[英]What is the recommended way to include properties in dataclasses in asdict or serialization?
請注意,這類似於How to get @property methods in asdict? .
我有一個(凍結的)嵌套數據結構,如下所示。 定義了一些(完全)依賴於字段的屬性。
import copy
import dataclasses
import json
from dataclasses import dataclass
@dataclass(frozen=True)
class Bar:
x: int
y: int
@property
def z(self):
return self.x + self.y
@dataclass(frozen=True)
class Foo:
a: int
b: Bar
@property
def c(self):
return self.a + self.b.x - self.b.y
我可以序列化數據結構如下:
class CustomEncoder(json.JSONEncoder):
def default(self, o):
if dataclasses and dataclasses.is_dataclass(o):
return dataclasses.asdict(o)
return json.JSONEncoder.default(self, o)
foo = Foo(1, Bar(2,3))
print(json.dumps(foo, cls=CustomEncoder))
# Outputs {"a": 1, "b": {"x": 2, "y": 3}}
但是,我還想序列化屬性( @property
)。 請注意,我不想使用__post_init__
將屬性轉換為字段,因為我想凍結數據類。 我不想使用obj.__setattr__
來處理凍結的字段。 我也不想預先計算 class 之外的屬性值並將它們作為字段傳遞。
我目前使用的解決方案是明確寫出每個 object 是如何序列化的,如下所示:
class CustomEncoder2(json.JSONEncoder):
def default(self, o):
if isinstance(o, Foo):
return {
"a": o.a,
"b": o.b,
"c": o.c
}
elif isinstance(o, Bar):
return {
"x": o.x,
"y": o.y,
"z": o.z
}
return json.JSONEncoder.default(self, o)
foo = Foo(1, Bar(2,3))
print(json.dumps(foo, cls=CustomEncoder2))
# Outputs {"a": 1, "b": {"x": 2, "y": 3, "z": 5}, "c": 0} as desired
對於幾層嵌套,這是可以管理的,但我希望有一個更通用的解決方案。 例如,這是一個(hacky)解決方案,它從數據類庫中猴子修補 _asdict_inner 實現。
def custom_asdict_inner(obj, dict_factory):
if dataclasses._is_dataclass_instance(obj):
result = []
for f in dataclasses.fields(obj):
value = custom_asdict_inner(getattr(obj, f.name), dict_factory)
result.append((f.name, value))
# Inject this one-line change
result += [(prop, custom_asdict_inner(getattr(obj, prop), dict_factory)) for prop in dir(obj) if not prop.startswith('__')]
return dict_factory(result)
elif isinstance(obj, tuple) and hasattr(obj, '_fields'):
return type(obj)(*[custom_asdict_inner(v, dict_factory) for v in obj])
elif isinstance(obj, (list, tuple)):
return type(obj)(custom_asdict_inner(v, dict_factory) for v in obj)
elif isinstance(obj, dict):
return type(obj)((custom_asdict_inner(k, dict_factory),
custom_asdict_inner(v, dict_factory))
for k, v in obj.items())
else:
return copy.deepcopy(obj)
dataclasses._asdict_inner = custom_asdict_inner
class CustomEncoder3(json.JSONEncoder):
def default(self, o):
if dataclasses and dataclasses.is_dataclass(o):
return dataclasses.asdict(o)
return json.JSONEncoder.default(self, o)
foo = Foo(1, Bar(2,3))
print(json.dumps(foo, cls=CustomEncoder3))
# Outputs {"a": 1, "b": {"x": 2, "y": 3, "z": 5}, "c": 0} as desired
是否有推薦的方法來實現我想要做的事情?
它似乎與一個方便的dataclass
功能相矛盾:
Class(**asdict(obj)) == obj # only for classes w/o nested dataclass attrs
如果你沒有找到任何相關的 pypi 包,你總是可以像這樣添加一個 2-liner:
from dataclasses import asdict as std_asdict
def asdict(obj):
return {**std_asdict(obj),
**{a: getattr(obj, a) for a in getattr(obj, '__add_to_dict__', [])}}
然后,您可以以自定義但簡短的方式在 dicts 中指定您想要的內容:
@dataclass
class A:
f: str
__add_to_dict__ = ['f2']
@property
def f2(self):
return self.f + '2'
@dataclass
class B:
f: str
print(asdict(A('f')))
print(asdict(B('f')))
:
{'f2': 'f2', 'f': 'f'}
{'f': 'f'}
沒有“推薦”的方式來包含我所知道的它們。
這里有一些似乎有效的東西,我認為它可以滿足您的眾多要求。 它定義了一個自定義編碼器,當對象是dataclass
而不是猴子修補(私有) dataclasses._asdict_inner()
時調用其自己的_asdict()
方法。 _asdict()
函數並將代碼封裝(捆綁)在使用它的客戶編碼器中.
像你一樣,我使用dataclasses.asdict()
的當前實現作為指南/模板,因為你所要求的基本上只是一個定制版本。 作為property
的每個字段的當前值是通過調用其__get__
方法獲得的。
import copy
import dataclasses
from dataclasses import dataclass, field
import json
import re
from typing import List
class MyCustomEncoder(json.JSONEncoder):
is_special = re.compile(r'^__[^\d\W]\w*__\Z', re.UNICODE) # Dunder name.
def default(self, obj):
return self._asdict(obj)
def _asdict(self, obj, *, dict_factory=dict):
if not dataclasses.is_dataclass(obj):
raise TypeError("_asdict() should only be called on dataclass instances")
return self._asdict_inner(obj, dict_factory)
def _asdict_inner(self, obj, dict_factory):
if dataclasses.is_dataclass(obj):
result = []
# Get values of its fields (recursively).
for f in dataclasses.fields(obj):
value = self._asdict_inner(getattr(obj, f.name), dict_factory)
result.append((f.name, value))
# Add values of non-special attributes which are properties.
is_special = self.is_special.match # Local var to speed access.
for name, attr in vars(type(obj)).items():
if not is_special(name) and isinstance(attr, property):
result.append((name, attr.__get__(obj))) # Get property's value.
return dict_factory(result)
elif isinstance(obj, tuple) and hasattr(obj, '_fields'):
return type(obj)(*[self._asdict_inner(v, dict_factory) for v in obj])
elif isinstance(obj, (list, tuple)):
return type(obj)(self._asdict_inner(v, dict_factory) for v in obj)
elif isinstance(obj, dict):
return type(obj)((self._asdict_inner(k, dict_factory),
self._asdict_inner(v, dict_factory)) for k, v in obj.items())
else:
return copy.deepcopy(obj)
if __name__ == '__main__':
@dataclass(frozen=True)
class Bar():
x: int
y: int
@property
def z(self):
return self.x + self.y
@dataclass(frozen=True)
class Foo():
a: int
b: Bar
@property
def c(self):
return self.a + self.b.x - self.b.y
# Added for testing.
d: List = field(default_factory=lambda: [42]) # Field with default value.
foo = Foo(1, Bar(2,3))
print(json.dumps(foo, cls=MyCustomEncoder))
輸出:
{"a": 1, "b": {"x": 2, "y": 3, "z": 5}, "d": [42], "c": 0}
如果適用於您的解決方案,您可以在基礎 class 上定義屬性,並讓具體類實現這些屬性。 這適用於asdict
。
from dataclasses import asdict, dataclass, field
@dataclass
class Liquid:
volume: int
price: int
total_cost: int = field(init=False)
class Milk(Liquid):
volume: int
price: int
@property
def total_cost(self):
return self.volume * self.price
milk = Milk(10)
print(asdict(milk))
>>>{'volume': 10, 'price': 3, 'total_cost': 30}
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