《转》python学习（3）

转自http://www.cnblogs.com/BeginMan/archive/2013/06/03/3114974.html

1、print语句调用str()函数显示，交互式解释器调用repr()函数来显示对象

>>> s='python'
>>> s
'python'         #repr()，显示结果呈现单引号
>>> print s    #str().没有单引号
python
>>> repr(s)
"'python'"
>>> str(s)
'python'

str()主要显示给人看，repr()显示个机器和畜生看。
print语句会默认给每一行加上换行符，只要在print语句的最后添加一个逗号(,)就可让结果排列在一行。

2、raw_input():

读取标准输入，并把结果给指定变量，如:name=raw_input('your name:')

3、一些语句

(1)、if、if .. else ..、if ..elif..else..

elif即‘else if ’，注意在Django中不存在 elif 模板标签

（2）、while循环
循环控制，最好依赖 ..True..Flase，如下：(《DjangoBook第八章例子》)

#coding=utf-8
'''
Created on 2013-4-17
@author: BeginMan
'''
db={}
def newuser():
    prompt='login desired:'
    while True:
        name=raw_input(prompt)
        if db.has_key(name):
            prompt='name taken,try another'
            continue
        else:
            break
    pwd=raw_input('password:')
    db[name]=pwd

def olduser():
    name=raw_input('name:')
    pwd=raw_input('password:')
    if pwd==db.get(name):
        print 'welecom back ',name
    else:
        print 'login error'

def showmenu():
    prompt="""
    -----------------
    (N) new user login
    (E) existing user login
    (Q) quit
    -----------------
    Enter choice:
    """
    done=False
    while not done:
        chosen=False
        while not chosen:
            try:
                choice=raw_input(prompt).strip()[0].lower()
            except(EOFError,KeyboardInterrupt):
                choice='q'
            print '\n you picked:[%s]' %choice
            if choice not in 'neq':
                print 'invalid option,try again'
            else:
                chosen=True
        if choice=='q':done=True
        if choice=='n':newuser()
        if choice=='e':olduser()

if __name__=='__main__':
    showmenu()

(3)、for循环

不同C#、java、C、等编程语言，如js中：for(var i=0;i<s.length;i++){....};python中它更像C#中的foreach()：

>>> dic={'name':'BeginMan','job':'pythoner','age':22}
>>> for obj in dic.items():
    print obj

('age', 22)
('job', 'pythoner')
('name', 'BeginMan')

（4）、range()/len()使用

这两个方法用的很多，如：

>>> for obj in range(5):
    print obj,

0 1 2 3 4
>>> for obj in [0,1,2,3,4]:
    print obj,

0 1 2 3 4

首先了解下range()。它很像JavaScript里面随机函数，在python里也这样称呼。

>>> help(range)
Help on built-in function range in module __builtin__:

range(...)
    range([start,] stop[, step]) -> list of integers

    Return a list containing an arithmetic progression of integers.
    range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
    When step is given, it specifies the increment (or decrement).
    For example, range(4) returns [0, 1, 2, 3].  The end point is omitted!
    These are exactly the valid indices for a list of 4 elements.

当然，我们也可以这样：

>>> for obj in range(5,10):
>>> print obj,

5 6 7 8 9

range()经常和len()函数一起使用用于字符串索引，如：

>>> name='BeginMan'
>>> for obj in range(len(name)):
    print '(%d)' %obj,name[obj]

(0) B
(1) e
(2) g
(3) i
(4) n
(5) M
(6) a
(7) n

enumerate()的强势围攻，
上面的例子循环有些约束，Python2.3推出了enumerate()函数来解决这一问题，enumerate:枚举 的意思：

>>> for i,j in enumerate(name):
>>> print i,j

B
e
g
i
n
M
a
n

 4、列表解析

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5、文件操作

打开文件：handle=open(file_name,access_mode='r')
如果打开成功，一个文件对象的句柄将会被返回，就可以通过它的句柄进行一系列的操作。

>>> dic={'name':'BeginMan','job':'pythoner','age':22}
>>> for obj in dic.items():
    print obj

('age', 22)
('job', 'pythoner')
('name', 'BeginMan')

附：遍历数组的两种方式

第一种，最常用的，通过for in遍历数组

>>> name='BeginMan'
>>> for obj in range(len(name)):
    print '(%d)' %obj,name[obj]

(0) B
(1) e
(2) g
(3) i
(4) n
(5) M
(6) a
(7) n

第二种，先获得数组的长度，然后根据索引号遍历数组，同时输出索引号

colours = ["red","green","blue"]  

for i in range(0, len(colours)):
    print i, colour[i]  

# 0 red
# 1 green
# 2 blue