Python內置函數(37)——len

英文文檔：

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).

說明：

　　

　　1. 返回對象的長度，參數可以是序列(比如字符串、字節數組、元組、列表和range對象)，或者是集合(比如字典、集合、不可變集合)

>>> len('abcd') # 字符串
4
>>> len(bytes('abcd','utf-8')) # 字節數組
4
>>> len((1,2,3,4)) # 元組
4
>>> len([1,2,3,4]) # 列表
4
>>> len(range(1,5)) # range對象
4
>>> len({'a':1,'b':2,'c':3,'d':4}) # 字典
4
>>> len({'a','b','c','d'}) # 集合
4
>>> len(frozenset('abcd')) #不可變集合
4

　　2. 如果參數為其它類型，則其必須實現__len__方法，並返回整數，否則報錯。

>>> class A:
    def __init__(self,name):
        self.name = name
    def __len__(self):
        return len(self.name)

>>> a = A('')
>>> len(a)
0
>>> a = A('Aim')
>>> len(a)
3
>>> class B:
    pass

>>> b = B()
>>> len(b)
Traceback (most recent call last):
  File "<pyshell#65>", line 1, in <module>
    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 "<pyshell#71>", line 1, in <module>
    len(c)
TypeError: 'str' object cannot be interpreted as an integer