st-error-boundary 0.1.5

A tiny, typed error-boundary decorator for Streamlit apps (UI-safe fallback + pluggable hooks)

st-error-boundary

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A minimal, type-safe error boundary library for Streamlit applications with pluggable hooks and safe fallback UI.

Motivation

Streamlit's default behavior displays detailed stack traces in the browser when exceptions occur. While client.showErrorDetails = "none" prevents information leakage, it shows only generic error messages, leaving users confused. The typical solution—scattering st.error() and st.stop() calls throughout your code—severely degrades readability and maintainability, and creates a risk of forgetting exception handling in critical places.

This library solves the problem with the decorator pattern: a single "last line of defense" decorator that separates exception handling (cross-cutting concern) from business logic. Just decorate your main function, and all unhandled exceptions are caught and displayed with user-friendly messages—no need to pollute your code with error handling boilerplate everywhere.

This pattern is extracted from production use and open-sourced to help others build robust Streamlit applications without sacrificing code clarity. For the full architectural context, see the PyConJP 2025 presentation.

Features

• Minimal API: Just two required arguments (on_error and fallback)
• PEP 561 Compatible: Ships with py.typed for full type checker support
• Callback Protection: Protect both decorated functions and widget callbacks (on_click, on_change, etc.)
• Pluggable Hooks: Execute side effects (audit logging, metrics, notifications) when errors occur
• Safe Fallback UI: Display user-friendly error messages instead of tracebacks

Installation

pip install st-error-boundary

Quick Start

Basic Usage (Decorator Only)

For simple cases where you only need to protect the main function:

import streamlit as st
from st_error_boundary import ErrorBoundary

# Create error boundary
boundary = ErrorBoundary(
    on_error=lambda exc: print(f"Error logged: {exc}"),
    fallback="An error occurred. Please try again later."
)

@boundary.decorate
def main() -> None:
    st.title("My App")

    if st.button("Trigger Error"):
        raise ValueError("Something went wrong")

if __name__ == "__main__":
    main()

⚠️ Important: The @boundary.decorate decorator alone does not protect on_click/on_change callbacks—you must use boundary.wrap_callback() for those (see Advanced Usage below).

Advanced Usage (With Callbacks)

To protect both decorated functions and widget callbacks:

import streamlit as st
from st_error_boundary import ErrorBoundary

def audit_log(exc: Exception) -> None:
    # Log to monitoring service
    print(f"Error: {exc}")

def fallback_ui(exc: Exception) -> None:
    st.error("An unexpected error occurred.")
    st.link_button("Contact Support", "https://example.com/support")
    if st.button("Retry"):
        st.rerun()

# Single ErrorBoundary instance for DRY configuration
boundary = ErrorBoundary(on_error=audit_log, fallback=fallback_ui)

def handle_click() -> None:
    # This will raise an error
    result = 1 / 0

@boundary.decorate
def main() -> None:
    st.title("My App")

    # Protected: error in if statement
    if st.button("Direct Error"):
        raise ValueError("Error in main function")

    # Protected: error in callback
    st.button("Callback Error", on_click=boundary.wrap_callback(handle_click))

if __name__ == "__main__":
    main()

Why ErrorBoundary Class?

Streamlit executes on_click and on_change callbacks before the script reruns, meaning they run outside the decorated function's scope. This is why @boundary.decorate alone cannot catch callback errors.

Execution Flow:

1. User clicks button with on_click=callback
2. Streamlit executes callback() -> Not protected by decorator
3. Streamlit reruns the script
4. Decorated function executes -> Protected by decorator

Solution: Use boundary.wrap_callback() to explicitly wrap callbacks with the same error handling logic.

API Reference

ErrorBoundary

ErrorBoundary(
    on_error: ErrorHook | Iterable[ErrorHook],
    fallback: str | FallbackRenderer
)

Parameters:

• on_error: Single hook or list of hooks for side effects (logging, metrics, etc.)
• fallback: Either a string (displayed via st.error()) or a callable that renders custom UI
  • When fallback is a str, it is rendered using st.error() internally
  • To customize rendering (e.g., use st.warning() or custom widgets), pass a FallbackRenderer callable instead

Methods:

• .decorate(func): Decorator to wrap a function with error boundary
• .wrap_callback(callback): Wrap a widget callback (on_click, on_change, etc.)

ErrorHook Protocol

def hook(exc: Exception) -> None:
    """Handle exception with side effects."""
    ...

FallbackRenderer Protocol

def renderer(exc: Exception) -> None:
    """Render fallback UI for the exception."""
    ...

Examples

Multiple Hooks

def log_error(exc: Exception) -> None:
    logging.error(f"Error: {exc}")

def send_metric(exc: Exception) -> None:
    metrics.increment("app.errors")

boundary = ErrorBoundary(
    on_error=[log_error, send_metric],  # Hooks execute in order
    fallback="An error occurred."
)

Custom Fallback UI

def custom_fallback(exc: Exception) -> None:
    st.error(f"Error: {type(exc).__name__}")
    st.warning("Please try again or contact support.")

    col1, col2 = st.columns(2)
    with col1:
        if st.button("Retry"):
            st.rerun()
    with col2:
        st.link_button("Report Bug", "https://example.com/bug-report")

boundary = ErrorBoundary(on_error=lambda _: None, fallback=custom_fallback)

Important Notes

Callback Error Rendering Position

TL;DR: Errors in callbacks appear at the top of the page, not near the widget. Use the deferred rendering pattern (below) to control error position.

When using wrap_callback(), errors in widget callbacks (on_click, on_change) are rendered at the top of the page instead of near the widget. This is a Streamlit architectural limitation.

Deferred Rendering Pattern

Store errors in session_state during callback execution, then render them during main script execution:

import streamlit as st
from st_error_boundary import ErrorBoundary

# Initialize session state
if "error" not in st.session_state:
    st.session_state.error = None

# Store error instead of rendering it
boundary = ErrorBoundary(
    on_error=lambda exc: st.session_state.update(error=str(exc)),
    fallback=lambda _: None  # Silent - defer to main script
)

def trigger_error():
    raise ValueError("Error in callback!")

# Main app
st.button("Click", on_click=boundary.wrap_callback(trigger_error))

# Render error after the button
if st.session_state.error:
    st.error(f"Error: {st.session_state.error}")
    if st.button("Clear"):
        st.session_state.error = None
        st.rerun()

Result: Error appears below the button instead of at the top.

For more details, see Callback Rendering Position Guide.

Development

# Install dependencies
make install

# Run linting and type checking
make

# Run tests
make test

# Run example app
make example

License

MIT

Contributing

Contributions are welcome! Please open an issue or submit a pull request.