Python与GUI（PyQt）程序中的语句

Question

I'm trying to combine python resource management within a PyQt GUI application.

Target:

• Allocate a resource (eg open a file) as a reaction to GUI event
• Keep a handle to this resource within my application (maybe the GUI object)
• Clean up resource properly, when it is no longer needed (latest when the application closes, potentially in reaction to a GUI event)

How do I combine this in a nice way?

As it is now programmed, widget.getResource will allocate and release the resource right away. However, I cannot yield from the function either, as then the allocation would never be executed. I could place the context in the main part, but I want to use some information from GUI (eg which resource to allocate), so I need a main event loop before I can enter.

Example code:

# Resource context manager:
import contextlib
def allocateResource():
    print('allocated')

def releaseResource():
    print('released')

@contextlib.contextmanager
def resourceContext():
    allocateResource()
    try:
        yield 'special resource'
    finally:
        releaseResource()

# PyQt Widget:
from PyQt4.QtGui import QWidget, QPushButton, QApplication, QVBoxLayout

class widget(QWidget):

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

        self.button1 = QPushButton('Get Resource')
        self.button1.clicked.connect(self.getResource)

        self.setLayout(QVBoxLayout())
        self.layout().addWidget(self.button1)

    def getResource(self):
        print('getting resource')
        with resourceContext() as r:
            print('now I can use ', r)

        print('Ouch. Leaving context again')

if __name__ == '__main__':
    import sys
    app = QApplication(sys.argv)
    main = widget()
    main.show()
    # directly emulate click
    main.button1.clicked.emit(True)

    sys.exit(app.exec_())

Answer 1

You're right, you can't use a context manager for this since the resource allocation is open-ended. You'll have to keep track of the resource and deallocate it manually whenever you're done with it.

If it's a resource that typically won't be deallocated until you close your application then you could do that by overriding the closeEvent method.

class widget(QWidget):
    ...
    def getResource(self):
        if not self.resource:
            self.resource = allocateResource()
        return self.resource

    def closeEvent(self, event):
        if self.resource:
            releaseResource()
        super(widget, self).closeEvent(event)