PySide6 Code Reviewer

Expert code review for modern PySide6/Qt 6.8+ applications.

Review Process

1. Identify Qt version assumptions — Verify code targets Qt 6.8+ (no Qt5 compat)
2. Check thread safety — All GUI operations on main thread, proper worker patterns
3. Validate signal/slot usage — Modern connection syntax, proper signatures
4. Assess Model/View implementation — Role usage, data method patterns, index validity
5. Review resource management — Parent-child ownership, prevent leaks
6. Evaluate async patterns — QThread, QtConcurrent, asyncio integration
7. Check QML integration — Property bindings, type registration, context exposure

Critical Anti-Patterns (Always Flag)

# WRONG: GUI operation from worker thread
class Worker(QThread):
    def run(self):
        self.label.setText("Done")  # CRASH: Cross-thread GUI access

# WRONG: Blocking the event loop
def on_button_click(self):
    time.sleep(5)  # FREEZES UI
    requests.get(url)  # FREEZES UI

# WRONG: Old-style signal connection (Qt4/5 legacy)
self.connect(button, SIGNAL("clicked()"), self.handler)

# WRONG: String-based slot connection
button.clicked.connect("self.handler")  # Should be callable

# WRONG: No parent = memory leak risk
label = QLabel("text")  # Should have parent or be assigned to layout

# WRONG: Deleting QObject while signals pending
obj.deleteLater()  # OK
del obj  # WRONG if signals/slots active

# WRONG: Direct widget manipulation in QThread.run()
class BadWorker(QThread):
    def run(self):
        self.progress_bar.setValue(50)  # Thread violation!

Modern Patterns (Require These)

Signal/Slot Connections

# Qt 6 style - always use this
button.clicked.connect(self.on_click)
button.clicked.connect(lambda: self.handler(arg))

# Typed signals with modern syntax
class Worker(QObject):
    progress = Signal(int)          # Single type
    result = Signal(str, list)      # Multiple types
    error = Signal(Exception)       # Exception passing
    finished = Signal()             # No arguments

Thread-Safe Worker Pattern

class Worker(QObject):
    finished = Signal()
    progress = Signal(int)
    result = Signal(object)
    error = Signal(str)

    @Slot()
    def run(self):
        try:
            for i in range(100):
                # Do work
                self.progress.emit(i)
            self.result.emit(data)
        except Exception as e:
            self.error.emit(str(e))
        finally:
            self.finished.emit()

# Usage
thread = QThread()
worker = Worker()
worker.moveToThread(thread)
thread.started.connect(worker.run)
worker.finished.connect(thread.quit)
worker.finished.connect(worker.deleteLater)
thread.finished.connect(thread.deleteLater)
thread.start()

Async Integration (Qt 6.8+)

import asyncio
from PySide6.QtAsyncio import QAsyncioEventLoopPolicy

# Set up asyncio with Qt event loop
asyncio.set_event_loop_policy(QAsyncioEventLoopPolicy())

class AsyncWidget(QWidget):
    async def fetch_data(self):
        async with aiohttp.ClientSession() as session:
            async with session.get(url) as response:
                return await response.json()
    
    def start_fetch(self):
        asyncio.ensure_future(self.fetch_data())

Detailed Reference Files

• references/signals-slots.md — Signal/slot patterns, connection types, thread-safe emission
• references/model-view.md — QAbstractItemModel, roles, proxies, delegates
• references/threading.md — QThread, QtConcurrent, async patterns, thread pools
• references/widgets.md — Widget lifecycle, layouts, styling, high-DPI
• references/qml-integration.md — QML/Python bridge, properties, type registration
• references/performance.md — Paint optimization, model efficiency, lazy loading
• references/anti-patterns.md — Comprehensive anti-pattern catalog with fixes

Review Checklist (Use for PRs)

Thread Safety

• All widget/GUI calls on main thread only
• Workers use signal/slot for UI updates
• No time.sleep() or blocking calls in main thread
• QThread subclass only overrides run(), not constructor work
• moveToThread() used correctly (object has no parent)

Memory Management

• Widgets have parents OR are added to layouts
• deleteLater() used for QObjects, never del
• Lambda connections don't capture stale references
• Circular signal connections avoided
• Model data returned by value, not reference

Signal/Slot Correctness

• Modern connection syntax (no strings)
• Signal signatures match slot signatures
• @Slot() decorator on all slots
• Qt.QueuedConnection for cross-thread signals
• Signals defined as class attributes

Model/View

• beginInsertRows()/endInsertRows() bracket changes
• dataChanged emitted for updates
• index.isValid() checked before access
• Custom roles use Qt.UserRole + n
• roleNames() override for QML compatibility

Performance

• No widget creation in paint events
• update() not repaint() for redraws
• Large lists use QAbstractItemModel (not QListWidget)
• Lazy loading for expensive data
• blockSignals() during batch updates

Qt 6.8+ Specifics

• Using new enum scoping (Qt.AlignmentFlag.AlignCenter)
• QtAsyncio for async patterns (not third-party)
• QML_ELEMENT macro equivalents for type registration
• Property bindings via QProperty where appropriate
• New QPermission API for platform permissions