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2014-08-24 Python Interpreter.md

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  • tags: python
  • date: 2014-08-24

Python Interpreter

Invoking the Interprer

The Python Interpreter is usually installed as /usr/local/bin/python3.4 on those machines where it is available.

Typing an end-of-file character (C-d on Unix, C-z on windows) at the primary prompt causes the interpreter to exit with a zero exit status. if thar doesn't work, you can exit the interpreter by typing the following command: quit().

1.1 Argument Passing

Arguments will assigned to the argv variable in the sys modules. You can access arguments list by executing import sys. The length of the list is at least one. Here list some cases about this:

  • no script and no arguments: sys.argv[0] is an empty string
  • script name '-'(meaning standard input): sys.argv[0] is set to '-'
  • -c command is used: sys.argv[0] is set to '-c'
  • -m command is used: sys.argv[0] is set to full name of the loacated modules

1.2 Interactive Mode

The Interpreter and Its Environment

2.1 Error Handling

When an error occurs, the interpreter prints an error message and a stack trace. Exceptions handled by an except clause in a try statement are not errors in this context.

2.2 Excutable Python Scripts

On Unix like systems, Python script can be made derectly executable, like the shell script, by putting the line:

#! /usr/bin/env python3.4

The script can be a executable mode, or permission, using the chmod command:

chmod +x myscript.py

2.3 Source Code Encoding

It is possible to specify a different encoding for source file. In order to do this, put one more special comment line right after the #! line to define the source file encoding：

# -*- coding: encoding -*-

2.4 The Interactive Startup File

You can do this by setting an variable named PYTHONSTARTUP to the name of a file containing your start-up command. This is similar to the .profile feature of the unix shells.

If you want to read an additional start-up file frome the current directory, you can program this in the global start-up file using code like this:

import os
filename = os.environ.get('PYTHONSTARTUP')
if filename and os.path.isfile(filename)
    exec(open(filename).read())

2.5 The Customization Modules

Python provides two hooks to let you customize it: sitecustomize and user customize. To see how it works, you need first to find the location of your site-packages directory. Start python and run this code:

>>> import site
>>> site.getusersitepackages()

sitecustomize works in the same way, but is typically created by an administrator of the computer in the global site-packages directory.