`Lazy`¶

Load a component fragment when its placeholder enters the document.


Import	`from hedron import Lazy`
Distribution	`hedron`
Backend activity	Immediately after load
Normal render mode	`RenderMode.FRAGMENT`

Live demo¶

DemoCode

Docs simulation — not a running Hedron server. Interactive demos show a “Simulated HTMX” trace when applicable.

Loading account activity…

Minimal runnable app.py that reproduces this demo (real Hedron, not the docs simulator):

app.py

import os

from hedron import ComponentRef, Hedron, Lazy, Loading, Page, html, swap

app = Hedron(
    title="Lazy demo",
    security="standard",
    explorer="off",
    session_secret=os.environ.get("HEDRON_SESSION_SECRET", "dev-only"),
)

box = app.region("lazy-box")
ref = ComponentRef(
    logical_id="activity-feed",
    path="/activity-feed",
    target=box.selector,
    swap="innerHTML",
)


@app.page("/")
def home() -> Page:
    return Page(
        Lazy(
            ref=ref,
            placeholder=Loading("Loading account activity…"),
            target_id=box.id,
        ),
        title="Lazy",
    )


@app.view("/activity-feed", fragment_regions=(box,))
def feed():
    return swap(
        html.div(
            html.strong("3 recent events"),
            html.span("Deployment, approval, and release notes loaded."),
        )
    )

Basic use¶

from hedron import ComponentRef, Lazy, Skeleton
component = Lazy(ref=ComponentRef('activity-feed', '/activity-feed'), placeholder=Skeleton(lines=3), target_id='activity-feed')

Compose under Page for full documents, or return from a fragment route for HTMX swaps.

How it works¶

Lazy emits a load-triggered HTMX request that swaps into an inner #…-body wrapper so a template[data-hedron-error-template] survives a successful load. hedron-ui.mjs (core and FastAPI copies, kept byte-identical) rematerializes the template on htmx:responseError / htmx:sendError.

This component can initiate or represent a backend interaction. The live documentation intercepts that interaction with JavaScript and shows the same pending, success, or replacement states without making a real request. In an application, keep the URL, authorization, validation, and returned fragment on the server; JavaScript is only progressive enhancement.

Constructor and parameters¶

Lazy(*, ref, placeholder=None, target_id=None, error=None)

Parameter	Type	Meaning
`ref`	`ComponentRef`	Typed fragment endpoint.
`placeholder`	`NodeLike \| None`	Initial content; defaults to Loading.
`target_id`	`str \| None`	Explicit host ID; generated collision-free by default.
`error`	`NodeLike \| None`	Error template kept outside the inner `#…-body` swap target.

Composition and backend behavior¶

Keep Lazy at the smallest semantic boundary. Fragment routes should return only the replaced region and preserve stable target IDs across success, validation, empty, loading, and error responses.

Mutating flows must use POST, validate CSRF, authorize on the server, re-validate typed input, and return a bounded fragment. GET remains safe and repeatable; native submit should still work without HTMX.

Accessibility¶

Choose a placeholder that reserves approximately the final space and provide meaningful loading text for material waits.

Security¶

Escaping and SafeUrl / TrustedHtml are framework concerns; authorization and data exposure remain application code. Redact secrets before rendering.

Common mistakes¶

Do not lazy-load content needed to understand or operate the initial page without a robust failure state.
Do not copy docs-preview JavaScript into an application server.

Testing¶

from hedron import RenderMode, render

result = render(component, mode=RenderMode.FRAGMENT)
assert result.html
assert not result.diagnostics

All component demos · Built-in API · Testing

Lazy¶