Python基础（1）

1、"Hello World"
    --print....
        print 'Hello World'   #(2.x)
        print ('Hello World')    #(3.x)
    --stdout
        import sys
        sys.stdout.write('Hello World!/n')
2、general rules
    --use '#' to start a comment
    --one statement per line,unless semicolon used more than one statement per line
    --code blocks delimited by indentation
    --use '/' to break up long lines
3、simple statements fit on one line,no code block following
4、compound statements have code block
    --fist line called a header
    --indented sub-block of code called a suite
    --header plus ':' followed by indented suite
    --indentation allows any whiltespace
        ==use spaces not tabs
        ==use same # of spaces for each line
5、Variables
    --dynamic typing....variables not declared
        ==type inferred on assignment
        ==var = expression
    --flexible assignment syntax
        ==var1 = var2 = xepression
        ==var1, var2 = exp1, exp2        #it equals to var1=exp1;var2=exp2
6、Assignment
    --primary assignment operator:=
    --augmented assignment:+=,*=,.....
    --++,-- are not permitted
7、identifier rules (similar to other language)
    --fist character must be alphabetic
    --any others can be alphanumeric
    --treat underscores (_) as alphabetic
    --case-sensitive
8、Python gotchas
    --special names begin/end with underscores
        ==avoid using until you know what...
            **they are used for
            **you are doing
    --do not use built-in function/data names

   
   
     --reserved words,or keywords,can't be used
   
9、Function
    --declared using def  keyword
   
    --declarations:
        ==header line and at least one for body
        ==can have any number of arguments
    --if no value explicitly returned...
        ==Python returns None
            **None: Python's NULL or void
            **None: constant Boolean False
10、Importing & Modules
    --Allows use of outside code
    --What are Modules
        ==Self-contained Python code
        ==Bring in new functionality/objects
        ==Import only features that you need
    --two different ways to access module attributes
        == Use module elements with name (I preferred)

              
        == Can import individual elements into "namespace"              
              
11、Keyboard input
    -- get keyboard input from user
        == use Python's raw_input() function
        == renamed to input() in Python 3.x
    -- Syntax:
        
    -- Example: