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.. _tut-appendix:

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Appendix

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.. _tut-interac:

Interactive Mode

================

.. _tut-error:

Error Handling

--------------

When an error occurs, the interpreter prints an error message and a stack trace.

In interactive mode, it then returns to the primary prompt; when input came from

a file, it exits with a nonzero exit status after printing the stack trace.

(Exceptions handled by an :keyword:`except` clause in a :keyword:`try` statement

are not errors in this context.) Some errors are unconditionally fatal and

cause an exit with a nonzero exit; this applies to internal inconsistencies and

some cases of running out of memory. All error messages are written to the

standard error stream; normal output from executed commands is written to

standard output.

Typing the interrupt character (usually Control-C or DEL) to the primary or

secondary prompt cancels the input and returns to the primary prompt. [#]_

Typing an interrupt while a command is executing raises the

:exc:`KeyboardInterrupt` exception, which may be handled by a :keyword:`try`

statement.

.. _tut-scripts:

Executable Python Scripts

-------------------------

On BSD'ish Unix systems, Python scripts can be made directly executable, like

shell scripts, by putting the line ::

#!/usr/bin/env python3.4

(assuming that the interpreter is on the user's :envvar:`PATH`) at the beginning

of the script and giving the file an executable mode. The ``#!`` must be the

first two characters of the file. On some platforms, this first line must end

with a Unix-style line ending (``'\n'``), not a Windows (``'\r\n'``) line

ending. Note that the hash, or pound, character, ``'#'``, is used to start a

comment in Python.

The script can be given an executable mode, or permission, using the

:program:`chmod` command.

.. code-block:: bash

$ chmod +x myscript.py

On Windows systems, there is no notion of an "executable mode". The Python

installer automatically associates ``.py`` files with ``python.exe`` so that

a double-click on a Python file will run it as a script. The extension can

also be ``.pyw``, in that case, the console window that normally appears is

suppressed.

.. _tut-startup:

The Interactive Startup File

----------------------------

When you use Python interactively, it is frequently handy to have some standard

commands executed every time the interpreter is started. You can do this by

setting an environment variable named :envvar:`PYTHONSTARTUP` to the name of a

file containing your start-up commands. This is similar to the :file:`.profile`

feature of the Unix shells.

This file is only read in interactive sessions, not when Python reads commands

from a script, and not when :file:`/dev/tty` is given as the explicit source of

commands (which otherwise behaves like an interactive session). It is executed

in the same namespace where interactive commands are executed, so that objects

that it defines or imports can be used without qualification in the interactive

session. You can also change the prompts ``sys.ps1`` and ``sys.ps2`` in this

file.

If you want to read an additional start-up file from the current directory, you

can program this in the global start-up file using code like ``if

os.path.isfile('.pythonrc.py'): exec(open('.pythonrc.py').read())``.

If you want to use the startup file in a script, you must do this explicitly

in the script::

import os

filename = os.environ.get('PYTHONSTARTUP')

if filename and os.path.isfile(filename):

with open(filename) as fobj:

startup_file = fobj.read()

exec(startup_file)

.. _tut-customize:

The Customization Modules

-------------------------

Python provides two hooks to let you customize it: :mod:`sitecustomize` and

:mod:`usercustomize`. To see how it works, you need first to find the location

of your user site-packages directory. Start Python and run this code::

>>> import site

>>> site.getusersitepackages()

'/home/user/.local/lib/python3.4/site-packages'

Now you can create a file named :file:`usercustomize.py` in that directory and

put anything you want in it. It will affect every invocation of Python, unless

it is started with the :option:`-s` option to disable the automatic import.

:mod:`sitecustomize` works in the same way, but is typically created by an

administrator of the computer in the global site-packages directory, and is

imported before :mod:`usercustomize`. See the documentation of the :mod:`site`

module for more details.

.. rubric:: Footnotes

.. [#] A problem with the GNU Readline package may prevent this.

.. _tut-intro:

**********************

开胃菜

**********************

如果你要用计算机做很多工作，最后你会发现有一些任务你更希望用自动化的方式进行处理。比如，你想要在大量的文本文件中执行查找/替换，或者以复杂的方式对大量的图片进行重命名和整理。也许你想要编写一个小型的自定义数据库、一个特殊的 GUI 应用程序或一个简单的小游戏。

如果你是一名专业的软件开发者，可能你必须使用几种 C/C++/JAVA 类库，并且发现通常编写/编译/测试/重新编译的周期是如此漫长。也许你正在为这些类库编写测试用例，但是发现这是一个让人烦躁的工作。又或者你已经完成了一个可以使用扩展语言的程序，但你并不想为此重新设计并实现一套全新的语言。

那么 Python 正是你所需要的语言。

虽然你能够通过编写 Unix shell 脚本或 Windows 批处理文件来处理其中的某些任务，但 Shell 脚本更适合移动文件或修改文本数据，并不适合编写 GUI 应用程序或游戏；虽然你能够使用 C/C++/JAVA 编写程序，但即使编写一个简单的 first-draft 程序也有可能耗费大量的开发时间。相比之下，Python 更易于使用，无论在 Windows、Mac OS X 或 Unix 操作系统上它都会帮助你更快地完成任务。

虽然 Python 易于使用，但它却是一门完整的编程语言；与 Shell 脚本或批处理文件相比，它为编写大型程序提供了更多的结构和支持。另一方面，Python 提供了比 C 更多的错误检查，并且作为一门 *高级语言*，它内置支持高级的数据结构类型，例如：灵活的数组和字典。因其更多的通用数据类型，Python 比 Awk 甚至 Perl 都适用于更多问题领域，至少大多数事情在 Python 中与其他语言同样简单。

Python 允许你将程序分割为不同的模块，以便在其他的 Python 程序中重用。Python 内置提供了大量的标准模块，你可以将其用作程序的基础，或者作为学习 Python 编程的示例。这些模块提供了诸如文件 I/O、系统调用、Socket 支持，甚至类似 Tk 的用户图形界面（GUI）工具包接口。

Python 是一门解释型语言，因为无需编译和链接，你可以在程序开发中节省宝贵的时间。Python 解释器可以交互的使用，这使得试验语言的特性、编写临时程序或在自底向上的程序开发中测试方法非常容易。你甚至还可以把它当做一个桌面计算器。

Python 让程序编写的紧凑和可读。用 Python 编写的程序通常比同样的 C、C++ 或 Java 程序更短小，这是因为以下几个原因：

* 高级数据结构使你可以在一条语句中表达复杂的操作；

* 语句组使用缩进代替开始和结束大括号来组织；

* 变量或参数无需声明。

Python 是 *可扩展* 的：如果你会 C 语言编程便可以轻易地为解释器添加内置函数或模块，或者为了对性能瓶颈作优化，或者将 Python 程序与只有二进制形式的库（比如某个专业的商业图形库）连接起来。一旦你真正掌握了它，你可以将 Python 解释器集成进某个 C 应用程序，并把它当作那个程序的扩展或命令行语言。

顺便说一句，这个语言的名字来自于 BBC 的 “Monty Python’s Flying Cirecus” 节目，和爬行类动物没有任何关系。在文档中引用 Monty Python 的典故不仅可行，而且值得鼓励！

现在你已经为 Python 兴奋不已了吧，大概想要领略一些更多的细节！学习一门语言最好的方法就是使用它，本指南推荐你边读边使用 Python 解释器练习。

下一节中，我们将解释 Python 解释器的用法。这是很简单的一件事情，但它有助于试验后面的例子。

本手册剩下的部分将通过示例介绍 Python 语言及系统的诸多特性，开始是简单的语法、数据类型和表达式，接着介绍函数与模块，最后涉及异常和自定义类这样的高级内容。