嘗試使用metaclasses創建只讀插槽

Question

我根據讀取的內容編寫了一些代碼here，here和here。

#! /usr/bin/env python3 

class ROSlotsType(type): 
    def __new__(cls, name, bases, namespace, **kwds): 
     roprops = namespace.pop("__roslots__") 
     namespace["__slots__"] = tuple(("_" + propname) for propname in roprops) 
     for propname in roprops: 
      namespace[propname] = property(lambda self: getattr(self, "_" + propname)) # can't use self.__dict__ since it doesn't exist 
     return type.__new__(cls, name, bases, namespace) 

class Location(metaclass = ROSlotsType): 
    __roslots__ = ["lat", "lon", "alt"] 
    def __init__(self, lat, lon, alt = 0): 
     self._lat = lat ; self._lon = lon ; self._alt = alt 
    def __repr__(self): 
     return "Location({}, {}, {})".format(self._lat, self._lon, self._alt) 

place = Location(25.282, 82.956, 77.0) 
print("Created object {}".format(place)) 

print("Accessing its attributes:", place.lat, place.lon, place.alt) 

print("Trying to access its __dict__...") 
try: place.__dict__ 
except: 
    print("Caught exception; object has only __slots__: {}".format(place.__slots__)) 

print("Trying to set new property...") 
try: place.name = "Varanasi" 
except: 
    print("Caught exception; cannot add new property") 

print("Trying to modify read-only property...") 
try: place.alt += 1 
except: 
    print("Caught exception; cannot modify read-only property") 

執行上面給出：

Created object Location(25.282, 82.956, 77.0) 
Accessing its attributes: 77.0 77.0 77.0 
Trying to access its __dict__... 
Caught exception; object has only __slots__: ('_lat', '_lon', '_alt') 
Trying to set new property... 
Caught exception; cannot add new property 
Trying to modify read-only property... 
Caught exception; cannot modify read-only property 

的插槽和只讀行爲做工精細，但顯然有一些問題與物業干將，因爲雖然__repr__它採用_lat和_lon是直接給出正確的值時，使用place.lat和place.lon的屬性訪問取而代之的值爲place.alt。

請告訴我我的代碼有什麼問題以及如何解決它。

Answer 1

的lambda這裏創建一個匿名函數：

namespace[propname] = property(lambda self: getattr(self, "_" + propname)) 

該函數引用propname，這是在它被定義的函數的局部變量。不幸的是，它不是在那個複製propname值一刻，它保持參考到propname變量，一旦你得到解決實際使用功能，for循環已經完成，propname留在roprops的最後一個值;即alt。

爲了解決這個問題，你可以使用一種有點古怪但廣爲人知的方法來通過值來捕獲它，而不是通過引用：創建一個影響另一個變量的參數，但默認值爲你的值想：

namespace[propname] = property(lambda self, propname=propname: getattr(self, "_" + propname)) 

由於Karl Knechtel mentions in the comments，你也可以使用operator.attrgetter，從而消除了哈克位乾脆：

namespace[propname] = property(operator.attrgetter('_' + propname)) 

最後，你的問題最初發布在代碼審查，我注意到你應該可能運行你的c通過pep8頌。