Using the Python Interpreter
2.1. Invoking the Interpreter¶
The Python interpreter is usually installed as /usr/local/bin/python on those machines where it is available; putting /usr/local/bin in your Unix shell’s search path makes it possible to start it by typing the command
python
to the shell. Since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E.g., /usr/local/python is a popular alternative location.)
On Windows machines, the Python installation is usually placed in C:\Python27, though you can change this when you’re running the installer. To add this directory to your path, you can type the following command into the command prompt in a DOS box:
set path=%path%;C:\python27
Typing an end-of-file character (Control-D on Unix, Control-Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn’t work, you can exit the interpreter by typing the following command: quit().
The interpreter’s line-editing features usually aren’t very sophisticated. On Unix, whoever installed the interpreter may have enabled support for the GNU readline library, which adds more elaborate interactive editing and history features. Perhaps the quickest check to see whether command line editing is supported is typing Control-P to the first Python prompt you get. If it beeps, you have command line editing; see Appendix Interactive Input Editing and History Substitution for an introduction to the keys. If nothing appears to happen, or if ^P is echoed, command line editing isn’t available; you’ll only be able to use backspace to remove characters from the current line.
The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes a script from that file.
A second way of starting the interpreter is python -c command [arg] ..., which executes the statement(s) in command, analogous to the shell’s -c option. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote command in its entirety with single quotes.
Some Python modules are also useful as scripts. These can be invoked using python -m module [arg] ..., which executes the source file for module as if you had spelled out its full name on the command line.
When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing -i before the script. (This does not work if the script is read from standard input, for the same reason as explained in the previous paragraph.)
2.1.1. Argument Passing¶
When known to the interpreter, the script name and additional arguments thereafter are turned into a list of strings and assigned to the argv variable in the sys module. You can access this list by executing import sys. The length of the list is at least one; when no script and no arguments are given, sys.argv[0] is an empty string. When the script name is given as '-' (meaning standard input), sys.argv[0] is set to '-'. When -c command is used, sys.argv[0] is set to '-c'. When -m module is used, sys.argv[0] is set to the full name of the located module. Options found after -c command or -m module are not consumed by the Python interpreter’s option processing but left in sys.argv for the command or module to handle.
2.1.2. Interactive Mode¶
When commands are read from a tty, the interpreter is said to be in interactive mode. In this mode it prompts for the next command with the primary prompt, usually three greater-than signs (>>>); for continuation lines it prompts with the secondary prompt, by default three dots (...). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt:
python
Python 2.7 (#1, Feb 28 2010, 00:02:06)
Type "help", "copyright", "credits" or "license" for more information.
>>>
Continuation lines are needed when entering a multi-line construct. As an example, take a look at this if statement:
>>> the_world_is_flat = 1
>>> if the_world_is_flat:
... print "Be careful not to fall off!"
...
Be careful not to fall off!
2.2. The Interpreter and Its Environment¶
2.2.1. 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 except clause in a 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. [1] Typing an interrupt while a command is executing raises the KeyboardInterrupt exception, which may be handled by a try statement.
2.2.2. 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 python
(assuming that the interpreter is on the user’s 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 chmod command:
$ 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.
2.2.3. Source Code Encoding¶
It is possible to use encodings different than ASCII in Python source files. The best way to do it is to put one more special comment line right after the #! line to define the source file encoding:
# -*- coding: encoding -*-
With that declaration, all characters in the source file will be treated as having the encoding encoding, and it will be possible to directly write Unicode string literals in the selected encoding. The list of possible encodings can be found in the Python Library Reference, in the section on codecs.
For example, to write Unicode literals including the Euro currency symbol, the ISO-8859-15 encoding can be used, with the Euro symbol having the ordinal value 164. This script will print the value 8364 (the Unicode codepoint corresponding to the Euro symbol) and then exit:
# -*- coding: iso-8859-15 -*-
currency = u"€"
print ord(currency)
If your editor supports saving files as UTF-8 with a UTF-8 byte order mark (aka BOM), you can use that instead of an encoding declaration. IDLE supports this capability if Options/General/Default Source Encoding/UTF-8 is set. Notice that this signature is not understood in older Python releases (2.2 and earlier), and also not understood by the operating system for script files with #! lines (only used on Unix systems).
By using UTF-8 (either through the signature or an encoding declaration), characters of most languages in the world can be used simultaneously in string literals and comments. Using non-ASCII characters in identifiers is not supported. To display all these characters properly, your editor must recognize that the file is UTF-8, and it must use a font that supports all the characters in the file.
2.2.4. 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 PYTHONSTARTUP to the name of a file containing your start-up commands. This is similar to the .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 /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'): execfile('.pythonrc.py'). 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):
execfile(filename)
Footnotes
| [1] |
A problem with the GNU Readline package may prevent this. |
使用Python解释器
2.1调用解释器
Python 解释器通常位于路径/usr/local/bin/python下，在UNIX的命令行输入python就能够将其打开。当然也可以将Python安装在其它路径下。在Windows操作系统中，Python常位于C:\Python27路径下。
解释器的编辑功能通常都不够强大，而GNU readline library 能够为编程人员提供更为强大的交互式编程工具。
确认是否支持命令行编辑的简便方法是在你的第一个Python程序中点击Ctrl+P，如果如果蜂鸣器响了，说明其支持命令行编辑模式。
Python解释器的工作方式有点像Unix的脚本程序：如果调用了标准输入输出，程序就会从命令行输入输出、数据；如果调用了以文件名命名的变量或者直接掉偶那个了一个文件，程序就读取并运行该脚本。
第二种打开Python解释器的方法是在命令行输入：
python -c command [arg] ...
它通过命令行来运行脚本。因为Python程序经常包含空格或者其它对解释器来说非常特别的字符，因此强烈建议您使用单引号引用命令。
一些Python模块也能够作为非常有用的脚本。他们可以通过python -m module [arg] ...语句进行调用。
当调用一个脚本文件的时候，有时通过交互式运行方法非常有用。它可以通过传递 -i参数来实现。(但如果程序使用的不是标准输入输出，交互式方法将无法实现。)
2.1.1 参数传递
当指令被解释器检测出来的时候，脚本的名字和参数也被转换成了一个字符串的列表并且被赋值给了sys模块中的argv变量，你可以通过执行import sys来查看这个长度至少为1的列表。当没有脚本也没有参数的时候，sys.argv[0]是一个空的字符串。
2.1.2交互式模式
当命令是通过标准输入输出方式执行的时候，我们说它运行在交互式模式下。
python
Python 2.7 (#1, Feb 28 2010, 00:02:06)
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> the_world_is_flat = 1
>>> if the_world_is_flat:
... print "Be careful not to fall off!"
...
Be careful not to fall off!
2.2解释器及其工作工作环境
2.2.1错误处理
当错误发生的时候，解释器打印一条错误信息及其堆栈调用。在交互式模式下，它返回到主提示；在输入来自文件的时候，程序打印出堆栈调用情况之后返回一个非零值。(在这里不把异常处理当做错误处理。)
所有的错误信息都被写入到standard output stream流。
在命令行引入中断会引发KeyboardInterrupt exception，需要通过try处理。
2.2.2可执行的Python脚本
在BSD UNIX操作系统中，Python脚本能够直接运行。在源文件的开头写上
#! /usr/bin/env python
可以使该文件具备直接运行的能力。
使用chmod能够使脚本可运行。
$ chmod +x myscript.py
2.2.3 代码编写
在Python源代码中可以使用与ASCII不同的编码方式。最好的方法是在#!写上注释。
# -*- coding: encoding -*-
通过这种生命方法，源文件中的所有字符都被当做encoding编码方法处理，也可以直接使用UNICODE编码方法。Python的所有支持编码方法可以在the Python Library Reference,中查到。
2.2.4在交互模式下运行程序
当你在交互式模式下使用Python的时候，在解释器运行是总会调用一些指令，你可以通过定义名为PYTHONSTARTUP环境变量来实现。它和UNIX中的.profile有些类似。
这种文件只在交互式模式下进行读取，而不再Python读取脚本文件时进行。他们执行在相同的命名空间之下，所以对象可以在交互式模式下使用。
如果你希望在当前的目录下读取一个额外的启动文件，你可以在全局启动文件中这样定义：
import os
filename = os.environ.get('PYTHONSTARTUP')
if filename and os.path.isfile(filename):
execfile(filename)
【附】
GNU readline 是由 GNU 工程创建和维护的一个软件包。它提供行编辑功能。例如，在 readline 授权的应用程序中，按下 C-b (CTRL-B) 将指针向后移到一个位置，然而 C-f 向前移动这个指针到一个位置；C-r 在历史纪录中搜索；这些组合键（这些是默认的，尽管像组合 vi's 是选择性的）来自于 GNU 的最早和最流行的工程，文本编辑器 Emacs 。Readline 支持多种高级特征，包括一个 kill ring（ 复制/粘贴剪切板的一种更灵活的版本）和 tab completion 。作为一个跨平台的库，readline 允许多种系统上很多应用程序来展示行编辑行为。它是在 GNU 通用公共许可证(GPL)下的分发的一个自由包。