PyQt6 UI Development Rules Skill

Overview

This skill enforces rules for building production-quality PyQt6 desktop applications. The core principles are: strict MVC separation via signals/slots, never blocking the UI thread, centralized theming via QSS, and layout-manager-driven responsive design. These rules prevent the most common PyQt6 failures: frozen UIs, untestable coupling, and platform-specific rendering bugs.

When to Use

• When building new PyQt6 desktop applications
• When refactoring existing PyQt/PySide code to PyQt6
• When debugging frozen or unresponsive Qt UIs
• When implementing custom widgets or complex layouts
• When setting up cross-platform desktop application builds

Iron Laws

1. ALWAYS use Qt's signal/slot mechanism for UI-to-logic communication -- direct method calls between UI and business logic layers break MVC separation and cause untestable coupling.
2. NEVER perform long-running operations on the main UI thread -- blocking the Qt event loop makes the interface unresponsive and triggers OS "not responding" dialogs.
3. ALWAYS apply QSS stylesheets at the QApplication level rather than per-widget -- per-widget inline styles create inconsistent themes and unmaintainable styling sprawl.
4. NEVER use absolute pixel coordinates for widget layout -- use Qt layout managers (QVBoxLayout, QHBoxLayout, QGridLayout) to ensure DPI-aware and cross-platform rendering.
5. ALWAYS test the UI on all target platforms before release -- PyQt6 rendering, font scaling, and widget sizing differ between Windows, macOS, and Linux.

Anti-Patterns

Anti-Pattern	Why It Fails	Correct Approach
Calling business logic directly from UI slots	Couples UI to logic; makes testing impossible and breaks MVC architecture	Emit signals from UI; connect to controller/service methods via slot
Running network or file I/O on the main thread	Blocks the Qt event loop; UI freezes until operation completes	Use QThread, QRunnable, or asyncio with qasync for background operations
Hardcoding pixel sizes and positions	Breaks on high-DPI displays and different OS DPI scaling settings	Use layout managers and size policies; use logicalDpiX() for DPI-aware sizing
Setting styles inline on individual widgets	Creates visual inconsistency; extremely difficult to theme or maintain	Define a single QSS stylesheet at QApplication level and use object names/classes
Ignoring cross-platform rendering differences	Widget sizes, fonts, and margins differ significantly between Windows/macOS/Linux	Test on all target platforms; use platform-conditional logic where rendering diverges

Workflow

Step 1: Application Architecture (MVC)

# model.py -- Business logic, no Qt dependencies
class DataModel:
    def __init__(self):
        self._items = []

    def add_item(self, item: str) -> bool:
        if item and item not in self._items:
            self._items.append(item)
            return True
        return False

# controller.py -- Mediates between Model and View
from PyQt6.QtCore import QObject, pyqtSignal

class Controller(QObject):
    items_changed = pyqtSignal(list)
    error_occurred = pyqtSignal(str)

    def __init__(self, model: DataModel):
        super().__init__()
        self._model = model

    def add_item(self, item: str) -> None:
        if self._model.add_item(item):
            self.items_changed.emit(self._model._items.copy())
        else:
            self.error_occurred.emit(f"Could not add: {item}")

Step 2: Signal/Slot Wiring

# view.py -- UI only, connects via signals/slots
from PyQt6.QtWidgets import QMainWindow, QVBoxLayout, QWidget, QLineEdit, QPushButton, QListWidget

class MainView(QMainWindow):
    def __init__(self, controller: Controller):
        super().__init__()
        self._controller = controller

        # Wire signals to slots
        self._controller.items_changed.connect(self._on_items_changed)
        self._controller.error_occurred.connect(self._on_error)

        # UI emits to controller -- never calls model directly
        self._add_btn.clicked.connect(lambda: self._controller.add_item(self._input.text()))

    def _on_items_changed(self, items: list) -> None:
        self._list.clear()
        self._list.addItems(items)

Step 3: Background Operations

from PyQt6.QtCore import QThread, pyqtSignal

class WorkerThread(QThread):
    progress = pyqtSignal(int)
    finished_with_result = pyqtSignal(object)
    error = pyqtSignal(str)

    def __init__(self, task_fn, parent=None):
        super().__init__(parent)
        self._task_fn = task_fn

    def run(self):
        try:
            result = self._task_fn(self.progress.emit)
            self.finished_with_result.emit(result)
        except Exception as e:
            self.error.emit(str(e))

Step 4: QSS Theming

# Apply at QApplication level
app = QApplication(sys.argv)
app.setStyleSheet(Path("styles/dark-theme.qss").read_text())

# QSS file
"""
QMainWindow {
    background-color: #2b2b2b;
    color: #e0e0e0;
}
QPushButton {
    background-color: #3c3f41;
    border: 1px solid #555;
    border-radius: 4px;
    padding: 6px 16px;
    color: #e0e0e0;
}
QPushButton:hover {
    background-color: #4c5052;
}
"""

Step 5: Layout Management

# Use layout managers -- never setGeometry() or move()
layout = QVBoxLayout()
layout.addWidget(self._toolbar)
layout.addWidget(self._content, stretch=1)  # stretch fills available space
layout.addWidget(self._status_bar)

# For responsive grids
grid = QGridLayout()
grid.addWidget(label, 0, 0)
grid.addWidget(input_field, 0, 1)
grid.setColumnStretch(1, 1)  # input stretches, label stays fixed

Complementary Skills

Skill	Relationship
modern-python	Project setup with uv, ruff, ty, pytest
python-backend-expert	Backend service patterns for desktop app backends
tdd	Test-driven development for Qt widget testing
accessibility	Accessibility audit patterns applicable to desktop apps

Memory Protocol (MANDATORY)

Before starting:

Read .claude/context/memory/learnings.md for prior PyQt6 patterns and platform-specific workarounds.

After completing: Record any platform-specific rendering issues, signal/slot patterns, or QThread gotchas to .claude/context/memory/learnings.md.

ASSUME INTERRUPTION: Your context may reset. If it's not in memory, it didn't happen.