Conditional attributes with `ABSENT`¶

Problem. You want an attribute to appear sometimes and disappear other times, without two parallel branches that construct the tag differently.

Solution. Set the attribute to a callable that returns :data:ABSENT when you want it gone, and a value otherwise. tagz omits the attribute entirely when the result is ABSENT.

from tagz import html, ABSENT

state = {"open": True}

def aria_expanded():
    return "true" if state["open"] else ABSENT

panel = html.div("...", aria_expanded=aria_expanded)
assert 'aria-expanded="true"' in panel.to_string()

state["open"] = False
assert "aria-expanded" not in panel.to_string()

Why a sentinel, not `None`?¶

None is reserved for boolean attributes — it renders the attribute as a bare name (<input checked>). If you want the attribute removed, ABSENT is the explicit signal.

Callable returns	Rendering
`"value"`	`attr="value"`
`None`	bare `attr` (boolean)
`ABSENT`	omitted entirely
`True`	bare `attr` (boolean)
`False`	omitted entirely

Static example¶

You can also pass ABSENT directly (not via a callable) — handy when computing attributes inline:

from tagz import html, ABSENT

def label_for(disabled: bool):
    return html.input(
        type="text",
        disabled=None if disabled else ABSENT,
    )

assert "disabled" in label_for(True).to_string()
assert "disabled" not in label_for(False).to_string()

See also¶