# Lazy Loading

URL: /docs/usage/lazy/
Section: usage
Tags: lazy, performance, imports

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Lazy loading defers the import of command handler modules until the command is actually invoked. This keeps CLI startup fast even with dozens of commands that have heavy dependencies. lazy_command Register a command with a dotted import path instead of a function reference: from milo import CLI cli = CLI(name="myapp") cli.lazy_command( "deploy", "myapp.commands.deploy:deploy_handler", description="Deploy to an environment", ) The module myapp.commands.deploy is not imported until someone runs myapp deploy. All other commands start instantly. Pre-computed schemas By default, the schema is generated by importing the handler and inspecting its signature. To avoid even that import, provide the schema upfront: cli.lazy_command( "deploy", "myapp.commands.deploy:deploy_handler", description="Deploy to an environment", schema={ "type": "object", "properties": { "target": {"type": "string"}, "dry_run": {"type": "boolean"}, }, "required": ["target"], }, ) With a pre-computed schema, --llms-txt and --mcp tools/list work without importing any handler modules. Lazy commands in groups Groups support lazy loading too: site = cli.group("site", description="Site operations") site.lazy_command( "build", "myapp.commands.site:build_handler", description="Build the site", ) How it works LazyCommandDef stores the import path and defers resolution: On registration, only the name, description, and optional schema are stored On first invocation, resolve() imports the module and extracts the handler The result is cached as a full CommandDef — subsequent calls skip the import Resolution is thread-safe (uses a lock with double-check pattern). When to use lazy loading CLIs with many commands where only one runs per invocation Commands that import heavy dependencies (cloud SDKs, ML libraries) Plugin systems where third-party command modules may not be installed For small CLIs with lightweight imports, the @cli.command decorator is simpler and equally fast. Tip Tip Combine pre-computed schemas with --llms-txt and --mcp to let AI agents discover all your commands without triggering any imports.

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Metadata:
- Author: lbliii
- Word Count: 282
- Reading Time: 1 minutes