PEP 810 Implementation

Python Lazy Imports

Explore the semantics from the proposal with this interactive WebAssembly demo. Write Python code in the editor, run it in your browser, or experiment in the interactive REPL.

📜 Read PEP 810 Specification

🎯 Available Demo Module

A special lazy_demo package is included in this demo. It contains submodules that print messages when imported, making it easy to see when lazy imports are reified:

lazy_demo - Main package (prints "📦 lazy_demo package loaded!")
lazy_demo.heavy - Heavy computation module (prints "⚙️ lazy_demo.heavy loaded!")
lazy_demo.utils - Utility functions (prints "🔧 lazy_demo.utils loaded!")
lazy_demo.data - Data module (prints "📊 lazy_demo.data loaded!")
lazy_demo.broken - Demonstrates deferred error handling: imports successfully but has a function that triggers a lazy import of a failing module

💡 Try: lazy import lazy_demo.heavy then access lazy_demo.heavy.compute() to see the lazy import in action!

💡 Test deferred errors: import lazy_demo.broken (succeeds), then call lazy_demo.broken.bar() (fails when lazy import triggers).

Code Editor

# Demonstrating lazy imports with lazy_demo package import sys # Lazy imports - modules NOT loaded yet! lazy import lazy_demo.heavy lazy import lazy_demo.utils print("After lazy imports:") print(f" 'lazy_demo.heavy' loaded? {'lazy_demo.heavy' in sys.modules}") print(f" 'lazy_demo.utils' loaded? {'lazy_demo.utils' in sys.modules}") print() # First access triggers loading (watch for print output!) print("Calling lazy_demo.heavy.compute()...") result = lazy_demo.heavy.compute() print(f" Result: {result}") print(f" 'lazy_demo.heavy' loaded? {'lazy_demo.heavy' in sys.modules}") print(f" 'lazy_demo.utils' still lazy? {'lazy_demo.utils' not in sys.modules}")

Terminal Output

Interactive REPL

About Lazy Imports (PEP 810)

PEP 810 introduces explicit syntax for lazy imports: lazy import module and lazy from module import name. Unlike normal imports which eagerly load modules at the import statement, lazy imports defer loading and execution until the first time the imported name is actually used. This reduces startup time, memory usage, and unnecessary work, especially beneficial for command-line tools, test suites, and applications with large dependency graphs. The feature is opt-in, explicit, and preserves full backwards compatibility.

Key benefits: Real-world deployments have demonstrated 50-70% startup time improvements and 30-40% memory savings in production workloads. Type annotation imports impose no runtime penalty, and unused code paths never trigger module loading. The design is local (affects only marked imports), explicit (clear lazy keyword), controlled (library authors decide), and granular (import-by-import basis).

Note: This is an experimental WebAssembly build of Python. Some features like networking, subprocesses, and threading are not available. For more information, see PEP 810 and the implementation repository .