QListWidget::currentTextChanged ( const QString & currentText ) 

// In Constructor:
connect(listWidget,SIGNAL(currentTextChanged(const QString&)),this,SLOT(doSomething(const QString&)));

// Slot
WidgetClassName::doSomething(const QString& str)
{
   // do what you want with str
}

QListWidgetItem * QListWidget::currentItem () const

QString QListWidgetItem::text () const

// In constructor
connect(btn,SIGNAL(clicked()),this,SLOT(btnClicked()));

ClassName::btnClicked()
{
   QString str = list->currentItem()->text();
}

import sys
from PyQt4 import QtCore, QtGui


class TWidget(QtGui.QWidget):
   
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
        self.vbox=QtGui.QVBoxLayout(self)
        self.qlistwidget=QtGui.QListWidget(self)
        map(self.qlistwidget.addItem, [str(x) for x in range(5)])
       
        self.vbox.addWidget(self.qlistwidget)
        self.btn=QtGui.QPushButton("Click me..", self)
        self.vbox.addWidget(self.btn)
        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('TWidget')
       
        self.setupConnections()
       
    def setupConnections(self):
        self.connect(self.qlistwidget,QtCore.SIGNAL("currentTextChanged(const QString&)"),self.onCurrentTextChanged);
        self.connect(self.btn, QtCore.SIGNAL("clicked()"), self.onBtnClicked)
       
    def onBtnClicked(self):
        item=self.qlistwidget.currentItem().text()
        QtGui.QMessageBox.information(None, "res", item)
       
    def onCurrentTextChanged(self, text):
        print text

app = QtGui.QApplication(sys.argv)
w = TWidget()
w.show()
sys.exit(app.exec_())