[Zope] TAL: test() or condition Howto?

Hi, I have the followong TAL-expression: <td tal:content="structure python:here.test(myImportantCondition and myExpensiveMethod(), myExpensiveMethod(), 'myDefaultString')">myDefaultString</td> To execute 'myExpensiveMethod()' 'myImportantCondition' must be 'true'. But I want to avoid to execute 'myExpensiveMethod()' two times. How can I achive that? As default-value myDefaultString should be served. I don't know a way with tal:condition 'cause on 'false' there is no way to get 'myDefaultString', isn't it? -- many thanks for your replies, Elena