CODEWITHSUNDEEP
python-3.xmetaclass
Kirill Muchow
Kirill Muchow

Reputation: 103

Metaclass does not initialize new class

all! I have a big problem with my code. I try to write simple code as simple framework for understanding meta programming. And I have the next problem.

doc1 = Document()
doc1.id = 20
doc2 = Document()
print (doc2.id) # show 20, but it's wrong!!! It should be 0

This is my code list

class BaseModel(type):
def __new__(cls, name, bases, attrs,  **kwargs):
    instance = super().__new__

    parents = [b for b in bases if isinstance(b, BaseModel)]
    if not parents:
        return instance(cls, name, bases, attrs)

    module_ = attrs.pop('__module__')
    attrs_ = {'__module__': module_}

    classcell_ = attrs.pop('__classcell__', None)
    if classcell_ is not None:
        attrs_['__classcell__'] = classcell_

    for key, value in attrs.items():
        if not isinstance(value, Field):
            continue

        print (value.__dict__)
        attrs_[key] = value

    instance = instance(cls, name, bases, attrs_, **kwargs)

    return instance


class Model(metaclass=BaseModel):
    pass

class Field:
    def __init__(self, value=0):
        self._value = value

    def __get__(self, instance, owner):
        return self._value

    def __set__(self, instance, value):
        self._value = value

class Document(Model):
    id = Field()

What's going on? I think the main issue is new method in metaclass. Am I right? How to fix it?

Upvotes: 0

Views: 98

Answers (1)

Olivier Melançon
Olivier Melançon

Reputation: 22324

What you are trying to accomplish with your metaclass is confusing, although it is not the source of the issue. Implementing unique ids does not require a metaclass, it only requires a well-crafted descriptor.

In your case, your Field descriptor keeps track of a single value instead of existing pairs of instances and ids.

To fix this you need to keep weak references of the already taken values.

import weakref, itertools

class UniqueId:
    def __init__(self):
        self._values = weakref.WeakKeyDictionary()

    def __get__(self, instance, owner):
        if instance is None:
            return self
        else:
            return self._values[instance]

    def __set__(self, instance, value):
        if value not in self._values.values():
            self._values[instance] = value
        else:
            raise ValueError('Unique id {} already taken'.format(value))

    def get_unique_id(self):
        existing_ids = set(self._values.values())
        return next(i for i in itertools.count() if i not in existing_ids)

class Model:
    def __new__(cls, *args, **kwargs):
        instance = super().__new__(cls, *args, **kwargs)

        for name, attr in cls.__dict__.items():
            if isinstance(attr, UniqueId):
                setattr(instance, name, attr.get_unique_id())

        return instance

class Document(Model):
    id = UniqueId()

Example

doc1 = Document()
print(doc1.id) # 0

doc2= Document()
print(doc2.id) # 1

doc2.id = 0 # ValueError: Unique id 0 already taken

Upvotes: 2

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