- python的特点
- 2024-07-06
- Although it has been around for well over fifteen years, some feel that Python is still relatively new to the general software development industry. We should, however, use caution with our use of the word “relatively,” as a few years seem like decades when developing on “Internet time.” When people ask, “What is Python?” it is difficult to say any one thing. The tendency is to want to blurt out all the things that you feel Python is in one breath. Python is (fill-in-the-blanks here) . Just what are some of those features? For your sanity, we will elucidate each here . . . one at a time. 1.3.1 High Level It seems that with every generation of languages, we move to a higher level. Assembly was a godsend for those who struggled with machine code, then came FORTRAN, C, and Pascal, which took computing to another plane and created the software development industry. Through C came more modern compiled languages, C++ and Java. And further still we climb, with powerful, system-accessible, interpreted scripting languages like Tcl, Perl, and Python. ptg 1.3 Features 7 Each of these languages has higher-level data structures that reduce the “framework” development time that was once required. Useful types like Python’s lists (resizeable arrays) and dictionaries (hash tables) are built into the language. Providing these crucial building blocks in the core language encourages their use and minimizes development time as well as code size, resulting in more readable code. Because there is no one standard library for heterogeneous arrays (lists in Python) and hash tables (Python dictionaries or “dicts” for short) in C, they are often reimplemented and copied to each new project. This process is messy and error prone. C++ improves the situation with the standard template library, but the STL can hardly compare to the simplicity and readability of Python’s built-in lists and dicts. 1.3.2 Object Oriented Object-oriented programming (OOP) adds another dimension to structured and procedural languages where data and logic are discrete elements of programming. OOP allows for associating specific behaviors, characteristics, and/or capabilities with the data that they execute on or are representative of. Python is an object-oriented (OO) language, all the way down to its core. However, Python is not just an OO language like Java or Ruby. It is actually a pleasant mix of multiple programming paradigms. For instance, it even borrows a few things from functional languages like Lisp and Haskell. 1.3.3 Scalable Python is often compared to batch or Unix shell scripting languages. Simple shell scripts handle simple tasks. They may grow (indefinitely) in length, but not truly in depth. There is little code-reusability and you are confined to small projects with shell scripts. In fact, even small projects may lead to large and unwieldy scripts. Not so with Python, where you can grow your code from project to project, add other new or existing Python elements, and reuse code at your whim. Python encourages clean code design, high-level structure, and “packaging” of multiple components, all of which deliver the flexibility, consistency, and faster development time required as projects expand in breadth and scope. The term “scalable” is most often applied to measuring hardware throughput and usually refers to additional performance when new hardware is added to a system. We would like to differentiate this comparison with ours here, which tries to reflect the notion that Python provides basic building ptg 8 Chapter 1 Welcome to Python! blocks on which you can build an application, and as those needs expand and grow, Python’s pluggable and modular architecture allows your project to flourish as well as maintain manageability. 1.3.4 Extensible As the amount of Python code increases in your project, you will still be able to organize it logically by separating your code into multiple files, or modules, and be able to access code from one module and attributes from another. And what is even better is that Python’s syntax for accessing modules is the same for all modules, whether you access one from the Python standard library, one you created just a minute ago, or even an extension you wrote in another language! Using this feature, you feel like you have just “extended” the language for your own needs, and you actually have. The most critical portions of code, perhaps those hotspots that always show up in the profiler or areas where performance is absolutely required, are candidates for being rewritten as a Python extension written in C. But again, the interface is exactly the same as for pure Python modules. Access to code and objects occurs in exactly the same way without any code modification whatsoever. The only thing different about the code now is that you should notice an improvement in performance. Naturally, it all depends on your application and how resource-intensive it is. There are times where it is absolutely advantageous to convert application bottlenecks to compiled code because it will decidedly improve overall performance. This type of extensibility in a language provides engineers with the flexibility to add-on or customize their tools to be more productive, and to develop in a shorter period of time. Although this feature is self-evident in mainstream third-generation languages (3GLs) such as C, C++, and even Java, the ease of writing extensions to Python in C is a real strength of Python. Furthermore, tools like PyRex, which understands a mix of C and Python, make writing extensions even easier as they compile everything to C for you. Python extensions can be written in C and C++ for the standard implementation of Python in C (also known as CPython). The Java language implementation of Python is called Jython, so extensions would be written using Java. Finally, there is IronPython, the C# implementation for the .NET or Mono platforms. You can extend IronPython in C# or Visual Basic.NET. 1.3.5 Portable Python can be found on a wide variety of systems, contributing to its continued rapid growth in today’s computing domain. Because Python is written in C, ptg 1.3 Features 9 and because of C’s portability, Python is available on practically every type of platform that has an ANSI C compiler. Although there are some platformspecific modules, any general Python application written on one system will run with little or no modification on another. Portability applies across multiple architectures as well as operating systems. 1.3.6 Easy to Learn Python has relatively few keywords, simple structure, and a clearly defined syntax. This allows the student to pick up the language in a relatively short period of time. What may perhaps be new to beginners is the OO nature of Python. Those who are not fully versed in the ways of OOP may be apprehensive about jumping straight into Python, but OOP is neither necessary nor mandatory. Getting started is easy, and you can pick up OOP and use when you are ready to. 1.3.7 Easy to Read Conspicuously absent from the Python syntax are the usual mandatory symbols found in other languages for accessing variables, code block definition, and pattern-matching. These include dollar signs ( $ ), semicolons ( ; ), tildes ( ~ ), and so on. Without all these distractions, Python code is much more clearly defined and visible to the eye. In addition, much to many programmers’ dismay (and relief), Python does not give as much flexibility to write obfuscated code compared to other languages, making it easier for others to understand your code faster and vice versa. Readability usually helps make a language easy to learn, as we described above. We would even venture to claim that Python code is fairly understandable even to a reader who has never seen a single line of Python before. Take a look at the examples in the next chapter, “Getting Started,” and let us know how well you fare. 1.3.8 Easy to Maintain Maintaining source code is part of the software development lifecycle. Your software usually continues to evolve until it is replaced or obsoleted. Quite often it lasts longer than a programmer’s stay at a company. Much of Python’s success is that source code is fairly easy to maintain, dependent, of course, on size and complexity. However, this conclusion is not difficult to draw given that Python is easy to learn and easy to read. Another motivating advantage of Python is that upon reviewing a script you wrote six months ago, ptg 10 Chapter 1 Welcome to Python! you are less likely to get lost or pull out a reference book to get reacquainted with your software. 1.3.9 Robust Nothing is more powerful than allowing a programmer to recognize error conditions and provide a software handler when such errors occur. Python provides “safe and sane” exits on errors, allowing the programmer to be in the driver’s seat. When your Python crashes due to errors, the interpreter dumps out a “stack trace” full of useful information such as why your program crashed and where in the code (file name, line number, function call, etc.) the error took place. These errors are known as exceptions. Python even gives you the ability to monitor for errors and take an evasive course of action if such an error does occur during runtime. These exception handlers can take steps such as defusing the problem, redirecting program flow, perform cleanup or maintenance measures, shutting down the application gracefully, or just ignoring it. In any case, the debugging part of the development cycle is reduced considerably. Python’s robustness is beneficial for both the software designer and the user. There is also some accountability when certain errors occur that are not handled properly. The stack trace that is generated as a result of an error reveals not only the type and location of the error, but also in which module the erroneous code resides. 1.3.10 Effective as a Rapid Prototyping Tool We’ve mentioned before how Python is easy to learn and easy to read. But, you say, so is a language like BASIC. What more can Python do? Unlike selfcontained and less flexible languages, Python has so many different interfaces to other systems that it is powerful enough in features and robust enough that entire systems can be prototyped completely in Python. Obviously, the same systems can be completed in traditional compiled languages, but Python’s simplicity of engineering allows us to do the same thing and still be home in time for supper. Also, numerous external libraries have already been developed for Python, so whatever your application is, someone may have traveled down that road before. All you need to do is “plug-and-play” (some assembly required, as usual). There are Python modules and packages that can do practically anything and everything you can imagine. The Python Standard Library is fairly complete, and if you cannot find what you need there, chances are there is a third-party module or package that can do the job. ptg 1.4 Downloading and Installing Python 11 1.3.11 A Memory Manager The biggest pitfall with programming in C or C++ is that the responsibility of memory management is in the hands of the developer. Even if the application has very little to do with memory access, memory modification, and memory management, the programmer must still perform those duties, in addition to the original task at hand. This places an unnecessary burden and responsibility upon the developer and often provides an extended distraction. Because memory management is performed by the Python interpreter, the application developer is able to steer clear of memory issues and focus on the immediate goal of just creating the application that was planned in the first place. This leads to fewer bugs, a more robust application, and shorter overall development time. 1.3.12 Interpreted and (Byte-) Compiled Python is classified as an interpreted language, meaning that compile-time is no longer a factor during development. Traditionally, purely interpreted languages are almost always slower than compiled languages because execution does not take place in a system’s native binary language. However, like Java, Python is actually byte-compiled, resulting in an intermediate form closer to machine language. This improves Python’s performance, yet allows it to retain all the advantages of interpreted languages
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