Python內建函數len

Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, listor or range) such as a dictionary, set, or frozen set).

說明：

　1. 傳回物件的長度，參數可以是序列(例如字串、位元組陣列、元組、列表和range物件)，或是集合(例如字典、集合、不可變集合)

>>> 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. 如果參數為其它類型，則其必須實現__len__方法，並傳回整數，否則報錯。

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

>>> a = A('')
>>> len(a)
>>> a = A('Aim')
>>> len(a)
>>> 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 &#39;B&#39; has no len()
>>> class C:
    def __len__(self):
        return &#39;len&#39;

>>> c = C()
>>> len(c)
Traceback (most recent call last):
  File "<pyshell#71>", line 1, in <module>
    len(c)
TypeError: &#39;str&#39; object cannot be interpreted as an integer