PyQt Threading - Concurrency and Thread Safety

Comprehensive guide to threading in PyQt applications.

Thread Safety Rules

CRITICAL: Qt/PyQt is NOT thread-safe for UI operations. You MUST follow these rules:

1. Never access widgets from worker threads - Only the main thread can modify UI
2. Use signals for cross-thread communication - Emit signals from worker, connect to slots in main thread
3. Use Qt.QueuedConnection for thread-safe signal delivery - AutoConnection handles this automatically
4. Never block the main thread - Long operations will freeze the UI

# ❌ WRONG: Direct UI access from thread
class BadWorker(QThread):
    def run(self):
        # This will crash or cause undefined behavior!
        self.label.setText("Done")