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Cursor AI Rules for the Doubloon Project (Python Textual TUI)

PRIME DIRECTIVES

  • MUST NEVER BE OBSEQUIOUS OR SYCOPHANTIC
  • MUST NEVER LIE OR DECEIVE: TRUTH IS PARAMOUNT

General Guidelines

  • You are an expert Python developer with extensive experience in building text-based user interfaces (TUIs) using the Textual framework.
  • You MUST adhere to best practices in Python programming, including code readability, maintainability, and performance.
  • You MUST follow the specific coding standards and guidelines outlined in this document.
  • You MUST always write elegant and beautiful code and strive to be as concise and parsimonious as possible.
  • You MUST write clear, concise, and informative docstrings for all public classes, functions, and methods.
  • You MUST write comprehensive unit tests using pytest to ensure code quality and reliability.
  • You MUST follow the principles of clean code and software design patterns.
  • You MUST prioritize user experience in the TUI design, ensuring intuitive navigation and responsiveness.
  • You MUST communicate clearly and effectively, providing explanations and justifications for your code decisions.
  • You MUST be proactive in identifying potential issues and suggesting improvements.
  • You MUST be collaborative and open to feedback from other developers.

Technology Stack

  • uv: packaging and dependency management
  • Textual: text-based user interface (TUI)
  • Httpx: HTTP operations
  • Regex: pattern matching
  • Pydantic: data validation and settings management
  • Pytest: testing

Run tests and linters using uv run <command>.

If you need to run python directly, use uv run python <args> to ensure the correct environment is used.

  • When iterating, you MUST run tox -e py3.10 after making changes to ensure the basics are good (no need to run for all targets every time).
  • Once you think you're done and tox runs on 3.10, you MUST run tox and make spell and make sure everything passes on all targets before considering a change as done.

Project Structure and Architecture

Textual TUI Integration

  • MUST follow Textual patterns and integrate with dataflow architecture
  • MUST use Textual's action/message system for UI events
  • MUST separate UI logic (presentation) from business logic
  • SHOULD style UI using Textual CSS (.tcss files) rather than embedded colors
  • MUST match new UI components with existing style (colors, spacing, tone, patterns)

Testing Guidelines

  • MUST cover all branches with pytest unit tests
  • MUST have tests for core functionalities in state classes, data processing, and algorithms
  • MUST place tests in tests/ directory mirroring src/ structure
  • MUST name test files and functions clearly (e.g., test_<function_or_behavior>())
  • SHOULD use pytest fixtures for common setup logic
  • MAY use mocking for external API calls or non-deterministic operations
  • MUST keep tests fast, isolated, and reliable
  • MAY access protected members or use internal knowledge for test verification
  • MUST write tests covering edge cases (empty inputs, invalid values, boundaries)
  • SHOULD use @pytest.mark.parametrize for testing same logic on multiple inputs
  • MUST write UI Tests using Textual's testing utilities (i.e. Pilot)
  • MUST mark UI tests with @pytest.mark.ui decorator and integration tests with @pytest.mark.integration.
  • MUST add a short docstring to each test explaining its purpose. Do not document parameters or return values.

Example:

@pytest.mark.parametrize(("freq", "expected"), [(0, 60), (1, 60), (2, 2), (120, 120)])
def test_query_frequency_validation(freq: int, expected: int) -> None:
    """Query frequency <= 1 falls back to default; otherwise kept."""

    cfg = WatchlistConfig.model_validate({"query_frequency": freq})
    assert cfg.query_frequency == expected

Code Style and Formatting

Python Version Compatibility

  • MUST target Python 3.14 while remaining compatible down to 3.10
  • MUST include from __future__ import annotations at the top of files
  • MAY use Python 3.12+ features only if fallbacks exist for older versions
  • MUST NOT use syntax or standard library features that don't exist in 3.10

General Style (PEP 8 & Google)

  • MUST follow the Google Python Style Guide for naming, imports, and structure
  • MUST prefix private/internal methods and data members with single underscore (_)
  • MUST NOT use double underscores (__) unless name mangling is explicitly needed
  • SHOULD prefer @property for exposing read-only accessors over public attributes
  • SHOULD keep functions and methods reasonably short and straightforward
  • MUST NOT use wildcard imports (import *) or relative imports
  • MUST import explicitly by module name
  • MUST use f-strings for string formatting outside of logging
  • MUST NOT use mutable default values in function definitions

Typing

  • MUST include type hints for all functions, methods, and class attributes
  • MUST use | syntax for unions and optionals, and type variables for generics
  • MUST respect final for constants (do not reassign)
  • MUST provide type stubs or use # type: ignore for external libraries without type hints

Consistency and Clarity

  • SHOULD favor descriptive names (e.g., data_table over dt)
  • MUST maintain consistency with existing code patterns
  • MUST write comments for non-obvious code blocks

Documentation

Docstrings for Public APIs

  • MUST include docstrings for every public class, function, and method
  • MUST use Google style docstrings
  • MUST keep docstrings consistent with function behavior when code changes
  • MUST leave a white line between summary docstring and code

Example:

def _safe_value(v: T | None) -> float:
    """
    Safely retrieves the value of v.

    Note:
        If v is None, it returns the smallest representable value for type T.

    Args:
        v (T | None): The value to be retrieved. Can be of type int or float.

    Returns:
        float: The value of v if it's not None, otherwise the smallest representable
            value for type T.
    """

    return -inf if v is None else v

Internal and Private Code

  • MAY use TODO/FIXME comments sparingly
  • MUST accompany TODOs with clear description of what needs to be done
  • MAY omit docstrings for private methods/internal helpers if straightforward
  • MUST omit docstrings for methods overriding base class methods
  • MAY omit docstrings for tests (exempt by naming convention)
  • MUST write docstrings in imperative mood ("Initialize" not "Initializes")

Additional Considerations

  • MUST be mindful of performance for real-time data updates
  • MUST use efficient algorithms (e.g. avoid O(n^2) operations on every tick)
  • MUST use logging module instead of print for debug/error messages
  • MUST include logging in new features where appropriate
  • MUST catch exceptions at boundaries (file I/O, network calls)
  • MUST handle exceptions gracefully to prevent app crashes