15. 附录 Python v3.4.3 官方教程

15.1.1. 错误处理

当错误发生时，解释器会输出错误信息和堆栈追踪。在交互模式下，Python解释器会返回主提示符；如果是从文件输入，它会在输出堆栈追踪后以非零状态退出。(Exceptions handled by an except clause in a try statement are not errors in this context.) 一些错误是非常致命的，同时导致以非零状态退出。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.

在主从提示符下输入中断符（通常是Control-C或者DEL键），取消输入然后回到主提示符。[1]在命令执行的时候输入中断符会导致 KeyboardInterrupt 异常,这个异常会被 try 单元处理。

15.1.2. Executable Python Scripts

在类UNIX系统上，Python脚本可以被直接执行，就像直接输入shell脚本一样。

#!/usr/bin/env python3.4 

(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 (' '), not a Windows ('r ') line ending. Note that the hash, or pound, character, '#', is used to start a comment in Pyth

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.

15.1.3. 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'): 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) 

15.1.4. The Customization Modules

Python provides two hooks to let you customize it: sitecustomize and 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 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 -s option to disable the automatic import.

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 usercustomize. See the documentation of the site module for more details.

Footnotes