The expr attribute defines an expression that is evaluated to generate a lookup table key for the parent <al-lookup> (5.3.8) tag. When the parent <al-lookup> is executed all of the expr expressions are evaluated to build a dictionary of items.
For example:
>>> import albatross >>> ctx = albatross.SimpleContext('.') >>> ctx.locals.key = 2 >>> albatross.Template(ctx, '<magic>', ... '''<al-lookup name="look"> ... <al-item expr="1">item expr="1" key is <al-value expr="key"></al-item> ... <al-item expr="key">item expr="key" key is <al-value expr="key"></al-item> ... </al-lookup>''').to_html(ctx) >>> ctx.locals.key = 1 >>> t = albatross.Template(ctx, '<magic>', ''' ... <al-value expr="key" lookup="look" whitespace> ... ''') >>> t.to_html(ctx) >>> ctx.flush_content() item expr="1" key is 1 >>> ctx.locals.key = 2 >>> t.to_html(ctx) >>> ctx.flush_content() item expr="key" key is 2