Talks -Toby Ho: Rewind: Python Time-Travel Debugger

PyRewind is a time-travel debugger for Python that allows stepping backwards and forwards through program execution

The debugger stores program state history in a SQLite database containing:

• Python code files
• Runtime errors
• Function calls
• Variable values
• Stack traces
• Print output

Two main approaches to time-travel debugging:

• Record/replay of program execution and inputs
• Immutable state updates with history preservation

Key benefits over traditional debuggers:

• No need to set breakpoints
• Can inspect program state at any point in time
• Eliminates Heisenbugs by providing deterministic replay
• Allows live code editing during debugging

Technical implementation details:

• Logs all state changes and function calls
• Captures communication with external world
• Creates deterministic replay from recorded logs
• Built as a fork of Python 3.10 interpreter

Inspired by:

• Brett Victor’s “Inventing on Principle” talk
• Redux developer tools
• Mozilla’s RR debugger
• Omniscient debugging concepts

Limitations:

• Complex to implement at interpreter level
• Storage requirements for program state history
• Handling infinite loops remains challenging
• Non-deterministic inputs need special handling