如何通过类变量查找类的特定实例

Question

class Class():
    id_list = []
    def __init__(self, name, id):
        self.name = name
        self.id = id
        Class.id_list.append(self.id)
class1 = Class("Bob", 0)
class2 = Class("John", 1)

我想做的是遍历 Class 的每个实例的 ID，如果它与某个 ID 匹配，那么它会告诉我类的名称，这可能吗？例如：如果我正在寻找"John"，我会查找ID号1

Answer 1

如果仅将每个实例的id值存储在id_list类变量中，则没有简单的方法可以获取相应的名称。但是，如果可以更改逻辑以在列表中存储对整个实例的引用而不是id，则可以在其上查找所需的任何属性。如果按id进行查找很常见，您可能希望使用字典而不是列表来获得更快的查找(尽管如果id值始终从0开始，每次增加1，您也可以在O(1)时间内索引列表(。

尝试这样的事情：

class Class:
    instances = []
    def __init__(self, name): # no id arg needed
        self.name = name  # your code probably shouldn't have quotation marks around "name"
        self.id = len(self.instances) # automatically use the next available id value
        self.instances.append(self) # append a reference to the whole instance to the list
    @classmethod
    def lookup_class_name_by_id(cls, id):
        if 0 <= id < len(cls.instances):
            return cls.instances[id].name
        raise ValueError("Invalid ID {}".format(id))

Answer 2

如果我理解正确，您想创建一个按 id 搜索函数，以便 id 链接到实例，您可以为此创建一个字典，键作为 id，值作为实例。

class Class():
    id_list = {} # id_list will be a dict instead
    def __init__(self, name, id):
        self.name = name # no quotation marks...
        self.id = id
        Class.id_list[self.id] = self # save the id as key and self as he value
    @classmethod
    def search_by_id(cls, id):
        try:
            return cls.id_list[id] # return the instance with the id 
        except KeyError: # error check for if id does not exist
            raise KeyError("No user with id %s" % str(id))
class1 = Class("Bob", 0)
class2 = Class("John", 1)
print(Class.search_by_id(0).name) # prints Bob
print(Class.search_by_id(1).name) # prints John
print(Class.search_by_id(2).name) # prints raised a KeyError since id 2 doesn't exist yet: KeyError: 'No user with id 2'

Answer 3

如果您希望每次实例化一个人时他们都有一个新 ID，您可以这样做：

class MyClass():
    ids = 0
    id_list = {}
    def __init__(self, name):
        self.name = "name"
        MyClass.ids += 1
        self.myId = MyClass.ids
        MyClass.id_list[self.myId] = self
   def search_by_id(id):
      return MyClass.id_list[id]
bob = MyClass("bob")
kyle = MyClass("kyle")
print(bob.myId)
print(kyle.myId)
print(MyClass.search_by_id(2).name)
>1
>2
>kyle