返回类实例，而不是创建一个新的实例（如果已经存在） [英] Return class instance instead of creating a new one if already existing

问题描述

我为正在进行的一些实验室实验的结果定义了一个名为 Experiment 的类。这个想法是创建一种数据库：如果我添加一个实验，它将在退出前被腌制到db中，并在启动时重新加载（并添加到类注册表中）。

I defined a class named Experiment for the results of some lab experiments I am conducting. The idea was to create a sort of database: if I add an experiment, this will be pickled to a db before at exit and reloaded (and added to the class registry) at startup.

我的班级定义是：

class IterRegistry(type):
    def __iter__(cls):
        return iter(cls._registry)


class Experiment(metaclass=IterRegistry):
    _registry = []
    counter = 0

    def __init__(self, name, pathprotocol, protocol_struct, pathresult, wallA, wallB, wallC):
        hashdat = fn.hashfile(pathresult)
        hashpro = fn.hashfile(pathprotocol)
        chk = fn.checkhash(hashdat)
        if chk:
            raise RuntimeError("The same experiment has already been added")
        self._registry.append(self)
        self.name = name
        [...]

而 fn .checkhash 是用于检查包含t的文件的哈希值的函数他的结果是：

While fn.checkhash is a function that checks the hashes of the files containing the results:

def checkhash(hashdat):
    for exp in cl.Experiment:
        if exp.hashdat == hashdat:
            return exp
    return False

我添加了一个以前添加的实验，它不会被覆盖。

So that if I add a previously added experiment, this won't be overwritten.

是否可以以某种方式返回现有实例（如果已经存在）而不是引发错误？ （我知道在 __ init __ 块中是不可能的）

Is it possible to somehow return the existing instance if already existant instead of raising an error? (I know in __init__ block it is not possible)

推荐答案

您如果您要自定义创建内容，而不仅仅是在新创建的对象中进行初始化，则可以使用 __ new __ ：

You can use __new__ if you want to customize the creation instead of just initializing in newly created object:

class Experiment(metaclass=IterRegistry):
    _registry = []
    counter = 0

    def __new__(cls, name, pathprotocol, protocol_struct, pathresult, wallA, wallB, wallC):
        hashdat = fn.hashfile(pathresult)
        hashpro = fn.hashfile(pathprotocol)
        chk = fn.checkhash(hashdat)
        if chk:                      # already added, just return previous instance
            return chk
        self = object.__new__(cls)   # create a new uninitialized instance
        self._registry.append(self)  # register and initialize it
        self.name = name
        [...]
        return self                  # return the new registered instance

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