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