Python与Qt快速构建GUI应用：实战开源技术

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Introduction

This book teaches how to write GUI applications using the Python program-

ming language and the Qt application development framework. The only

essential prior knowledge is that you can program in some object-oriented pro-

gramming language, such as C

, C#, Java, or of course, Python itself. For the

rich text chapter, some familiarity with HTML and with regular expressions is

assumed, and the databases and threading chapters assume some basic knowl-

edge of those topics. A knowledge of GUI programming is not required, since

all the key concepts are covered.

The book will be useful to people who program professionally as part of their

job, whether as full-time software developers, or those from other disciplines,

including scientists and engineers, who need to do some programming in sup-

port of their work. It is also suitable for undergraduate and post-graduate stu-

dents, particularly those doing courses or research that includes a substantial

computing element. The exercises (with solutions) are provided especially to

help students.

Python is probably the easiest to learn and nicest scripting language in

widespread use, and Qt is probably the best library for developing GUI applica-

tions. The combination of Python and Qt, “PyQt”, makes it possible to develop

applications on any supported platform and run them unchanged on all the

supported platforms—for example, all modern versions of Windows, Linux,

Mac OS X, and most Unix-based systems. No compilation is required thanks

to Python being interpreted, and no source code changes to adapt to different

operating systems are required thanks to Qt abstracting away the platform-

speciﬁc details. We only have to copy the source ﬁle or ﬁles to a target machine

that has both Python and PyQt installed and the application will run.

If you are new to Python: Welcome! You are about to discover a language that

is clear to read and write, and that is concise without being cryptic. Python

supports many programming paradigms, but because our focus is on GUI

programming, we will take an object-oriented approach everywhere except in

the very early chapters.

Python is a very expressive language, which means that we can usually write

far fewer lines of Python code than would be required for an equivalent appli-

cation written in, say, C

or Java. This makes it possible to show some small

but complete examples throughout the text, and makes PyQt an ideal tool for

rapidly and easily developing GUI applications, whether for prototyping or for

production use.

Introduction 3

programming teams varying in size from just one person to more than a dozen

people. Many in-house tools are written using PyQt, but because these are of-

ten used to gain competitive advantage, the companies involved generally do

not permit their use of PyQt to be made public. PyQt is also widely used in the

open source world, with games, utilities, visualization tools, and IDEs all writ-

ten using it.

This book is speciﬁcally about PyQt4, the Python bindings for the Qt 4 C

application development framework.

★

PyQt4 is provided in the form of ten

Python modules which between them contain around 400 classes and about

6 000 methods and functions. All the example programs have been tested on

Windows, Linux, and Mac OS X, using Python 2.5, Qt 4.2, and PyQt 4.2, and

additionally on Windows and Linux using Qt 4.3 and PyQt 4.3. Backporting to

earlier versions is possible in some cases, but we recommend using the most

up-to-date versions of Python, Qt, and PyQt.

Python, PyQt, and Qt can be used free of charge for noncommercial purposes,

but the license used by Python is different from that used by PyQt and Qt.

Python is available with a very liberal license that allows it to be used to de-

velop both commercial and noncommercial applications. Both PyQt and Qt are

dual-licensed: This essentially allows them to be used to develop noncommer-

cial applications—which must in turn be licensed using an acceptable open

source license such as the GNU General Public License (GPL); or to be used to

develop commercial applications—in this case, a commercial PyQt license and

a commercial Qt license must be purchased.

The Structure of the Book

The book is divided into four parts. Part I is primarily a rapid conversion course

aimed at non-Python programmers who are familiar with an object-oriented

language, although it also has some (clearly marked) PyQt content. Because

the core Python language is mostly simple and is quite small, these chapters

can teach the basics of Python to a sufﬁcient extent that real Python applica-

tions can be written.

If you think that you can pick up the Python syntax simply through reading

it, you might be tempted to skip Part I and dive straight into the GUI pro-

gramming that begins in Part II. The early chapters in Part II include back-

references to the relevant pages in Part I to support readers who choose this

approach. However, even for readers familiar with Python, we recommend

reading about

QString

in Chapter 1. If you are unfamiliar with partial function

application (currying), it is important to read the subsection that covers this in

Chapter 2, since this technique is sometimes used in GUI programming.

★

There are also Python bindings for the older Qt 3 library, but there is no reason to use that library

for new projects, especially since Qt 4 offers far more functionality and is easier to use.

4 Introduction

Part II begins by showing three tiny PyQt GUI applications to give an initial

impression of what PyQt programming is like. It also explains some of the

fundamental concepts involved in GUI programming, including PyQt’s high-

level signals and slots communication mechanism. Chapter 5 shows how to

create dialogs and how to create and lay out widgets (“controls” in Windows-

speak—the graphical elements that make up a user interface such as buttons,

listboxes, and such) in a dialog. Dialogs are central to GUI programming: Most

GUI applications have a single main window, and dozens or scores of dialogs,

so this topic is covered in depth.

After the dialogs chapter comes Chapter 6, which covers main windows,

including menus, toolbars, dock windows, and keyboard shortcuts, as well as

loading and saving application settings. Part II’s ﬁnal chapters show how to

create dialogs using Qt Designer, Qt’s visual design tool, and how to save data

in binary, text, and XML formats.

Part III gives deeper coverage of some of the topics covered in Part II, and in-

troduces many new topics. Chapter 9 shows how to lay out widgets in quite

sophisticated ways, and how to handle multiple documents. Chapter 10 covers

low-level event handlers, and how to use the clipboard as well as drag and drop,

text, HTML, and binary data. Chapter 11 shows how to modify and subclass

existing widgets, and how to create entirely new widgets from scratch, with

complete control over their appearance and behavior. This chapter also shows

how to do basic graphics. Chapter 12 shows how to use Qt 4.2’s new graphics

view architecture,which is particularly suited to handling large numbers of in-

dependent graphical objects. Qt’s HTML-capable rich text engine is covered in

Chapter 13. This chapter also covers printing both to paper and to PDF ﬁles.

Part III concludes with two chapters on model/view programming: Chapter 14

introduces the subject and shows how to use Qt’s built-in views and how to

create custom data models and custom delegates, and Chapter 15 shows how

to use the model/view architecture to perform database programming.

Part IV continues the model/view theme, with coverage of three different

advanced model/view topics in Chapter 16. The ﬁrst section of Chapter 17

describes the techniques that can be used for providing online help, and the

second section explains how to internationalize an application, including how

to use Qt’s translation tools to create translation ﬁles. The Python standard

library provides its own classes for networking and for threading, but in the

last two chapters of Part IV we show how to do networking and threading us-

ing PyQt’s classes.

Appendix A explains where Python, PyQt, and Qt can be obtained, and how to

install them on Windows, Mac OS X, and Linux. PyQt is much easier to learn

if you install it and try out some of the exercises, and if you inspect some of

the example code. Appendix B presents screenshots and brief descriptions

of selected PyQt widgets; this is helpful for those new to GUI programming.

Appendix C presents diagrams of some of PyQt’s key class hierarchies; this

Introduction 5

is useful for getting to know what classes PyQt has to offer and how they

are related.

If you have never used Python before, you should begin by reading Chapters

1–6 in order. If you already know Python, at least read the string policy (in

bullet points on page 28), and skim the material in Chapter 2 (apart from the

ﬁrst section, which you’ll know well). Make sure that you are comfortable with

lambda

and partial function application, both of which are covered in Chapter 2.

It is probably also worth skimming Chapter 3 as well. Then read Chapters 4,

5, and 6 in order.

Once you have covered the ﬁrst six chapters, you have covered the essentials

of Python and the fundamentals of PyQt.

Chapter 7 is useful if you want to know how to create dialogs using a visual

design tool rather than purely by hand coding, something that can save a lot

of time. For ﬁle handling, at least read the ﬁrst three sections of Chapter 8. If

you plan to write and read text ﬁles, also read Chapter 8’s fourth section, and

similarly the ﬁfth section if you are going to use XML ﬁles.

For Part III, at the least read Chapter 10’s ﬁrst section, on event handling, and

all of Chapter 11. Chapter 12 and the ﬁrst section of Chapter 13 assume that

you have read about PyQt’s event handling,and that you have read Chapter 11.

Chapters 9 and 14 can be read stand-alone in this part,but Chapter 15 assumes

that you have read Chapter 14.

In Part IV, Chapter 16 assumes that you have read Chapters 14 and 15, but the

other chapters can be read independently.

If you ﬁnd errors in the text or in the examples, or have other comments,

please write to

mark@qtrac.eu

quoting “PyQt book” in the subject line. The

book’s home page, where any corrections will be published, and from where the

examples and exercise solutions can be downloaded, is

http://www.qtrac.eu/

pyqtbook.html

If you want to participate in the PyQt community, it is worthwhile joining the

mailing list. Go to

http://www.riverbankcomputing.com/mailman/listinfo/pyqt

ﬁnd a link to the archive, so that you can see what the mailing list is like, and

also for a form for joining. Python also has mailing lists and other community

activities. For these, go to

http://www.python.org/community

Acknowledgments

I have many people to thank, and I will begin with those who have been

intimately involved with the book.

Jasmin Blanchette is a senior software developer at Trolltech, a Qt expert, and

a ﬁne editor and writer in his own right. I have cowritten two C

/Qt books

with him. Jasmin has made a huge number of suggestions and criticisms that

have immensely improved the quality of this book.

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