如何修复 Python 中的循环导入错误

Question

我已经为目前 returns 原始 JSON 数据的 REST API 构建了一个 Python 包装器。我的下一步是将 JSON 数据转换为 Python 对象。出于这个原因，我制作了一个资源 class，其中包含将字典转换为相关对象的所有必要方法。

如何修复此循环导入错误？我考虑过从日历 class 中删除 periods 字段，但我限制了 API.

的功能

# test.py
from my_project.endpoints import Calendars
from my_project.resources import Calendar

calendars = Calendars(username, password).list()
calendars = [Calendar(**calendar) for calendar in calendars]

# ImportError: cannot import name 'Calendar' from partially initialized module 'my_project.model.resources.calendar' (most likely due to a circular import)

# my_project/client.py
from dataclasses import dataclass, fields

@dataclass
class Resource:
    def __post_init__(self, *_):
        for f in fields(self):
            value = getattr(self, f.name)
            if f.type is list and value is not None:
                astype = f.metadata.get("type", str)
                setattr(self, f.name, [self.__cast(v, astype) for v in value])
            else:
                setattr(self, f.name, self.__cast(value, f.type))

    @classmethod
    def __cast(cls, value, astype):
        if value is None:
            return None
        if astype is datetime:
            return datetime.fromisoformat(value)
        elif astype is date:
            return datetime.fromisoformat(value).date()
        elif astype == cls.__name__:
            key = cls._key()
            return cls(**{key: value})
        elif issubclass(astype, Resource) and isinstance(value, dict):
            return astype(**value)
        elif issubclass(astype, Resource) and isinstance(value, (str, int)):
            return astype(**{astype._key(): value})
        else:
            return astype(value)

    @classmethod
    def _key(cls) -> tuple:
        for f in fields(cls):
            if f.metadata.get("key"):
                return f.name

# my_project/resources.py
from my_project.model.resources.calendar import Calendar
from my_project.model.resources.period import Period
from my_project.model.resources.periodType import PeriodType

__all__ = ["Calendar", "Period", "PeriodType"]

# my_project/model/resources/calender.py
from my_project.model.resources.period import Period

@dataclass
class Calendar(Resource):
    calenderSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    periods: list = field(default=None, metadata={"type": Period})

# my_project/model/resources/period.py
from my_project.model.resources.calendar import Calendar
from my_project.model.resources.periodType import PeriodType

@dataclass
class Period(Resource):
    periodSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    periodType: PeriodType = field(default=None)
    calendar: Calendar = field(default=None)

编辑根据戴维斯的评论（感谢您的建议），我尝试将所有文件组合成一个 resources.py。尽管 from __future__ import annotations，仍然出现错误 NameError: name 'Period' is not defined。

# my_project/resources.py
from __future__ import annotations

from dataclasses import InitVar, dataclass, field
from datetime import date, datetime

from sapcommissions import Resource


@dataclass
class Calendar(Resource):
    calenderSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    periods: list = field(default=None, metadata={"type": Period}) # error here


@dataclass
class Period(Resource):
    periodSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    periodType: PeriodType = field(default=None)
    calendar: Calendar = field(default=None)


@dataclass
class PeriodType(Resource):
    periodTypeSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    level: int = field(default=None)

Answer 1

我们需要from __future__ import annotations推迟注释的评估并支持注释中的前向引用。

但这使得字段的类型成为字符串而不是数据类中的类型。

要解决此问题，我们必须 eval 字段的类型以获取它的类型。

并且因为 eval 需要访问它评估的对象并且为了使事情更简单，我将所有内容集成到一个模块中。

我检查一个字段的类型是否是 __post_init__ 中的 str 然后只有 eval 它以避免 eval 如果 Python 的未来版本是一个类型改变行为。这是针对 Python 3.8.

from __future__ import annotations
from dataclasses import dataclass, fields, field
from datetime import datetime, date


@dataclass
class Resource:
    def __post_init__(self, *_):
        for f in fields(self):
            value = getattr(self, f.name)
            field_type = f.type
            if isinstance(f.type, str):
                field_type = eval(f.type)
            # if f.type is list and value is not None:
            if field_type is list and value is not None:
                astype = f.metadata.get("type", str)
                setattr(self, f.name, [self.__cast(v, astype) for v in value])
            else:
                setattr(self, f.name, self.__cast(value, field_type))
                # setattr(self, f.name, self.__cast(value, f.type))

    @classmethod
    def __cast(cls, value, astype):
        if value is None:
            return None
        if astype is datetime:
            return datetime.fromisoformat(value)
        elif astype is date:
            return datetime.fromisoformat(value).date()
        elif astype == cls.__name__:
            key = cls._key()
            return cls(**{key: value})
        elif issubclass(astype, Resource) and isinstance(value, dict):
            return astype(**value)
        elif issubclass(astype, Resource) and isinstance(value, (str, int)):
            return astype(**{astype._key(): value})
        else:
            return astype(value)

    @classmethod
    def _key(cls) -> tuple:
        for f in fields(cls):
            if f.metadata.get("key"):
                return f.name


@dataclass
class Period(Resource):
    periodSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    periodType: PeriodType = field(default=None)
    calendar: Calendar = field(default=None)


@dataclass
class Calendar(Resource):
    calenderSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    periods: list = field(default=None, metadata={"type": Period})  # error here


@dataclass
class PeriodType(Resource):
    periodTypeSeq: str = field(default=None)
    name: str = field(default=None)
    description: str = field(default=None)
    level: int = field(default=None)

如何修复 Python 中的循环导入错误

How to fix this circular import error in Python

python

circular-dependency