st-components 0.1.6

React-style stateful components for Streamlit

st-components

st-components is a React-inspired component-based framework using Streamlit as its render engine.

Simply put, st-components adds a higher level component API on top of Streamlit.

It's still 100% Streamlit, same widgets, same runtime, same script-rerun pattern, but it changes the way you think about Streamlit components and how you will combine them to build an app.

Instead of thinking your components as functions, rendering immediately when called, returning their last known value from the UI, and combined in imperative fashion to achieve a more complex app logic (which is the standard Streamlit mental model):

import streamlit as st

def demo():
    st.header("Demo App")
    with st.container(key="pannel",border=True):
        name=st.text_input(key="name_input", label="Enter your name:", value="")
        if name:
            clicked = st.button(key="greet", label="Show greetings!")
            if clicked :    
                st.markdown(f"Hello {name}!")
                st.balloons()

def app():
    demo()

app()

In st-components, you think of components as nested objects, each with a render method returning the widgets it should show on screen, and who manage their state and logic internally via events callbacks. For those who already know React, it should sound quite familiar:

from st_components import App, Component
from st_components.elements import header, container, text_input, button, markdown, balloons

class Demo(Component):

    def __init__(self, **props):
        super().__init__(**props)
        self.state=dict(name=None, clicked=False)

    def on_change(self, name):
        self.state.name=name

    def on_click(self):
        self.state.clicked=True

    def render(self):
        try:
            return (
                header(key="h")("Demo App"),
                container(key="pannel",border=True)(
                    text_input(key="name_input", label="Enter your name:", value="", on_change=self.on_change),
                    button(key="greet", label="Show greetings!", on_click=self.on_click) if self.state.name else None,
                    markdown(key="m", body=f"Hello {self.state.name}!") if self.state.clicked else None,
                    balloons(key="b") if self.state.clicked else None
                )
            )
        finally:
            self.state.clicked = False

app=App()(
    Demo(key="demo")
)

app.render()

Admittedly, the second is a bit more declarative and verbose, so it's probably not that suitable for beginners. But it's much more powerful when it comes to turn components into stateful, reusable and composable building blocks, handling their own state and logic internally.

This short demo already shows the basic idea:

• The base Component class lets you declare custom reusable, stateful and reactive UI units by subclassing it.
• Elements like header, container, text_input etc. are ready-made thin wrappers around Streamlit primitives.
• App serves as the root entry point to render your components. It manages app-level states, render cycles, global config, theming, etc.

Table of Contents

• Installation
• Why This Exists
• Quick Start
• Mental Model
  • Component
  • Element
  • Keys
• Onboarding Path
  • Pattern 1: Simple component with local state
  • Pattern 2: Callbacks
  • Pattern 3: Refs
• API Reference
  • App
  • Elements
  • Component
  • State
  • Props
  • Functional Components
  • Hooks
    • use_state
    • use_context
    • use_memo
    • use_effect
    • use_ref
    • use_callback
    • use_previous
    • use_id
    • Hooks in class Components
  • get_state
  • reset_element
  • Fiber
• Theming and Config
• Built-ins
  • Flow helpers
    • Conditional
    • KeepAlive
    • Case
    • Switch, Match, Default
  • Router and Page
    • Router
    • Page
  • Theme tooling
    • ThemeEditor
    • ThemeEditorDialog
    • ThemeEditorButton
• Examples
• Usage Guidelines
  • Keep keys local and boring
  • Do not persist state manually
  • Think in paths/refs, not instances
• Non-Goals
• Dev
• Contributing
• License

Installation

pip install st-components

st-components builds on :

• Streamlit as its core runtime and UI engine.
• modict for its data models (State, Props, Fiber, Theme, Config, ...). All are still dicts, but they natively support attribute access, typed fields with defaults, runtime type checking/coercion, traversal utilities, etc.

Running an app

Run it like a normal Streamlit app:

streamlit run app.py

The library does not replace Streamlit's execution model. It just adds a component layer on top of the usual Streamlit API.

Why This Exists

Plain Streamlit is fast to start with, low effort for good results, but more sophisticated apps often drift toward:

• One st.session_state to do it all... state data is centralized and doesn't live close to where it's used.
• UI structure gets easily mixed with logic and data processing, dependant on script order/conditions/rerun logic, which can be harder to reason about in complex scenarios.
• Streamlit's contraint on absolute widget keys makes achieving fully autonomous reusable blocks pretty tough (requires careful st.session_state management, and to automate key prefixing to avoid namespace collisions when duplicating a same block)
• Reactive logic can live both in-line and in callbacks, with slightly different behaviour, leading to subtle timing issues

st-components gives you a more explicit structure:

• Components own their layout and local state
• layout logic lives in the render() method
• Reactive logic lives in callbacks and hooks
• keys stay purely local, only required to be unique amongst siblings
• the framework derives full tree paths automatically from the actual nesting of components
• using multiple instances of a same component becomes trivial

Quick Start

from st_components import App, Component
from st_components.elements import button, container


class Counter(Component):

    def __init__(self, **props):
        super().__init__(**props)
        self.state = dict(count=0)

    def increment(self):
        self.state.count += 1

    def render(self):
        return button(key="button", on_click=self.increment)(
            f"Clicked {self.state.count} times"
        )


app = App()(
    container(key="home")(
        Counter(key="counter_1"), # "app.home.counter_1.button" (internally)
        Counter(key="counter_2"), # "app.home.counter_2.button" no collision
    )
)

app.render()

Each Counter keeps its own state across reruns.

The creation syntax is intentionally two-step:

MyComponent(**props)(
    *children
)

First, __init__(...) receives props. Then __call__(...) receives children and puts them in self.props.children (this step can be ommitted if you don't want to pass any children). This is just syntactic sugar so that tree construction feels readable and close enough to JSX within Python language constraints (without having to implement a dedicated JSX parser).

children can still be passed as a prop if necessary, so these two forms are equivalent:

MyComponent(key="intro")("Hello")
MyComponent(key="intro", children=["Hello"])

In practice, the two-step style is the usual one because it makes nested UI trees much easier to read.

Mental Model

Component

A Component is a stateful unit.

• It has persistent local state.
• Its render() method returns Components, Elements, renderable values (anything supported by st.write) or tuples of these.
• A new Python instance is created on each rerun, but its state is restored from a Fiber stored in st.session_state.

Element

An Element is a render primitive.

• It renders into a corresponding Streamlit widget
• Its render() method returns nothing
• The actual value of the widget, if any, lives in st.session_state, accessible by element path or ref.
• You can't declare a custom state on it.

You'll generally use ready-made elements from st_components.elements (all streamlit widgets can be found there) and won't have to bother how they are implemented, unless you want to wrap a custom or third-party widget.

Any Component tree must recursively resolve into a tree of pure Elements.

Keys

Every Component and Element must have a key.

Keys are intentionally local:

• they only need to be unique among siblings
• they are not to be understood as global ids
• the framework will disambiguate from the render context

This means two nodes can both use key="counter" safely if they live in different branches.

In the final rendered tree, paths are derived structurally from the nesting of component keys.

Onboarding Path

If you're new to the library, this is the shortest useful path:

1. Start with App, Component, and a few elements.
2. Use self.state inside components for local UI state.
3. Pipe event handlers to deal with app logic.
4. Use Ref() only when you need path-based reachability later.
5. Add typed State and Props models once the shape stabilizes.

Pattern 1: Declare a simple component with local state

from st_components import Component
from st_components.elements import button, container, markdown


class Panel(Component):

    def __init__(self, **props):
        super().__init__(**props)
        self.state = dict(open=False)

    def toggle(self):
        self.state.open = not self.state.open

    def render(self):
        return container(key="panel", border=True)(
            button(key="toggle", on_click=self.toggle)(
                "Hide details" if self.state.open else "Show details"
            ),
            markdown(key="body")("Lots of details...") if self.state.open else None
        )

This is the preferred place for view state, local mode, and coordination between widgets.

Pattern 2: Callbacks

Callback payloads follow a simple rule:

• if an event carries a useful value, that value is injected into the handler
• otherwise the handler is called with no extra argument

In practice this means:

• on_change(value) for stateful widgets
• on_submit(value) for chat_input
• on_select(value) for selection-capable charts and dataframes
• on_click() for plain buttons

For example, a normal on_change handler receives the current widget value as value:

from st_components import Component
from st_components.elements import text_input


class NameForm(Component):

    def __init__(self, **props):
        super().__init__(**props)
        self.state = dict(name="")

    def sync_name(self, value):
        self.state.name = value

    def render(self):
        return text_input(
            key="name",
            value=self.state.name,
            on_change=self.sync_name,
        )("Name")

There is no separate args / kwargs callback plumbing layer on top of this. If a handler needs more context than the triggering payload, read it from component state, shared state, Refs, or get_state(...).

If the callback does nothing except copy the current widget value into one state field, you do not need to keep a dedicated handler like:

def set_name(self, value):
    self.state.name = value

Use sync_state(...) instead. It is just a convenience shortcut for that common pattern:

text_input(
    key="name",
    value=self.state.name,
    on_change=self.sync_state("name"),
)("Name")

Pattern 3: Use Ref() for logical reachability

Refs are path-based references to a given component or element in the tree. You attach one to a component or element when you want to access its state or value without having to provide its full path. They don't point to the instance directly, only to its location in the tree, which is enough to retrieve its state from the fiber (or its value from st.session_state directly in case of an element).

from st_components import App, Component, Ref, get_state
from st_components.elements import button, container, markdown, text_input


class Counter(Component):

    def __init__(self, **props):
        super().__init__(**props)
        self.state = dict(count=0)

    def increment(self):
        self.state.count += 1

    def render(self):
        return button(key="inc", on_click=self.increment)(
            f"Count: {self.state.count}"
        )


class RefDemo(Component):

    def __init__(self, **props):
        super().__init__(**props)
        self.state = dict(snapshot="")
        self.name_ref = Ref()
        self.counter_ref = Ref()

    def capture(self):
        self.state.snapshot = (
            f"name={get_state(self.name_ref).value or ''}, "
            f"count={get_state(self.counter_ref).count}"
        )

    def render(self):
        return container(key="demo", border=True)(
            text_input(key="name", ref=self.name_ref)("Name"),  # passing it the ref
            Counter(key="counter", ref=self.counter_ref),       # same
            button(key="capture", on_click=self.capture)("Read refs"),
            markdown(key="snapshot")(self.state.snapshot or "Nothing captured yet."),
        )


App()(RefDemo(key="refs")).render()

API Reference

App

App is the singleton root of the component tree and the entry point for every render cycle. Its key is always app, so all resolved component paths start with app.. Call get_app() to retrieve the instance from anywhere in the tree.

Constructor:

App(
    *,
    children=None,
    page_title=None,
    page_icon=None,
    layout=None,
    initial_sidebar_state=None,
    menu_items=None,
    theme=None,
    css=None,
    config=None,
    persist_theme=True,
    persist_config=True,
)

Props:

• children: the single root renderable. The two-step style App()(MyLayout(key="layout")) is preferred — it keeps props and tree structure visually separated.
• page_title, page_icon, layout, initial_sidebar_state, menu_items: forwarded to st.set_page_config(...). Use these instead of calling st.set_page_config directly.
• theme: app-level Streamlit theme. Accepts a Theme instance or a plain dict.
• css: extra CSS injected after the theme. Accepts a raw CSS string, a .css file path, a Path, or a list mixing those forms.
• config: selected Streamlit config values. Accepts a Config instance or a plain dict. Supported sections: client, runner, browser, server.
• persist_theme: if True (default), writes the current theme to .streamlit/config.toml on each render.
• persist_config: if True (default), writes the current config to .streamlit/config.toml on each render.

Methods:

• .render(): run the render cycle for the full app.
• .render_page(page_tree): render a page tree through the active app instance, preserving the current multipage path prefix. Used from file-backed page sources: get_app().render_page(...).
• .create_shared_state(name, spec): declare a named shared state namespace. spec must be a State instance or subclass. Idempotent — calling it again with the same name is a no-op. Read from anywhere with get_shared_state(name).
• .set_theme(theme): replace the in-memory theme. Accepts a Theme, a dict, or None.
• .save_theme(theme=None): optionally update, then persist the theme to .streamlit/config.toml.
• .set_css(css): replace the in-memory CSS string.
• .set_config(config): replace the in-memory Streamlit config. Accepts a Config, a dict, or None.
• .save_config(config=None): optionally update, then persist the config to .streamlit/config.toml.

For multipage apps, Router and Page are normal structural components. The active page lives in the path system as app.router.<page_key>.page....

Elements

All built-in Streamlit widgets are available as ready-made elements in st_components.elements, organized by sub-package:

• text — markdown, title, header, subheader, caption, code, latex, divider
• input — button, checkbox, radio, selectbox, multiselect, slider, text_input, text_area, number_input, date_input, time_input, color_picker, toggle, file_uploader, camera_input, chat_input, download_button
• layout — container, columns, tabs, expander, sidebar, popover, dialog, empty
• display — metric, json, dataframe, data_editor, table, image
• charts — line_chart, bar_chart, area_chart, scatter_chart, altair_chart, plotly_chart, pyplot, map, deck_gl_chart
• media — audio, video
• feedback — progress, spinner, status, toast, balloons, snow, success, info, warning, error, exception

All wrappers share the same two additions over the standard Streamlit signatures:

• key is always required — the framework uses it to derive the element's path in the tree
• ref is always accepted — attach a Ref() to access the element's state later via get_state(ref) or ref.state()

Writing a custom element wrapper

Subclass Element and implement render() when you need to integrate a third-party or custom Streamlit widget into the framework. The main primitive you need:

• KEY(local_key) — resolves local_key against the current path context and returns the full Streamlit widget key to pass to the underlying st.* call
• get_element_path() — returns the resolved path of the current element (its canonical key in the framework)
• set_element_value(path, value) — stores a post-processed value on the element's fiber, making it retrievable via get_state(path).value
• render(child) — renders a child component or element (the free function imported from st_components.core)

The built-in container element illustrates the pattern:

from st_components.core import Element, KEY, get_element_path, render, set_element_value

class container(Element):
    def __init__(self, *, key: str, ref=None, border=None, **kwargs):
        Element.__init__(self, key=key, ref=ref, border=border, **kwargs)

    def render(self):
        container_obj = st.container(key=KEY("raw"), **self.props.exclude("key", "children", "ref"))
        set_element_value(get_element_path(), container_obj)
        with container_obj:
            for child in self.children:
                render(child)

set_element_value is what makes the element's output retrievable later. For pure output widgets that produce no value (e.g. markdown, balloons), you can omit it. For widgets that natively write into st.session_state by Streamlit key (e.g. st.text_input), the value is already accessible there — KEY(...) ensures the key matches what the framework expects.

Component

Component is the base class for all stateful UI units. Subclass it and implement render() to return the subtree this component should produce — a Component, an Element, a tuple of those, a plain renderable value, or None.

Instance members:

• self.props — a Props modict populated from constructor arguments. Supports attribute access (self.props.color). None values in children are filtered out automatically.
• self.children — shorthand for self.props.children. A list of the positional arguments passed via MyComponent(key="x")(*children).
• self.state — the component's local state. Before mount, writes go to a temporary dict; after mount, reads and writes go through the fiber. Can be initialized by assigning a dict in __init__, or by declaring a typed nested State subclass.
• self.is_mounted — True if the component has an active fiber in the current session.

Methods:

• set_state(other=None, /, **kwargs) — update state fields. Accepts a dict positional argument, keyword arguments, or both. Works whether or not the component is mounted.
• sync_state(state_key) — returns a one-argument callback that writes its argument into self.state[state_key]. Shorthand for simple on_change handlers that just mirror a widget value into state.

Lifecycle methods (override as needed, default implementation is a no-op):

• component_did_mount() — called once when the fiber is first created (first render of this path).
• component_did_unmount() — called when the fiber is dropped (component left the tree for a full cycle without keep_alive).
• component_did_update(prev_state) — called at the end of each render cycle where self.state changed. prev_state is a snapshot of state from the previous cycle.

Fragment support:

Pass fragment=True as a class keyword to wrap the component's render in st.fragment, enabling partial reruns scoped to this component without rerunning the full app:

class LiveChart(Component, fragment=True, run_every="2s"):

    def render(self):
        ...

run_every accepts a duration string ("2s", "500ms") or a timedelta. When set, Streamlit re-renders the fragment on that interval automatically.

State

State is local, persistent per mounted component path, and restored automatically across reruns.

You can initialize state directly in __init__:

def __init__(self,**props):
    super().__init__(self,**props)
    self.state = dict(count=0, label="clicks)

Or skip the __init__ override and declare a typed nested subclass:

from st_components import Component, State
from st_components.elements import button, container, metric


class Counter(Component):

    class CounterState(State):
        _config=State.config(extra="ignore")
        count: int = 0
        label: str = "clicks"

    def increment(self):
        self.state.count += 1

    def render(self):
        return container(key="panel", border=True)(
            metric(key="metric", label=self.state.label, value=self.state.count),
            button(key="inc", on_click=self.increment)("Increment"),
        )

Typed state gives you defaults, validation, and a visible schema. It is worth introducing once a component's state shape stabilizes or when you want stronger guarantees than an ad hoc dict.

Props

Props work the same way: you can stay dynamic, or declare a typed nested Props subclass once the interface of a component becomes important enough to formalize.

You can declare typed props with a nested Props subclass:

from modict import modict
from st_components import Component, Props
from st_components.elements import markdown


class Badge(Component):

    class BadgeProps(Props):
        _config = Props.config(extra="forbid")
        label: str = "badge"
        color: str = "blue"

    def render(self):
        return markdown(key="body")(f":{self.props.color}[**{self.props.label}**]")

This is useful when you want defaults, validation, or stricter control over accepted inputs.

Functional Components

Use @component to turn functions into components. The function behaves as the render method and should accept a props parameter.

from st_components import App, component
from st_components.elements import container, markdown


@component
def Callout(props):
    return container(key="box", border=True)(
        markdown(key="body")(f"**{props.title}**\n\n{props.children[0]}")
    )

When calling the component, you still pass individual props and children normally rather than a props dict. The decorator wraps them into the framework Props object and passes it to the function when rendering.

app = App()(
    Callout(key="intro", title="Heads up")(
        "This is a functional component"
    )
)
app.render()

Hooks

Hooks are the general mechanism for storing and managing information on the mounted component fiber rather than on the transient Python instance recreated on each rerun.

This matters because component instances are not persistent across reruns, but fibers are. Hooks therefore give you a place to keep state, memoized values, technical refs, effects, and other render-adjacent data that must survive from one render cycle to the next.

Hooks are relevant for both functional components and class-based components:

• in functional components, they are the primary way to access persistent local state and render-cycle helpers
• in class components, they complement self.state when you need persistent technical data that should not live in ordinary instance attributes

Hooks are evaluated in call order during render, and their data persists on the mounted component fiber.

Available hooks:

• use_state(...): local render state for functional components. This is the functional equivalent of self.state.
• use_context(context): read a tree-scoped ambient value from the nearest matching provider.
• use_memo(factory, deps=None): memoize a computed value between renders.
• use_effect(effect, deps=None): run an effect after render, with optional cleanup support.
• use_ref(initial=None): keep a mutable technical value across renders through .current.
• use_callback(callback, deps=None): memoize a callback identity. This is a small convenience wrapper over use_memo(...).
• use_previous(value, initial=None): read the value from the previous render.
• use_id(): get a stable id for the current hook slot in the mounted component.

use_state

Use use_state() when a functional component needs local render state:

from st_components import component, use_state
from st_components.elements import button, markdown


@component
def Counter(props):
    state = use_state(count=0)

    def increment():
        state.count += 1

    return (
        markdown(key="value")(f"Count: **{state.count}**"),
        button(key="inc", on_click=increment)("Increment"),
    )

You can also pass a typed State instance:

from st_components import State, component, use_state
from st_components.elements import button, markdown


class CounterState(State):
    count: int = 0
    step: int = 1


@component
def Counter(props):

    state = use_state(CounterState(count=0, step=2))

    def increment():
        state.count += state.step

    return (
        markdown(key="value")(f"Count: **{state.count}**"),
        button(key="inc", on_click=increment)(f"+ {state.step}"),
    )

use_context

Use create_context(initial_context_data) to define a typed tree-scoped ambient value, wrap a subtree with MyContext.Provider(...), then read it from any descendant with use_context(MyContext).

This is useful when a value should be shared across a branch without being threaded manually through several layers of props.

Unlike shared_state, context is scoped by tree position rather than by global namespace. Two different branches can therefore provide different values for the same context at the same time.

Contexts are typed through ContextData. The initial context object is the default value returned by use_context(...) when no provider is present, and later providers replace the current context for their subtree with a new instance of the same schema.

You can pass either:

• a ContextData instance, if you want a custom typed subclass
• or a plain dict, which is automatically cast to the base ContextData class and then returned as such by use_context(...)

The same rule applies to Provider(data=...): it accepts either a ContextData instance or a plain dict, normalizes it to the context's original schema class, and rejects anything else. The provider does not implicitly merge with the previous scoped value.

Resolution follows the rendered tree:

• if a matching provider exists above the current component, use_context(...) returns the value from the nearest one
• otherwise it returns the context default
• nested providers override outer ones naturally for their own subtree

from st_components import ContextData, component, create_context, use_context


class ThemeData(ContextData):
    mode: str = "light"


ThemeContext = create_context(ThemeData(mode="light"))


@component
def Toolbar(props):
    theme = use_context(ThemeContext)
    return f"Theme: {theme.mode}"

App()(
    ThemeContext.Provider(key="theme_scope", data={"mode": "dark"})(
        Toolbar(key="toolbar")
    )
).render()

The provider is a component, so its key also appears in paths such as app.theme_scope.toolbar.

use_memo

Use use_memo(factory, deps) to reuse a computed value until its dependencies change.

• deps=None: recompute on every render
• deps=[]: compute once per mounted component
• otherwise: recompute only when the deps tuple changes between renders

from st_components import component, use_memo


@component
def Summary(props):
    total = use_memo(
        lambda: sum(props.values),
        deps=[tuple(props.values)],
    )
    return f"Total: {total}"

use_effect

Use use_effect(effect, deps) for post-render work.

• the effect runs after render
• if it returns a callable, that callable is used as cleanup
• cleanup runs before the effect reruns and when the component unmounts

from st_components import component, use_effect


@component
def Logger(props):
    use_effect(
        lambda: print(f"value changed to {props.value}"),
        deps=[props.value],
    )
    return None

use_ref

Use use_ref(initial) for mutable technical state that should persist across renders without being part of the render state.

from st_components import component, use_ref


@component
def PreviousTracker(props):
    previous = use_ref(None)
    seen = previous.current
    previous.current = props.value
    return f"Previous: {seen}"

use_callback

Use use_callback(callback, deps) when you want a stable callback identity between renders.

Conceptually:

use_callback(fn, deps) == use_memo(lambda: fn, deps)

use_previous

Use use_previous(value) when you want the previous render's value directly.

from st_components import component, use_previous


@component
def Delta(props):
    previous = use_previous(props.value)
    return f"Previous={previous}, Current={props.value}"

use_id

Use use_id() when you need a stable per-hook identifier for the mounted component.

Hooks in class Components

Hooks can also be used the same way in the render() method of class Components but, for some of them, the class has alternatives to using them:

• use_state(...) -> self.state
• use_effect(...) -> component_did_mount(), component_did_update(prev_state), component_did_unmount()
• use_callback(...) -> a normal instance method
• use_previous(...) -> a prev_state inside component_did_update(...)

The point is:

• hooks are used for persistent render-cycle data that must survive the short-lived component instance
• self.state and lifecycle methods of the class-oriented API surface can do the trick in some cases.

get_state(path_or_ref=None)

Returns the current state of any rendered Element or mounted Component, or None if the path doesn't resolve to a live fiber.

• for an Element, returns an ElementState with a single value field holding the current widget value (set by Streamlit or by set_element_value)
• for a Component, returns its State object, the same dict-like object as self.state during render
• path_or_ref may be omitted inside a running render or callback, defaulting to the current caller

Examples:

• element value: get_state("app.form.name").value
• component state: get_state("app.form.counter").count
• inside the current context: get_state()
• via a ref: get_state(ref) or ref.state()

reset_element(path_or_ref)

Forces a stateful Element to be recreated on the next rerun, so its declared default value is applied again.

reset_element(name_ref)

Fiber

A Fiber is the persistent record that gives a component its continuity across reruns.

Because Streamlit reruns the entire script from top to bottom on every user interaction, component Python instances are short-lived — they are created fresh on each rerun and discarded at the end of it. The fiber is what survives between reruns: it lives in st.session_state, keyed by the component's resolved path (e.g. app.home.counter), and is the actual source of a component's persistent identity.

What a fiber holds:

• state — the component's local state dict, restored into self.state at render time
• previous_state — a snapshot of state from the previous render cycle, used to detect changes and feed component_did_update(prev_state)
• component_id — a UUID linking the fiber to the current Python instance, so lifecycle callbacks (component_did_update, component_did_unmount) reach the right object
• hooks — an ordered list of HookSlot entries, one per use_* call, each carrying the hook's value, deps, and optional cleanup
• keep_alive — a flag set by flow helpers (Conditional, KeepAlive, Switch, ...) to prevent a hidden branch from being unmounted even though it didn't render in this cycle

The render cycle:

1. App.render() calls begin_render_cycle(): all fibers have keep_alive reset to False, and a snapshot of all current states is saved.
2. Each component renders: its path is derived from the nested key context, the fiber at that path is looked up (or created on first render), state is restored from it, hooks are replayed in order, and the fiber is marked as rendered for this cycle.
3. App.render() calls end_render_cycle():
   • Fibers that were not marked as rendered (and whose keep_alive is False) are unmounted: component_did_unmount() fires and the fiber is deleted.
   • Fibers whose state changed compared to the snapshot trigger component_did_update(prev_state).
   • Pending hook effects are flushed (effects run, previous cleanups are called first if deps changed).
   • previous_state is updated for the next cycle.

On first render, the fiber does not yet exist: mount() creates it and component_did_mount() fires.

Path derivation is purely structural: every key_context(self.key) call pushes the current key onto a thread-local stack, so a component nested as App > container("home") > Counter("c1") automatically resolves to app.home.c1 without any manual plumbing.

Hook slots are claimed by index in call order. The framework checks that the count and order of hook calls stays consistent between renders — calling hooks conditionally raises a RuntimeError, for the same reason as in React.

You generally do not interact with fibers directly. They are an implementation detail. But understanding them explains:

• why self.state survives reruns even though self does not
• why path-based reachability (Ref, get_state) works without holding object references
• why flow helpers need an explicit keep_alive mechanism rather than relying on plain Python conditionals
• why hook call order must be stable

Theming and Config

Use theme=..., css=..., and config=... on App(...) to control the visual shell of the app.

• theme covers Streamlit's theme tokens
• css covers custom styling outside those tokens
• config covers the supported Streamlit config sections exposed by the library

Use a Theme passed to App to control Streamlit theming (a plain dict also works):

from st_components import App, Theme


app = App(
    theme=Theme(
        base="dark",
        primaryColor="#2dd4bf",
        backgroundColor="#0f172a",
        textColor="#e2e8f0",
        sidebar={"backgroundColor": "#111827"},
    ),
)(
    MyLayout(key="layout"),
)

You can also use the built-in ThemeEditorButton to tune a theme visually while building your app:

from st_components import App, Component
from st_components.builtins import ThemeEditorButton
from st_components.elements import container, markdown


class Home(Component):

    def render(self):
        return container(key="page")(
            container(key="hero", border=True)(
                markdown(key="title")("# Hello"),
                markdown(key="body")("Use the built-in theme editor to tune the app live."),
                ThemeEditorButton(key="open", type="primary", title="Theme editor")(),
            )
        )


app = App()(
    Home(key="home")
)
app.render()

This is useful during development when you want to find a good theme quickly, then later replace it with a fixed theme in App(theme=...) or config.toml once the design is settled.

Notes:

• Theme fields name map to official Streamlit theme config keys
• theme persistence goes through .streamlit/config.toml
• live theme change is best-effort; some changes may require a complete rerun, or a restart.
• the persisted config in .streamlit/config.toml is the default source.
• CSS is injected after theme application, so CSS can intentionally override theme-driven styles

To see this live:

python -m st_components.examples theme_editor

Built-ins

st_components.builtins contains higher-level structural helpers built on top of the core component model.

from st_components.builtins import (
    Conditional, Case, Switch, Match, Default,
    KeepAlive, ThemeEditorButton, ThemeEditorDialog, Router, Page,
)

Flow helpers

Flow helpers let you express conditional and switched rendering declaratively, as part of the component tree.

The key property they share is state preservation: hidden branches are not unmounted — their fibers are kept alive so that state is not lost when a branch becomes visible again. A plain Python if would discard the hidden component's fiber on every rerun; flow helpers avoid that.

Conditional

Renders its first child when condition is True, its optional second child otherwise.

from st_components.builtins import Conditional

Conditional(key="toggle", condition=self.state.logged_in)(
    Dashboard(key="dashboard"),
    LoginForm(key="login"),
)

KeepAlive

Renders its single child when active=True, hides it when False, but keeps the fiber alive in both cases.

from st_components.builtins import KeepAlive

KeepAlive(key="panel", active=self.state.show_panel)(
    HeavyPanel(key="content"),
)

Useful when hiding a subtree that is expensive to reinitialize, or when you want to preserve its internal state without rendering it.

Case

Selects one child by integer index. All other branches have their fibers preserved.

from st_components.builtins import Case

Case(key="step", case=self.state.current_step)(
    StepOne(key="step_1"),
    StepTwo(key="step_2"),
    StepThree(key="step_3"),
)

Switch, Match, Default

Switch matches its value against a set of Match(when=...) children and renders the matching one, falling back to Default if present. All unmatched branches have their fibers preserved.

from st_components.builtins import Switch, Match, Default

Switch(key="view", value=self.state.active_tab)(
    Match(key="home",     when="home")(HomePage(key="page")),
    Match(key="settings", when="settings")(SettingsPage(key="page")),
    Default(key="fallback")(NotFound(key="page")),
)

Router and Page

Router and Page compile Streamlit's multipage navigation while keeping all pages inside the normal component path system.

Router

Declares the navigation structure. Its children must all be Page instances.

from st_components.builtins import Router, Page

Router(key="router", position="sidebar")(
    Page(key="home",     nav_title="Home",     default=True)(HomePage),
    Page(key="settings", nav_title="Settings")(SettingsPage),
)

Props:

• position: where the navigation is rendered — "sidebar" (default), "top", or "hidden".
• expanded: whether the sidebar nav is expanded by default.

Page

Wraps a page source and declares its navigation metadata. The source is passed as the single child and can be:

• a Component class or instance
• a callable
• a file path or path string (for file-backed pages)

Page(
    key="report",
    nav_title="Report",
    nav_icon=":material/bar_chart:",
    url_path="report",
    section="Analytics",
    default=False,
    visibility="visible",
    layout="wide",
)(ReportPage)

Props:

• nav_title, nav_icon: label and icon shown in the navigation.
• url_path: explicit URL path segment for this page.
• default: if True, this page is shown when no URL path matches.
• section: groups pages under a heading in the navigation.
• visibility: "visible" (default) or "hidden" — hides the page from navigation without removing it.
• layout, page_title, page_icon, initial_sidebar_state, menu_items: per-page overrides of Streamlit page config.

Pages live inside the component path system: a page component rendered from Page(key="report") under Router(key="router") produces paths like app.router.report.page....

Theme tooling

ThemeEditor

A full inline theme editor widget. Exposes controls for base theme, font, colors, corner radii, widget borders, and optional custom CSS. Changes are applied live and can be saved to .streamlit/config.toml.

from st_components.builtins import ThemeEditor

ThemeEditor(key="editor")()

ThemeEditorDialog

Wraps ThemeEditor in a Streamlit dialog. Useful when you want the editor accessible but out of the way.

from st_components.builtins import ThemeEditorDialog

ThemeEditorDialog(key="dialog", open=self.state.open, title="Theme editor", width="large")

Props: open, title, show_css, width.

ThemeEditorButton

A button that opens a ThemeEditorDialog. The most common entry point during development.

from st_components.builtins import ThemeEditorButton

ThemeEditorButton(key="theme_btn", type="primary")("Edit theme")

Props: label, title, show_css, width, type, help, icon, use_container_width, disabled, shortcut.

See the Theming and Config section for the recommended development workflow.

Examples

The repository includes several runnable examples. They are the fastest way to see the library's patterns in context:

• python -m st_components.examples basic
• python -m st_components.examples dashboard
• python -m st_components.examples functional
• python -m st_components.examples flow
• python -m st_components.examples hooks
• python -m st_components.examples multipage
• python -m st_components.examples theme_editor
• python -m st_components.examples primitives

What they are good for:

• basic: smallest class-component examples
• primitives: broad survey of available elements
• dashboard: larger composed UI with more realistic structure
• functional: @component and use_state() patterns
• hooks: compact overview of the hook system in one screen
• flow: structural built-ins such as switching and conditional rendering
• multipage: demonstrates a bit more advanced patterns: router, pages, component-backed pages, file-backed pages, shared state, and a lightweight provider above the router
• theme_editor: live theme tuning workflow

You can also run example files directly from the repository with streamlit run examples/<name>.py when that is more convenient.

If you want the fastest onboarding path, start with:

1. python -m st_components.examples basic
2. python -m st_components.examples primitives
3. python -m st_components.examples dashboard
4. python -m st_components.examples functional
5. python -m st_components.examples hooks
6. python -m st_components.examples theme_editor

Usage Guidelines

Keep keys local and boring

Keys identify siblings inside one branch, not global entities across the whole app.

Good:

• key="name"
• key="filters"
• key="save"

Bad:

• globally namespaced keys everywhere
• encoded hierarchy inside user keys

-> This is already done internally

Do not persist state manually in st.session_state

The framework already does that for you. Reach local component state with self.state, and reach any element or component state via get_state(...) or ref.state().

If you need custom state shared across several components, declare it once with app.create_shared_state("my_custom_state", State()) and consume it with get_shared_state("my_custom_state").

Think in paths/refs, not instances

Because every rerun recreates the tree, the stable identity of a component is its location in the tree materialized as a resolved path, or a Ref pointing to it, not the Python object from a previous run.

Component instances are short-lived and not persisted, they won't survive the current render cycle. They will be discarded in the garbage collector at the end of it.

From the point of view of the framework's internals, they are mostly wrappers around their render function and hold no precious state.

All that gives them a useful continuity (state, etc.) is persisted in and fetched from the Fiber at rendering time

This is why component coordination should preferably use the dedicated API:

• local state for behavior internal to one component
• context for ambient values shared by one subtree
• shared state for app-level coordination
• refs when you need path-based reachability to a specific mounted node

Any custom data that's attached only on the instance will die with it at the end of the current cyle.

Non-Goals

st-components is not trying to provide:

• a virtual DOM
• JSX syntax/parser
• a replacement for Streamlit's execution model

It is a structuring layer over Streamlit, not a different frontend runtime.

Dev

Install with dev dependencies:

pip install -e ".[dev]"

Run the test suite:

pytest

Tests cover the core component model, elements, hooks, functional components, context, and the examples runner. They run without a live Streamlit server.

Contributing

Contributions are welcome — don't wait for me to think of everything.

If you have an idea for a new built-in, a missing element wrapper, a hook, a better API surface, or just spotted something odd: open an issue or send a PR. The codebase is small and intentionally kept that way, so it is easy to navigate.

Things that are especially useful:

• bug reports with a minimal reproducible example
• missing element wrappers (anything from the Streamlit docs not yet covered)
• new built-ins for structural patterns you hit repeatedly
• docs improvements — if something was unclear to you, it is unclear to the next person too

License

MIT