《程序设计思想与方法》课程教学资源（课程教材）How to Think Like a Computer Scientist（Learning with Python）.pdf_大学文库

Foreword By David Beazley As an educator,researcher,and book author,I am delighted to see the completion of this book.Python is a fun and extremely easy-to-use programming language that has steadily gained in popularity over the last few years.Developed over ten years ago by Guido van Rossum,Python's simple syntax and overall feel is largely derived from ABC,a teaching language that was developed in the 1980's. However,Python was also created to solve real problems and it borrows a wide variety of features from programming languages such as C++,Java,Modula-3, and Scheme.Because of this,one of Python's most remarkable features is its broad appeal to professional software developers,scientists,researchers,artists, and educators. Despite Python's appeal to many different communities,you may still wonder "why Python?"or "why teach programming with Python?"Answering these questions is no simple task-especially when popular opinion is on the side of more masochistic alternatives such as C++and Java.However,I think the most direct answer is that programming in Python is simply a lot of fun and more productive. When I teach computer science courses,I want to cover important concepts in addition to making the material interesting and engaging to students.Unfortu- nately,there is a tendency for introductory programming courses to focus far too much attention on mathematical abstraction and for students to become frus- trated with annoying problems related to low-level details of syntax,compilation, and the enforcement of seemingly arcane rules.Although such abstraction and formalism is important to professional software engineers and students who plan to continue their study of computer science,taking such an approach in an intro- ductory course mostly succeeds in making computer science boring.When I teach a course,I don't want to have a room of uninspired students.I would much rather see them trying to solve interesting problems by exploring different ideas,taking unconventional approaches,breaking the rules,and learning from their mistakes

Foreword By David Beazley As an educator, researcher, and book author, I am delighted to see the completion of this book. Python is a fun and extremely easy-to-use programming language that has steadily gained in popularity over the last few years. Developed over ten years ago by Guido van Rossum, Python’s simple syntax and overall feel is largely derived from ABC, a teaching language that was developed in the 1980’s. However, Python was also created to solve real problems and it borrows a wide variety of features from programming languages such as C++, Java, Modula-3, and Scheme. Because of this, one of Python’s most remarkable features is its broad appeal to professional software developers, scientists, researchers, artists, and educators. Despite Python’s appeal to many different communities, you may still wonder “why Python?” or “why teach programming with Python?” Answering these questions is no simple task—especially when popular opinion is on the side of more masochistic alternatives such as C++ and Java. However, I think the most direct answer is that programming in Python is simply a lot of fun and more productive. When I teach computer science courses, I want to cover important concepts in addition to making the material interesting and engaging to students. Unfortunately, there is a tendency for introductory programming courses to focus far too much attention on mathematical abstraction and for students to become frustrated with annoying problems related to low-level details of syntax, compilation, and the enforcement of seemingly arcane rules. Although such abstraction and formalism is important to professional software engineers and students who plan to continue their study of computer science, taking such an approach in an introductory course mostly succeeds in making computer science boring. When I teach a course, I don’t want to have a room of uninspired students. I would much rather see them trying to solve interesting problems by exploring different ideas, taking unconventional approaches, breaking the rules, and learning from their mistakes

vi Foreword In doing so,I don't want to waste half of the semester trying to sort out obscure syntax problems,unintelligible compiler error messages,or the several hundred ways that a program might generate a general protection fault. One of the reasons why I like Python is that it provides a really nice balance between the practical and the conceptual.Since Python is interpreted,beginners can pick up the language and start doing neat things almost immediately with- out getting lost in the problems of compilation and linking.Furthermore,Python comes with a large library of modules that can be used to do all sorts of tasks rang- ing from web-programming to graphics.Having such a practical focus is a great way to engage students and it allows them to complete significant projects.How- ever,Python can also serve as an excellent foundation for introducing important computer science concepts.Since Python fully supports procedures and classes, students can be gradually introduced to topics such as procedural abstraction, data structures,and object-oriented programming-all of which are applicable to later courses on Java or C++.Python even borrows a number of features from functional programming languages and can be used to introduce concepts that would be covered in more detail in courses on Scheme and Lisp. In reading Jeffrey's preface,I am struck by his comments that Python allowed him to see a "higher level of success and a lower level of frustration"and that he was able to "move faster with better results."Although these comments refer to his introductory course,I sometimes use Python for these exact same reasons in advanced graduate level computer science courses at the University of Chicago. In these courses,I am constantly faced with the daunting task of covering a lot of difficult course material in a blistering nine week quarter.Although it is certainly possible for me to inflict a lot of pain and suffering by using a language like C++, I have often found this approach to be counterproductive-especially when the course is about a topic unrelated to just "programming."I find that using Python allows me to better focus on the actual topic at hand while allowing students to complete substantial class projects. Although Python is still a young and evolving language,I believe that it has a bright future in education.This book is an important step in that direction. David Beazley University of Chicago Author of the Python Essential Reference

vi Foreword In doing so, I don’t want to waste half of the semester trying to sort out obscure syntax problems, unintelligible compiler error messages, or the several hundred ways that a program might generate a general protection fault. One of the reasons why I like Python is that it provides a really nice balance between the practical and the conceptual. Since Python is interpreted, beginners can pick up the language and start doing neat things almost immediately without getting lost in the problems of compilation and linking. Furthermore, Python comes with a large library of modules that can be used to do all sorts of tasks ranging from web-programming to graphics. Having such a practical focus is a great way to engage students and it allows them to complete significant projects. However, Python can also serve as an excellent foundation for introducing important computer science concepts. Since Python fully supports procedures and classes, students can be gradually introduced to topics such as procedural abstraction, data structures, and object-oriented programming—all of which are applicable to later courses on Java or C++. Python even borrows a number of features from functional programming languages and can be used to introduce concepts that would be covered in more detail in courses on Scheme and Lisp. In reading Jeffrey’s preface, I am struck by his comments that Python allowed him to see a “higher level of success and a lower level of frustration” and that he was able to “move faster with better results.” Although these comments refer to his introductory course, I sometimes use Python for these exact same reasons in advanced graduate level computer science courses at the University of Chicago. In these courses, I am constantly faced with the daunting task of covering a lot of difficult course material in a blistering nine week quarter. Although it is certainly possible for me to inflict a lot of pain and suffering by using a language like C++, I have often found this approach to be counterproductive—especially when the course is about a topic unrelated to just “programming.” I find that using Python allows me to better focus on the actual topic at hand while allowing students to complete substantial class projects. Although Python is still a young and evolving language, I believe that it has a bright future in education. This book is an important step in that direction. David Beazley University of Chicago Author of the Python Essential Reference

viii Preface had to be a better language choice for our first-year class,I went looking for an alternative to C++. I needed a language that would run on the machines in our Linux lab as well as on the Windows and Macintosh platforms most students have at home.I wanted it to be free and available electronically,so that students could use it at home regardless of their income.I wanted a language that was used by professional programmers, and one that had an active developer community around it.It had to support both procedural and object-oriented programming.And most importantly,it had to be easy to learn and teach.When I investigated the choices with these goals in mind,Python stood out as the best candidate for the job. I asked one of Yorktown's talented students,Matt Ahrens,to give Python a try. In two months he not only learned the language but wrote an application called pyTicket that enabled our staff to report technology problems via the Web.I knew that Matt could not have finished an application of that scale in so short a time in C++,and this accomplishment,combined with Matt's positive assessment of Python,suggested that Python was the solution I was looking for. Finding a textbook Having decided to use Python in both of my introductory computer science classes the following year,the most pressing problem was the lack of an available textbook Free content came to the rescue.Earlier in the year,Richard Stallman had in- troduced me to Allen Downey.Both of us had written to Richard expressing an interest in developing free educational content.Allen had already written a first- year computer science textbook,How to Think Like a Computer Scientist.When I read this book,I knew immediately that I wanted to use it in my class.It was the clearest and most helpful computer science text I had seen.It emphasized the processes of thought involved in programming rather than the features of a particular language.Reading it immediately made me a better teacher. How to Think Like a Computer Scientist was not just an excellent book,but it had been released under a GNU public license,which meant it could be used freely and modified to meet the needs of its user.Once I decided to use Python, it occurred to me that I could translate Allen's original Java version of the book into the new language.While I would not have been able to write a textbook on my own,having Allen's book to work from made it possible for me to do so,at the same time demonstrating that the cooperative development model used so well in software could also work for educational content. Working on this book for the last two years has been rewarding for both my students and me,and my students played a big part in the process.Since I could

viii Preface had to be a better language choice for our first-year class, I went looking for an alternative to C++. I needed a language that would run on the machines in our Linux lab as well as on the Windows and Macintosh platforms most students have at home. I wanted it to be free and available electronically, so that students could use it at home regardless of their income. I wanted a language that was used by professional programmers, and one that had an active developer community around it. It had to support both procedural and object-oriented programming. And most importantly, it had to be easy to learn and teach. When I investigated the choices with these goals in mind, Python stood out as the best candidate for the job. I asked one of Yorktown’s talented students, Matt Ahrens, to give Python a try. In two months he not only learned the language but wrote an application called pyTicket that enabled our staff to report technology problems via the Web. I knew that Matt could not have finished an application of that scale in so short a time in C++, and this accomplishment, combined with Matt’s positive assessment of Python, suggested that Python was the solution I was looking for. Finding a textbook Having decided to use Python in both of my introductory computer science classes the following year, the most pressing problem was the lack of an available textbook. Free content came to the rescue. Earlier in the year, Richard Stallman had introduced me to Allen Downey. Both of us had written to Richard expressing an interest in developing free educational content. Allen had already written a firstyear computer science textbook, How to Think Like a Computer Scientist. When I read this book, I knew immediately that I wanted to use it in my class. It was the clearest and most helpful computer science text I had seen. It emphasized the processes of thought involved in programming rather than the features of a particular language. Reading it immediately made me a better teacher. How to Think Like a Computer Scientist was not just an excellent book, but it had been released under a GNU public license, which meant it could be used freely and modified to meet the needs of its user. Once I decided to use Python, it occurred to me that I could translate Allen’s original Java version of the book into the new language. While I would not have been able to write a textbook on my own, having Allen’s book to work from made it possible for me to do so, at the same time demonstrating that the cooperative development model used so well in software could also work for educational content. Working on this book for the last two years has been rewarding for both my students and me, and my students played a big part in the process. Since I could

i议 make instant changes whenever someone found a spelling error or difficult passage, I encouraged them to look for mistakes in the book by giving them a bonus point each time they made a suggestion that resulted in a change in the text.This had the double benefit of encouraging them to read the text more carefully and of getting the text thoroughly reviewed by its most important critics,students using it to learn computer science. For the second half of the book on object-oriented programming,I knew that someone with more real programming experience than I had would be needed to do it right.The book sat in an unfinished state for the better part of a year until the free software community once again provided the needed means for its completion. I received an email from Chris Meyers expressing interest in the book.Chris is a professional programmer who started teaching a programming course last year using Python at Lane Community College in Eugene,Oregon.The prospect of teaching the course had led Chris to the book,and he started helping out with it immediately.By the end of the school year he had created a companion project on our website at http://www.ibiblio.org/obp called Python for Fun and was working with some of my most advanced students as a master teacher,guiding them beyond where I could take them. Introducing programming with Python The process of translating and using How to Think Like a Computer Scientist for the past two years has confirmed Python's suitability for teaching beginning students.Python greatly simplifies programming examples and makes important programming ideas easier to teach. The first example from the text illustrates this point.It is the traditional "hello, world"program,which in the C++version of the book looks like this: #include void main() f cout <"Hello,world."<<endl; in the Python version it becomes: print"Hello,World！" Even though this is a trivial example,the advantages of Python stand out.York- town's Computer Science I course has no prerequisites,so many of the students

ix make instant changes whenever someone found a spelling error or difficult passage, I encouraged them to look for mistakes in the book by giving them a bonus point each time they made a suggestion that resulted in a change in the text. This had the double benefit of encouraging them to read the text more carefully and of getting the text thoroughly reviewed by its most important critics, students using it to learn computer science. For the second half of the book on object-oriented programming, I knew that someone with more real programming experience than I had would be needed to do it right. The book sat in an unfinished state for the better part of a year until the free software community once again provided the needed means for its completion. I received an email from Chris Meyers expressing interest in the book. Chris is a professional programmer who started teaching a programming course last year using Python at Lane Community College in Eugene, Oregon. The prospect of teaching the course had led Chris to the book, and he started helping out with it immediately. By the end of the school year he had created a companion project on our website at http://www.ibiblio.org/obp called Python for Fun and was working with some of my most advanced students as a master teacher, guiding them beyond where I could take them. Introducing programming with Python The process of translating and using How to Think Like a Computer Scientist for the past two years has confirmed Python’s suitability for teaching beginning students. Python greatly simplifies programming examples and makes important programming ideas easier to teach. The first example from the text illustrates this point. It is the traditional “hello, world” program, which in the C++ version of the book looks like this: #include void main() { cout << "Hello, world." << endl; } in the Python version it becomes: print "Hello, World!" Even though this is a trivial example, the advantages of Python stand out. Yorktown’s Computer Science I course has no prerequisites, so many of the students

中 Preface seeing this example are looking at their first program.Some of them are undoubt- edly a little nervous,having heard that computer programming is difficult to learn. The C++version has always forced me to choose between two unsatisfying op- tions:either to explain #include,void main(),{and }and risk confusing or intimidating some of the students right at the start,or to tell them,"Just don't worry about all of that stuff now;we will talk about it later,"and risk the same thing.The educational objectives at this point in the course are to introduce students to the idea of a programming language and to get them to write their first program,thereby introducing them to the programming environment.The Python program has exactly what is needed to do these things,and nothing more. Comparing the explanatory text of the program in each version of the book fur- ther illustrates what this means to the beginning student.There are thirteen paragraphs of explanation of "Hello,world!"in the C++version;in the Python version,there are only two.More importantly,the missing eleven paragraphs do not deal with the "big ideas"in computer programming but with the minutia of C++syntax.I found this same thing happening throughout the book.Whole paragraphs simply disappear from the Python version of the text because Python's much clearer syntax renders them unnecessary. Using a very high-level language like Python allows a teacher to postpone talking about low-level details of the machine until students have the background that they need to better make sense of the details.It thus creates the ability to put "first things first"pedagogically.One of the best examples of this is the way in which Python handles variables.In C++a variable is a name for a place that holds a thing.Variables have to be declared with types at least in part because the size of the place to which they refer needs to be predetermined.Thus,the idea of a variable is bound up with the hardware of the machine.The powerful and fundamental concept of a variable is already difficult enough for beginning students (in both computer science and algebra).Bytes and addresses do not help the matter.In Python a variable is a name that refers to a thing.This is a far more intuitive concept for beginning students and is much closer to the meaning of "variable"that they learned in their math courses.I had much less difficulty teaching variables this year than I did in the past,and I spent less time helping students with problems using them. Another example of how Python aids in the teaching and learning of programming is in its syntax for functions.My students have always had a great deal of difficulty understanding functions.The main problem centers around the difference between a function definition and a function call,and the related distinction between a parameter and an argument.Python comes to the rescue with syntax that is nothing short of beautiful.Function definitions begin with the keyword def,so I simply tell my students,"When you define a function,begin with def,followed by the name of the function that you are defining;when you call a function,simply

x Preface seeing this example are looking at their first program. Some of them are undoubtedly a little nervous, having heard that computer programming is difficult to learn. The C++ version has always forced me to choose between two unsatisfying options: either to explain #include, void main(), {, and }, and risk confusing or intimidating some of the students right at the start, or to tell them, “Just don’t worry about all of that stuff now; we will talk about it later,” and risk the same thing. The educational objectives at this point in the course are to introduce students to the idea of a programming language and to get them to write their first program, thereby introducing them to the programming environment. The Python program has exactly what is needed to do these things, and nothing more. Comparing the explanatory text of the program in each version of the book further illustrates what this means to the beginning student. There are thirteen paragraphs of explanation of “Hello, world!” in the C++ version; in the Python version, there are only two. More importantly, the missing eleven paragraphs do not deal with the “big ideas” in computer programming but with the minutia of C++ syntax. I found this same thing happening throughout the book. Whole paragraphs simply disappear from the Python version of the text because Python’s much clearer syntax renders them unnecessary. Using a very high-level language like Python allows a teacher to postpone talking about low-level details of the machine until students have the background that they need to better make sense of the details. It thus creates the ability to put “first things first” pedagogically. One of the best examples of this is the way in which Python handles variables. In C++ a variable is a name for a place that holds a thing. Variables have to be declared with types at least in part because the size of the place to which they refer needs to be predetermined. Thus, the idea of a variable is bound up with the hardware of the machine. The powerful and fundamental concept of a variable is already difficult enough for beginning students (in both computer science and algebra). Bytes and addresses do not help the matter. In Python a variable is a name that refers to a thing. This is a far more intuitive concept for beginning students and is much closer to the meaning of “variable” that they learned in their math courses. I had much less difficulty teaching variables this year than I did in the past, and I spent less time helping students with problems using them. Another example of how Python aids in the teaching and learning of programming is in its syntax for functions. My students have always had a great deal of difficulty understanding functions. The main problem centers around the difference between a function definition and a function call, and the related distinction between a parameter and an argument. Python comes to the rescue with syntax that is nothing short of beautiful. Function definitions begin with the keyword def, so I simply tell my students, “When you define a function, begin with def, followed by the name of the function that you are defining; when you call a function, simply