5.3.10.1 expr="..." attribute

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5.3.10.1 `expr="..."` attribute

The `expr` attribute defines an expression that is evaluated to generate a lookup table key for the parent `<al-lookup>` (5.3.9) 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