举例介绍Python中的25个隐藏特性

注:这里忽略了生成器，装饰器，交换变量等熟知技巧
1. 函数参数unpack

老生常谈的了:
 

def foo(x, y):  print x, y alist = [1, 2]adict = {'x': 1, 'y': 2} foo(*alist) # 1, 2foo(**adict) # 1, 2

2. 链式比较操作符
 

>>> x = 3>>> 1 < x < 5True>>> 4 > x >=3True

3. 注意函数的默认参数
 

>>> def foo(x=[]):...   x.append(1)...   print x...>>> foo()[1]>>> foo()[1, 1]

更安全的做法:
 

>>> def foo(x=None):...   if x is None:...     x = []...   x.append(1)...   print x...>>> foo()[1]>>> foo()[1]>>>

4. 字典有个get()方法

dct.get(key[,default_value]), 当字典dct中找不到key时，get就会返回default_value
 

sum[value] = sum.get(value, 0) + 1

5. 带关键字的格式化
 

>>> print "Hello %(name)s !" % {'name': 'James'}Hello James !>>> print "I am years %(age)i years old" % {'age': 18}I am years 18 years old

更新些的格式化:
 

>>> print "Hello {name} !".format(name="James")Hello James !

快有些模板引擎的味道了:)
6. for…else 语法
 

>>> for i in (1, 3, 5):...   if i % 2 == 0:...     break... else:...   print "var i is always an odd"...var i is always an odd>>>

else语句块会在循环结束后执行，除非在循环块中执行break
7. dict 的特殊方法__missing__

Python 2.5之后引入的。当查找不到key的时候，会执行这个方法。
 

>>> class Dict(dict):...  def __missing__(self, key):...   self[key] = []...   return self[key]...>>> dct = Dict()>>> dct["foo"].append(1)>>> dct["foo"].append(2)>>> dct["foo"][1, 2]

这很像collections.defaultdict不是吗?
 

>>> from collections import defaultdict>>> dct = defaultdict(list)>>> dct["foo"][]>>> dct["bar"].append("Hello")>>> dctdefaultdict(<type 'list'>, {'foo': [], 'bar': ['Hello']})

8. 切片操作的步长参数

还能用步长-1来反转链表:
 
9.另一种字符串连接
 

>>> Name = "Wang" "Hong">>> Name'WangHong'

连接多行:
 

>>> Name = "Wang" /... "Hong">>> Name'WangHong'10. Python解释器中的”_” >>> range(4)[0, 1, 2, 3]>>> _[0, 1, 2, 3]