Intro to Desktop Applications with Qt

(and Python, of course)

About Me

Amy Boyle

I create Graphical User Interfaces (GUIs) for scientific research

Why Qt?

Cross platform

Large user base

Good graphical designer

Great API and docs

Event Driven Programming

Programming paradigm where the flow of execution is directed by events, such as user clicks

Linear Paradigm

result = func0(inputs)
do_stuff_with_results

result1 = func1(other_inputs)
do_stuff_with_results

def stuff(sdfsd):
    pass

Event-Driven Paradigm

while not exit_called:

    while not event_queue.is_empty():
        dispatch_next_event()

    wait_for_more_events()

Where Do Events Come From?

• Key presses, mouse clicks
• Sockets activity
• Events posted from other threads
• Timers

In Qt, events are objects

• Represent things that have happened either within the application, or outside
• All derived from abstract QEvent class
• Events can be handled by any instance of QObject, almost always this is a QWidget
• Events can be passed among many handlers before the event is accepted

Event Handlers

Receive events, do something useful

def mousePressEvent(event):
   if event.button == 'left':
      do_that_cool_thing()

Widgets

• Basic UI elements that make up the part you see
• E.g. windows, buttons, input fields
• QWidget or a subclass of

Hello World

from PyQt4 import QtGui

app = QtGui.QApplication([])

hello_widget = QtGui.QPushButton("Hello world!")
hello_widget.show()

app.exec_()

Putting it Together

from PyQt4 import QtGui

class MyButton(QtGui.QPushButton):
    nclicks = 0
    def mousePressEvent(self, event):
        super(MyButton, self).mousePressEvent(event)
        self.nclicks += 1
        self.setText("pressed {}".format(self.nclicks))

if __name__ == '__main__':

    app = QtGui.QApplication([])

    hello_widget = MyButton("click me")
    hello_widget.show()

    app.exec_()

Widgets are containers

class MyWidget(QtGui.QWidget):
    def __init__(self):
        super(MyWidget, self).__init__()

        layout = QtGui.QVBoxLayout()

        self.field = QtGui.QLineEdit()
        self.prompt = QtGui.QLabel("Amy says:")
        self.label = QtGui.QLabel("")

        layout.addWidget(self.field)
        layout.addWidget(self.prompt)
        layout.addWidget(self.label)

        self.setLayout(layout)

Using events to edit other widgets

class MyWidget(QtGui.QWidget):
    def __init__(self):
        super(MyWidget, self).__init__()

        layout = QtGui.QVBoxLayout()

        self.field = QtGui.QLineEdit()
        self.prompt = QtGui.QLabel("Amy says:")
        self.label = QtGui.QLabel("")

        layout.addWidget(self.field)
        layout.addWidget(self.prompt)
        layout.addWidget(self.label)

        self.setLayout(layout)

        self.field.keyPressEvent = self.keyPressEvent

    def keyPressEvent(self, event):
        QtGui.QLineEdit.keyPressEvent(self.field, event)
        if event.key() == QtCore.Qt.Key_Backspace:
            self.label.setText(self.label.text()[:-1])
        else:
            self.label.setText(self.label.text() + event.text())

Signals and Slots

• "Signals" are emitted for common events appropriate for each type of widget
• Instead of re-implementing widgets we can connect functions to signals for common actions
• The functions we connect to are called "slots"
• We can also create our our signals, emitted on demand
• Allows for looser coupling

Much better...

from PyQt4 import QtGui, QtCore

class MyWidget(QtGui.QWidget):
    def __init__(self):
        super(MyWidget, self).__init__()

        layout = QtGui.QVBoxLayout()

        self.field = QtGui.QLineEdit()
        self.prompt = QtGui.QLabel("Amy says:")
        self.label = QtGui.QLabel("")

        layout.addWidget(self.field)
        layout.addWidget(self.prompt)
        layout.addWidget(self.label)

        self.setLayout(layout)

        self.field.textChanged.connect(self.label.setText)

Slots are any function with the appropriate parameters

class MyWidget(QtGui.QWidget):
    def __init__(self):
        super(MyWidget, self).__init__()

        self.field = QtGui.QLineEdit()
        self.prompt = QtGui.QLabel("Amy says:")
        self.label = QtGui.QLabel("")

        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.field)
        layout.addWidget(self.prompt)
        layout.addWidget(self.label)
        self.setLayout(layout)

        self.field.textChanged.connect(self.amySays)

    def amySays(self, text):
        self.label.setText(text + ' !!!')

* See my posts on installing in a virtualenv in Linux or Windows

Qt has classes for Web, Databases, and more!

QtWebKit, QtSql, QtNetwork, QtOpenGL...

Bare Bones Browser

import sys
from PySide import QtGui, QtCore, QtWebKit

class Browser(QtGui.QWidget):
    def __init__(self):
        super(Browser, self).__init__()

        self.page = QtWebKit.QWebView()
        self.addressBar = QtGui.QLineEdit("http://www.amyboyle.ninja")

        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.addressBar)
        layout.addWidget(self.page)
        self.setLayout(layout)

        self.addressBar.returnPressed.connect(self.loadAddress)

    def loadAddress(self):
        address = self.addressBar.text()
        self.page.load(QtCore.QUrl(address))

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    browser = Browser()
    browser.show()
    sys.exit(app.exec_())

Go Make Cool Things!

Presentation at amyboyle.ninja

Slides + examples on github