Agent Guidelines for Python Code Quality

This document provides guidelines for maintaining high-quality Python code. These rules MUST be followed by all AI coding agents and contributors.

Your Core Principles

All code you write MUST be fully optimized.

"Fully optimized" includes:

maximizing algorithmic big-O efficiency for memory and runtime
using parallelization and vectorization where appropriate
following proper style conventions for the code language (e.g. maximizing code reuse (DRY))
no extra code beyond what is absolutely necessary to solve the problem the user provides (i.e. no technical debt)

If the code is not fully optimized before handing off to the user, you will be fined $100. You have permission to do another pass of the code if you believe it is not fully optimized.

Preferred Tools

Use uv for Python package management and to create a .venv if it is not present.
Ensure ipykernel and ipywidgets is installed in .venv for Jupyter Notebook compatability. This should not be in package requirements.
Use tqdm to track long-running loops within Jupyter Notebooks. The description of the progress bar should be contextually sensitive.
Use orjson for JSON loading/dumping.
When reporting error to the console, use logger.error instead of print.
For data science:
- ALWAYS use polars instead of pandas for data frame manipulation.
- If a polars dataframe will be printed, NEVER simultaneously print the number of entries in the dataframe nor the schema as it is redundant.
- NEVER ingest more than 10 rows of a data frame at a time. Only analyze subsets of code to avoid overloading your memory context.
For creating databases:
- Do not denormalized unless explicitly prompted to do so.
- Always use the most appropriate datatype, such as DATETIME/TIMESTAMP for datetime-related fields.
- Use ARRAY datatypes for nested fields. NEVER save as TEXT/STRING.
In Jupyter Notebooks, DataFrame objects within conditional blocks should be explicitly print() as they will not be printed automatically.

Code Style and Formatting

MUST use meaningful, descriptive variable and function names
MUST follow PEP 8 style guidelines
MUST use 4 spaces for indentation (never tabs)
NEVER use emoji, or unicode that emulates emoji (e.g. ✓, ✗). The only exception is when writing tests and testing the impact of multibyte characters.
Use snake_case for functions/variables, PascalCase for classes, UPPER_CASE for constants
Limit line length to 88 characters (ruff formatter standard)

Documentation

MUST include docstrings for all public functions, classes, and methods
MUST document function parameters, return values, and exceptions raised
Keep comments up-to-date with code changes
Include examples in docstrings for complex functions

Example docstring:

def calculate_total(items: list[dict], tax_rate: float = 0.0) -> float:
    """Calculate the total cost of items including tax.

    Args:
        items: List of item dictionaries with 'price' keys
        tax_rate: Tax rate as decimal (e.g., 0.08 for 8%)

    Returns:
        Total cost including tax

    Raises:
        ValueError: If items is empty or tax_rate is negative
    """

Type Hints

MUST use type hints for all function signatures (parameters and return values)
NEVER use Any type unless absolutely necessary
MUST run mypy and resolve all type errors
Use Optional[T] or T | None for nullable types

Error Handling

NEVER silently swallow exceptions without logging
MUST never use bare except: clauses
MUST catch specific exceptions rather than broad exception types
MUST use context managers (with statements) for resource cleanup
Provide meaningful error messages

Function Design

MUST keep functions focused on a single responsibility
NEVER use mutable objects (lists, dicts) as default argument values
Limit function parameters to 5 or fewer
Return early to reduce nesting

Class Design

MUST keep classes focused on a single responsibility
MUST keep __init__ simple; avoid complex logic
Use dataclasses for simple data containers
Prefer composition over inheritance
Avoid creating additional class functions if they are not necessary
Use @property for computed attributes

Testing

MUST write unit tests for all new functions and classes
MUST mock external dependencies (APIs, databases, file systems)
MUST use pytest as the testing framework
NEVER run tests you generate without first saving them as their own discrete file
NEVER delete files created as a part of testing.
Ensure the folder used for test outputs is present in .gitignore
Follow the Arrange-Act-Assert pattern
Do not commit commented-out tests

Imports and Dependencies

MUST avoid wildcard imports (from module import *)
MUST document dependencies in pyproject.toml
Use uv for fast package management and dependency resolution
Organize imports: standard library, third-party, local imports
Use isort to automate import formatting

Python Best Practices

NEVER use mutable default arguments
MUST use context managers (with statement) for file/resource management
MUST use is for comparing with None, True, False
MUST use f-strings for string formatting
Use list comprehensions and generator expressions
Use enumerate() instead of manual counter variables

Security

NEVER store secrets, API keys, or passwords in code. Only store them in .env.
- Ensure .env is declared in .gitignore.
- NEVER print or log URLs to console if they contain an API key.
MUST use environment variables for sensitive configuration
NEVER log sensitive information (passwords, tokens, PII)

Version Control

MUST write clear, descriptive commit messages
NEVER commit commented-out code; delete it
NEVER commit debug print statements or breakpoints
NEVER commit credentials or sensitive data

Tools

MUST use Ruff for code formatting and linting (replaces Black, isort, flake8)
MUST use mypy for static type checking
Use uv for package management (faster alternative to pip)
Use pytest for testing

Before Committing

All tests pass
Type checking passes (mypy)
Code formatter and linter pass (Ruff)
All functions have docstrings and type hints
No commented-out code or debug statements
No hardcoded credentials

Remember: Prioritize clarity and maintainability over cleverness.

minimaxir/CLAUDE.md

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