Note this is similar to How to get @property methods in asdict?.
I have a (frozen) nested data structure like the following. A few properties that are (purely) dependent on the fields are defined.
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
I can serialize the data structure as follows:
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}}
However, I would like to also serialize the properties (@property). Note I do not want to turn the properties into fields using __post_init__ as I would like to keep the dataclass' frozen. I do not want to use obj.__setattr__ to work around the frozen fields. I also do not want to pre-compute the values of the properties outside the class and pass them in as fields.
The current solution I am using is to explicitly write out how each object is serialized as follows:
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
For a few levels of nesting, this is manageable but I am hoping for a more general solution. For example, here is a (hacky) solution that monkey-patches the _asdict_inner implementation from the dataclasses library.
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
Is there a recommended way to achieve what I am trying to do?
There's no "recommended" way to include them that I know of.
Here's something that seems to work and I think meets your numerous requirements. It defines a custom encoder that calls its own _asdict() method when the object is a dataclass instead of monkey-patching the (private) dataclasses._asdict_inner() function and encapsulates (bundles) the code within the customer encoder that makes use of it.
Like you, I used the current implementation of dataclasses.asdict() as a guide/template since what you're asking for is basically just a customized version of that. The current value of each field that's a property is obtained by calling its __get__ method.
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))
Output:
{"a": 1, "b": {"x": 2, "y": 3, "z": 5}, "d": [42], "c": 0}