So, are the parameter lines remembered as text and evaluated at the point write is called, or does it form a closure where the format is defined?

Eh... now we're getting into a grey error that I'm not really sure of. I think the variables in the format need to be declared prior to declaring the format, but they get evaluated each time write is called. I'm not sure if that's done using dynamic scoping or not.

In your use case, declaring the format inside your method is probably the cleanest thing to do.