How Do You Safely Share Data Between PyQt\'s Main Window and Threads?

In PyQt, Sharing Data Between Main Window and Threads

When creating GUI applications with PyQt, there may be instances where you need to share data between the main window and a thread. To achieve this, it's important to remember that widgets are not thread-safe. Only the main thread can interact with widgets directly.

Incorrect Practices

• Passing a Host Window Reference to a Thread: This allows the thread to access the main window's data, which can lead to thread-unsafe operations.
• Maintaining a Synchronized Copy: Keeping a copy of the data in the thread and synchronizing it with the main window via signals adds complexity and introduces potential race conditions.
• Using Global Variables: While technically feasible, global variables can lead to code dependency issues and make code harder to maintain.

Best Practice: Signals and Slots

The recommended approach for data sharing is to use Qt's signal-slot mechanism. Signals are emitted by objects to notify other objects of a specific event. Slots are functions that receive these signals and are executed in the main thread.

Example: Controlling Sleep Time from a Spinbox

Consider an example where a thread performs a repetitive task and needs to adjust its sleep time based on a user-controlled spinbox value in the main window.

<code class="python"># In the main window:

# Create worker thread
worker = Worker(spinbox.value())

# Connect the worker's beep signal to an update slot
worker.beep.connect(self.update)

# Connect the spinbox valueChanged signal to worker's update_value slot
spinbox.valueChanged.connect(worker.update_value)


class Worker(QtCore.QThread):
    beep = QtCore.pyqtSignal(int)

    def __init__(self, sleep_time):
        super(Worker, self).__init__()
        self.running = False
        self.sleep_time = sleep_time

    def run(self):
        # Thread execution logic here...

    def update_value(self, value):
        self.sleep_time = value</code>

In this example, the thread's update_value slot updates its sleep_time attribute whenever the main window's spinbox value changes. This allows the thread to dynamically adjust its sleep time based on user input. The updated sleep_time is then used in the thread's run method to control how long it sleeps between each task repetition.

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