January 2018
Intermediate to advanced
856 pages
17h 17m
Chinese
Content preview from Python 技術手冊 第三版
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第 4 章 物件導向的 Python
__len__ __len_ _(self)
呼叫
len(x)
會呼叫
x.__len_ _()
（其他內建函式需要知道容器
x
中有多少項目時也會）。
__len_ _
應該回傳一個
int
，也就是
x
中
項目的數量（number of items）。如果
__nonzero_ _
（v3 中的
__
bool__
）沒有出現，Python 也會在 Boolean 情境下呼叫
x.__len_ _
()
來估算
x
，在這種情況下，如果容器是空的（即容器的長度為
0
），容器就會被視為假（false）。所有的容器都應該有
__len_ _
，除
非判斷它含有多少項目的成本太過昂貴。
__setitem__ __setitem_ _(self,key,value)
對於繫結
x
一個項目或切片的請求（通常是一個指定
x[key]=value
）， Python 會呼叫
x.__setitem_ _(key,value)
。容器
x
應該只在它是可變的，所以其項目或切片可新增或重新繫結的時
候，才擁有
__setitem_ _
。
抽象基礎類別
抽象基礎類別（abstract base classes，ABCs）是物件導向（OO）設計中一
種重要的模式（pattern）：它們是無法直接被繼承的類別，存在只是為了讓
具體類別（concrete classes，一般的類別，可被繼承的那種）擴充它們。
OO 設計的一個推薦做法是，永遠都不要擴充（extend）一個具體類別：如
果兩個具體類別彼此的共通之處多到你想讓其中一個繼承另一個，那麼就 ...
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