Modular Code Organization
Write modular Python code with files sized for maintainability and AI-assisted development.
File Size Guidelines Lines Status Action 150-500 Optimal Sweet spot for AI code editors and human comprehension 500-1000 Large Look for natural split points 1000-2000 Too large Refactor into focused modules 2000+ Critical Must split - causes tooling issues and cognitive overload When to Split
Split when ANY of these apply:
File exceeds 500 lines Multiple unrelated concerns in same file Scroll fatigue finding functions Tests for the file are hard to organize AI tools truncate or miss context How to Split Natural Split Points By domain concept: auth.py → auth/login.py, auth/tokens.py, auth/permissions.py By abstraction layer: Separate interface from implementation By data type: Group operations on related data structures By I/O boundary: Isolate database, API, file operations Package Structure feature/ ├── init.py # Keep minimal, just exports ├── core.py # Main logic (under 500 lines) ├── models.py # Data structures ├── handlers.py # I/O and side effects └── utils.py # Pure helper functions
DO Use meaningful module names (data_storage.py not utils2.py) Keep init.py files minimal or empty Group related functions together Isolate pure functions from side effects Use snake_case for module names DON'T Split files arbitrarily by line count alone Create single-function modules Over-modularize into "package hell" Use dots or special characters in module names Hide dependencies with "magic" imports Refactoring Large Files
When splitting an existing large file:
Identify clusters: Find groups of related functions Extract incrementally: Move one cluster at a time Update imports: Fix all import statements Run tests: Verify nothing broke after each move Document: Update any references to old locations Current Codebase Candidates
Files over 2000 lines that need attention:
Math compute modules (scipy, mpmath, numpy) - domain-specific, may be acceptable patterns.py - consider splitting by pattern type memory_backfill.py - consider splitting by operation type Sources The Hitchhiker's Guide to Python Python Project Best Practices - Dagster Right-Sizing Python Files for AI Editors PEP 8 Style Guide