/modular-code

name: modular-code
description: Modular Code Organization
user-invocable: false

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

1. **By domain concept**: `auth.py` → `auth/login.py`, `auth/tokens.py`, `auth/permissions.py` 2. **By abstraction layer**: Separate interface from implementation 3. **By data type**: Group operations on related data structures 4. **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:

1. **Identify clusters**: Find groups of related functions 2. **Extract incrementally**: Move one cluster at a time 3. **Update imports**: Fix all import statements 4. **Run tests**: Verify nothing broke after each move 5. **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](https://docs.python-guide.org/writing/structure/)
• [Python Project Best Practices - Dagster](https://dagster.io/blog/python-project-best-practices)
• [Right-Sizing Python Files for AI Editors](https://medium.com/@eamonn.faherty_58176/right-sizing-your-python-files-the-150-500-line-sweet-spot-for-ai-code-editors-340d550dcea4)
• [PEP 8 Style Guide](https://peps.python.org/pep-0008/)