Monty

A 5 minutes lighting talk about Pydantic Monty I shared to Tokyo Python Meetup hosted by Ben.

These days I prefer give short talk like this, it’s mainly to keep my hacker spirit in the AI era.

Transcript

2. Monty Kir Chou (2026/5/20) note35.github.io/about 2
4. https://github.com/pydantic/monty 4
5. A brilliant idea? 1. Fast (in Rust) 2. Freedom (don't get bothered by CPython development) 3. Safe Prompt Request LLM Generated Python Code Monty (Python Interpreter in Rust) Prompt Response Monty's Limitation (as of 2026 May) 1. Use the rest of the standard library 2. Use third party libraries (like Pydantic) 3. Define classes (to-be-supported) 4. Use match statements (to-be-supported) 5
6. An example: LLM's code import pydantic_monty code = """ async def agent(prompt: str, messages: Messages): while True: print(f'messages so far: {messages}') output = await call_llm(prompt, messages) if isinstance(output, str): return output messages.extend(output) await agent(prompt, []) """ type_definitions = """...""" m = pydantic_monty.Monty( code, inputs=['prompt'], script_name='agent.py', type_check=True, type_check_stubs=type_definitions, ) async def call_llm(prompt, messages): if len(messages) < 2: return [{'role': 'system', 'content': 'resp'}] else: return f'example output, message count {len(messages)}' async def main(): output = await m.run_async( inputs={'prompt': 'testing'}, external_functions={'call_llm': call_llm}, ) print(output) #> example output, message count 2 if __name__ == '__main__': import asyncio asyncio.run(main()) 6
7. An example: Monty import pydantic_monty code = """ async def agent(prompt: str, messages: Messages): while True: print(f'messages so far: {messages}') output = await call_llm(prompt, messages) if isinstance(output, str): return output messages.extend(output) await agent(prompt, []) """ type_definitions = """...""" m = pydantic_monty.Monty( code, inputs=['prompt'], script_name='agent.py', type_check=True, type_check_stubs=type_definitions, ) async def call_llm(prompt, messages): if len(messages) < 2: return [{'role': 'system', 'content': 'resp'}] else: return f'example output, message count {len(messages)}' async def main(): output = await m.run_async( inputs={'prompt': 'testing'}, external_functions={'call_llm': call_llm}, ) print(output) #> example output, message count 2 if __name__ == '__main__': import asyncio asyncio.run(main()) 7
8. An example: Python's code import pydantic_monty code = """ async def agent(prompt: str, messages: Messages): while True: print(f'messages so far: {messages}') output = await call_llm(prompt, messages) if isinstance(output, str): return output messages.extend(output) await agent(prompt, []) """ type_definitions = """...""" m = pydantic_monty.Monty( code, inputs=['prompt'], script_name='agent.py', type_check=True, type_check_stubs=type_definitions, ) async def call_llm(prompt, messages): if len(messages) < 2: return [{'role': 'system', 'content': 'resp'}] else: return f'example output, message count {len(messages)}' async def main(): output = await m.run_async( inputs={'prompt': 'testing'}, external_functions={'call_llm': call_llm}, ) print(output) #> example output, message count 2 if __name__ == '__main__': import asyncio asyncio.run(main()) 8
9. An example: Run it $ pip install pydantic-monty # add ex1.py $ python ex1.py messages so far: [] messages so far: [{'role': 'system', 'content': 'example response'}] messages so far: [{'role': 'system', 'content': 'example response'}, {'role': 'system', 'content': 'example response'}] example output, message count 2 So it's still executed by CPython? (In fact, it can be called from also Rust and TS/JS) 9
10. A brilliant idea? 1. Fast (in Rust) 2. Freedom (don't get bothered by CPython development) 3. Safe That's what it said. 󰤇 10
11. https://github.com/pydantic/monty 11
12. https://hackmonty.com/ 12
13. https://pydantic.dev/articles/hack-monty-postmortem Hacked in 48 hours 65 submissions 13
14. The Value of Monty (for now) - A Nice Playground Features 1. Iterative Execution: start() and resume(). 2. Serialization: can be serialized to bytes and restored later. Sandbox • Application-level sandbox + Easy file mounting 14
15. sandbox competitor target 15
16. Next Step 16