如何在不使用任一類的對象實例的情況下覆蓋子類中的父類屬性？來自 Java/C++ 及其嚴格結構設計的世界，我發現自己受到 Python 做事方式的挑戰。我想保持相對靜止。例子：from urllib.parse import urljoinclass base: host = "/host/" path = "Override this in child classes" url = urljoin(host, path)class config(base): path = "config" @classmethod def print_url(cls): print(cls.url) # Currently prints "/host/Override this in child classes" # Would like to print "/host/config" insteadclass log(base): path = "log" @classmethod def print_url(cls): print(cls.url) # Currently prints "/host/Override this in child classes" # Would like to print "/host/log" instead所需用途：>>> config.print_url()/host/config>>> log.print_url()/host/log我希望config.path和log.path屬性能夠覆蓋base.path. 這樣我就可以url = urljoin(host, path)在類中一勞永逸地使用base（并且避免必須在每個派生類中復制/粘貼相同的屬性/計算）。我無法弄清楚如何在不構造對象的情況下完成此任務（我希望避免）。有人有什么建議嗎？提前致謝！
1 回答
守著一只汪
TA貢獻1872條經驗 獲得超4個贊
子path屬性會覆蓋base.path。您不覆蓋的是url屬性。當運行 的主體base來創建類對象時，就會計算一次。
未來你有幾個選擇。無論哪種方式，您都需要url動態地進行計算，或者每次訪問它時，或者每個子類至少一次。
最簡單的方法是制作url成classmethod：
class base:
host = "/host/"
path = "Override this in child classes"
@classmethod
def url(cls):
return urljoin(cls.host, cls.path)
@classmethod
def print_url(cls):
print(cls.url())
class config(base):
path = "config"
class log(base):
path = "log"
請注意，您現在指的是實際的類host并且path是動態的。您還只需要一種print_url方法，base而不是每個類中使用不同的方法。
base另一種選擇是為 及其所有子級提供一個元類，url其形式為property：
class url_meta(type):
@property
def url(cls):
return urljoin(cls.host, cls.path)
class base(metaclass=url_meta):
host = "/host/"
path = "Override this in child classes"
@classmethod
def print_url(cls):
print(cls.url)
class config(base):
path = "config"
class log(base):
path = "log"
這是可行的，因為 python 類也是對象。property您可以在類的類（元類）中定義 a ，它的行為與任何property對實例的行為一樣。這次實例本身就是一個類。
第三種選擇是確保url在每個子項中靜態但正確地定義它。該__init_subclass__方法允許您直接從以下位置非常方便地執行此操作base：
class base:
host = "/host/"
path = "Override this in child classes"
url = urljoin(host, path)
@classmethod
def __init_subclass__(cls):
cls.url = urljoin(cls.host, cls.path)
@classmethod
def print_url(cls):
print(cls.url)
class config(base):
path = "config"
class log(base):
path = "log"
您也可以使用元類完成同樣的事情：
class url_meta2(type):
def __init__(cls, *args, **kwargs):
cls.url = urljoin(cls.host, cls.path)
class base(metaclass=url_meta2):
host = "/host/"
path = "Override this in child classes"
@classmethod
def print_url(cls):
print(cls.url)
class config(base):
path = "config"
class log(base):
path = "log"
添加回答
舉報
0/150
提交
取消