我正在使用FastAPI和pydantic构建一个API。
由于我遵循DDD/clean体系结构，它将模型的定义与持久性层的定义分离开来，因此我在模型中使用标准库数据类，然后使用命令式映射(例如:sql)将它们映射到SQLAlchemy表。古典映射)。
@dataclass
class User:
name: str
age: int
@pydantic.dataclasses.dataclass
class PydanticUser(User):
...
但是，我在定义带有自引用的类时遇到了一个问题。
✅从Pydantic的BaseModel继承的类可以自引用
继承pydantic的BaseModel可以工作，但是这与SQLAlchemy命使式映射不兼容，我希望使用SQLAlchemy命使式映射来坚持干净的体系结构/DDD原则。
class BaseModelPerson(BaseModel):
name: str
age: int
parent_person: BaseModelPerson = None
BaseModelPerson.update_forward_refs()
john = BaseModelPerson(name="John", age=49, parent_person=None)
tim = BaseModelPerson(name="Tim", age=14, parent_person=john)
print(john)
# BaseModelPerson(name='John', age=49, parent_person=None)
print(tim)
# BaseModelPerson(name='Tim', age=14, parent_person=BaseModelPerson(name='John', age=49, parent_person=None))
✅标准自由dataclasses也可以自我参照
from __future__ import annotations
from dataclasses import dataclass
@dataclass
class StdlibPerson:
name: str
age: int
parent: StdlibPerson
john = StdlibPerson(name="John", age=49, parent=None)
tim = StdlibPerson(name="Tim", age=14, parent=john)
print(john)
# StdlibPerson(name='John', age=49, parent=None)
print(tim)
# StdlibPerson(name='Tim', age=14, parent=StdlibPerson(name='John', age=49, parent=None))
❌Pydantic数据类转换导致递归错误
当我尝试将标准库数据类转换为pydantic数据类时，出现问题。
定义一个Pydantic数据类:
PydanticPerson = pydantic.dataclasses.dataclass(StdlibPerson)
返回错误:
# output (hundreds of lines - that is recursive indeed)
# The name of an attribute on the class where we store the Field
File "pydantic/main.py", line 990, in pydantic.main.create_model
File "pydantic/main.py", line 299, in pydantic.main.ModelMetaclass.__new__
File "pydantic/fields.py", line 411, in pydantic.fields.ModelField.infer
File "pydantic/fields.py", line 342, in pydantic.fields.ModelField.__init__
File "pydantic/fields.py", line 456, in pydantic.fields.ModelField.prepare
File "pydantic/fields.py", line 673, in pydantic.fields.ModelField.populate_validators
File "pydantic/class_validators.py", line 255, in pydantic.class_validators.prep_validators
File "pydantic/class_validators.py", line 238, in pydantic.class_validators.make_generic_validator
File "/usr/lib/python3.9/inspect.py", line 3111, in signature
return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
File "/usr/lib/python3.9/inspect.py", line 2860, in from_callable
return _signature_from_callable(obj, sigcls=cls,
File "/usr/lib/python3.9/inspect.py", line 2323, in _signature_from_callable
return _signature_from_function(sigcls, obj,
File "/usr/lib/python3.9/inspect.py", line 2155, in _signature_from_function
if _signature_is_functionlike(func):
File "/usr/lib/python3.9/inspect.py", line 1883, in _signature_is_functionlike
if not callable(obj) or isclass(obj):
File "/usr/lib/python3.9/inspect.py", line 79, in isclass
return isinstance(object, type)
RecursionError: maximum recursion depth exceeded while calling a Python object
像这样定义StdlibPerson并不能解决问题:
@dataclass
class StdlibPerson
name: str
age: int
parent: "Person" = None
也不能使用pydantic文档提供的第二种方法:
@pydantic.dataclasses.dataclass
class PydanticPerson(StdlibPerson)
...
❌直接使用Pydantic数据类
from __future__ import annotations
from pydantic.dataclasses import dataclass
from typing import Optional
@pydantic.dataclasses.dataclass
class PydanticDataclassPerson:
name: str
age: int
parent: Optional[PydanticDataclassPerson] = None
john = PydanticDataclassPerson(name="John", age=49, parent=None)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 6, in __init__
File "pydantic/dataclasses.py", line 97, in pydantic.dataclasses._generate_pydantic_post_init._pydantic_post_init
# | False | | |
File "pydantic/main.py", line 1040, in pydantic.main.validate_model
File "pydantic/fields.py", line 699, in pydantic.fields.ModelField.validate
pydantic.errors.ConfigError: field "parent" not yet prepared so type is still a ForwardRef, you might need to call PydanticDataclassPerson.update_forward_refs().
>>> PydanticDataclassPerson.update_forward_refs()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: type object 'PydanticDataclassPerson' has no attribute 'update_forward_refs'
如何定义具有自引用对象的pydantic模型，使其与SQLAlchemy命令式映射兼容?
用FastAPI、自引用对象和SQLAlchemy命令式映射构建REST API似乎没有简单的解决方案。
我已经决定切换到FastAPI/GraphQL堆栈，使用FastAPI文档中明确推荐的Strawberry库。
到目前为止没有问题，Strawberry使构建GraphQL服务器变得很容易，并且它可以轻松地处理自引用对象。
#!/usr/bin/env python3.10
# src/my_app/entrypoints/api/schema.py
import typing
import strawberry
@strawberry.type
class Person:
name: str
age: int
parent: Person | None
from typing import ForwardRef
from pydantic import BaseModel
Foo = ForwardRef('Foo')
class Foo(BaseModel):
a: int = 123
b: Foo = None
Foo.update_forward_refs()
print(Foo())
#> a=123 b=None
print(Foo(b={'a': '321'}))
#> a=123 b=Foo(a=321, b=None)
你可以这样使用它。这被称为延迟注释。这里是阅读更多的链接https://pydantic-docs.helpmanual.io/usage/postponed_annotations/
继承也可以
class Person(BaseModel):
name: str
age: int
class StdlibPerson(Person):
parent: Person = None