SKILL.md
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
- 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.
- 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.
- ALWAYS apply QSS stylesheets at the QApplication level rather than per-widget -- per-widget inline styles create inconsistent themes and unmaintainable styling sprawl.
- NEVER use absolute pixel coordinates for widget layout -- use Qt layout managers (QVBoxLayout, QHBoxLayout, QGridLayout) to ensure DPI-aware and cross-platform rendering.
- 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 fixedComplementary 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.