5.4.6.2 expr="..." attribute

This attribute must the specified. It contains an expression that is evaluated when the template is executed and the result is written as a string to the output.

For example:

>>> import albatross
>>> ctx = albatross.SimpleContext('.')
>>> ctx.locals.items = ['pencil', 'eraser', 'lunchbox']
>>> albatross.Template(ctx, '<magic>', '''
... There are <al-value expr="len(items)" whitespace> items
... ''').to_html(ctx)
>>> ctx.flush_content()
There are 3 items