Python易忽视知识点小结

这里记录Python中容易被忽视的小问题

一、input(...)和raw_input(...)

#简单的差看帮助文档input(...)和raw_input(...)有如下区别 
>>> help(input) 
Help on built-in function input in module __builtin__: 
input(...) 
  input([prompt]) -> value 
  Equivalent to eval(raw_input(prompt)). 
>>> help(raw_input) 
Help on built-in function raw_input in module __builtin__: 
raw_input(...) 
  raw_input([prompt]) -> string 
    
  Read a string from standard input. The trailing newline is stripped. 
  If the user hits EOF (Unix: Ctl-D, Windows: Ctl-Z+Return), raise EOFError. 
  On Unix, GNU readline is used if enabled. The prompt string, if given, 
  is printed without a trailing newline before reading. 
  
#可见 input会根据输入的内容eval结果来返回值，即输入纯数字，则得到的就是纯数字 
#     raw_input返回的才是字符串 
#test： 
>>> a = input("输入数字") 
输入数字1 
>>> type(a) 
<type 'int'> 
>>> b=raw_input("输入数字") 
输入数字1 
>>> type(b) 
<type 'str'> 

ps:在python3.0以后的版本中,raw_input和input合体了,取消raw_input,并用input代替,所以现在的版本input接收的是字符串

二、python三目运算符

虽然Python没有C++的三目运算符(?:)，但也有类似的替代方案，

那就是
1、 true_part if condition else false_part

>>> 1 if True else 0 
1 
>>> 1 if False else 0 
0 
>>> "True" if True else "False" 
'True' 
>>> "True" if True else "False" 
'Falser' 

2、 (condition and [true_part] or [false_part] )[0]

>>> (True and ["True"] or ["False"])[0] 
'True' 
>>> (False and ["True"] or ["False"])[0] 
'False' 
>>>  

三、获得指定字符串在整个字符串中出现第N次的索引

# -*- coding: cp936 -*- 
def findStr(string, subStr, findCnt): 
  listStr = a.split(subStr,findCnt) 
  if len(listStr) <= findCnt: 
    return -1 
  return len(string)-len(listStr[-1])-len(subStr) 
#test 
a = "12345(1)254354(1)3534(1)14" 
sub = "(1)" 
N = 2   #查找第2次出现的位置 
print findStr(a,sub,N) 
N = 10   #查找第10次出现的位置 
print findStr(a,sub,N) 
#结果 
#>>>  
#14 
#-1 

四、enumerate用法：

遍历序列的时候，可能同时需要用到序列的索引和对应的值，这时候可以采用enumerate方法进行遍历

enumerate的说明如下：

>>> help(enumerate) 
Help on class enumerate in module __builtin__: 
 
class enumerate(object) 
 | enumerate(iterable[, start]) -> iterator for index, value of iterable 
 |  
 | Return an enumerate object. iterable must be another object that supports 
 | iteration. The enumerate object yields pairs containing a count (from 
 | start, which defaults to zero) and a value yielded by the iterable argument. 
 | enumerate is useful for obtaining an indexed list: 
 |   (0, seq[0]), (1, seq[1]), (2, seq[2]), ... 
 |  
 | Methods defined here: 
 |  
 | __getattribute__(...) 
 |   x.__getattribute__('name') <==> x.name 
 |  
 | __iter__(...) 
 |   x.__iter__() <==> iter(x) 
 |  
 | next(...) 
 |   x.next() -> the next value, or raise StopIteration 
 |  
 | ----------------------------------------------------------------------
 | Data and other attributes defined here: 
 |  
 | __new__ = <built-in method __new__ of type object> 
 |   T.__new__(S, ...) -> a new object with type S, a subtype of T 

五、遍历序列的方法

>>> List = ['a','b','c'] 
>>> for index, value in enumerate(List): 
  print index, value 
0 a 
1 b 
2 c 
>>>  

六、使用python random模块的sample函数从列表中随机选择一组元素

import 
List = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]  
slice = random.sample(List, 5)
#从List中随机获取5个元素，作为一个片断返回  
print slice  
print List #原有序列并没有改变。

七、用json打印包含中文的列表字典等

# -*- coding:utf-8 -*- 
import json 
#你的列表 
listA = [{'path': ['[AWS] \xe7\xbe\x8e\xe5\xb0\x91\xe5\xa5\xb3\xe6\x88\x98\xe5\xa3\xab Sailor Moon Crystal - Moon Pride MV[BIG5][BDrip 1080p x264 AAC][6E5CFE86].mp4'], 'length': 131248608L}, {'path': ['[AWS] \xe7\xbe\x8e\xe5\xb0\x91\xe5\xa5\xb3\xe6\x88\x98\xe5\xa3\xab Sailor Moon Crystal - Moon Pride MV[BIG5][BDrip 720p x264 AAC][639D304A].mp4'], 'length': 103166306L}, {'path': ['[AWS] \xe7\xbe\x8e\xe5\xb0\x91\xe5\xa5\xb3\xe6\x88\x98\xe5\xa3\xab Sailor Moon Crystal - Moon Pride MV[BIG5][BDrip 480p x264 AAC][5A81BACA].mp4'], 'length': 75198408L}]
#打印列表
print json.dumps(listA, encoding='UTF-8', ensure_ascii=False) 

输出结果：

>>>  
[{"path": ["[AWS] 美少女战士 Sailor Moon Crystal - Moon Pride MV[BIG5][BDrip 1080p x264 AAC][6E5CFE86].mp4"], "length": 131248608}, {"path": ["[AWS] 美少女战士 Sailor Moon Crystal - Moon Pride MV[BIG5][BDrip 720p x264 AAC][639D304A].mp4"], "length": 103166306}, {"path": ["[AWS] 美少女战士 Sailor Moon Crystal - Moon Pride MV[BIG5][BDrip 480p x264 AAC][5A81BACA].mp4"], "length": 75198408}] 

希望本文所述对大家的Python程序设计有所帮助。