英文文档：
len(s)
return the length (the number of items) of an object. the argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).
说明：
1. 返回对象的长度，参数可以是序列(比如字符串、字节数组、元组、列表和range对象)，或者是集合(比如字典、集合、不可变集合)
>>> len(‘abcd’) # 字符串
>>> len(bytes(‘abcd’,’utf-8′)) # 字节数组
>>> len((1,2,3,4)) # 元组
>>> len([1,2,3,4]) # 列表
>>> len(range(1,5)) # range对象
>>> len({‘a’:1,’b’:2,’c’:3,’d’:4}) # 字典
>>> len({‘a’,’b’,’c’,’d’}) # 集合
>>> len(frozenset(‘abcd’)) #不可变集合
2. 如果参数为其它类型，则其必须实现__len__方法，并返回整数，否则报错。
>>> class a:
def __init__(self,name):
self.name = name
def __len__(self):
return len(self.name)
>>> a = a(”)
>>> len(a)
>>> a = a(‘aim’)
>>> len(a)
>>> class b:
pass
>>> b = b()
>>> len(b)
traceback (most recent call last):
file “”, line 1, in
len(b)
typeerror: object of type ‘b’ has no len()
>>> class c:
def __len__(self):
return ‘len’
>>> c = c()
>>> len(c)
traceback (most recent call last):
file “”, line 1, in
len(c)
typeerror: ‘str’ object cannot be interpreted as an integer