3c.pt, we should have the ZPT behavior.

\malthe

[Sidnei da Silva (sidnei)]

I believe a self-closing <td/> *might* break some browsers. There are only some tags which are allowed to be self-closing, according to the HTML4 spec (If I remember correctly).

We should definitely have the same behavior as ZPT for z3c.pt. It is fine to be different in chameleon.zpt.

[Malthe Borch (mborch)]

2009/5/29 Sidnei da Silva <email address hidden>:
> I believe a self-closing <td/> *might* break some browsers. There are
> only some tags which are allowed to be self-closing, according to the
> HTML4 spec (If I remember correctly).

Definitely, but so be it ('fix the template, not the engine").

> We should definitely have the same behavior as ZPT for z3c.pt. It is
> fine to be different in chameleon.zpt.

I agree. I'll look into implementing this.

[Malthe Borch (mborch)]

It turns out to be difficult to implement the self-closing variant, so I will go with Sidnei's proposal here.

[Gary Poster (gary)]

We are using the newest chameleon package, *and* we are not actually using chameleon. I can't verify that any problem has been fixed in Launchpad so marking Invalid.