Python dataclass from a nested dict

I'm the author of dacite - the tool that simplifies creation of data classes from dictionaries.

This library has only one function from_dict - this is a quick example of usage:

from dataclasses import dataclass
from dacite import from_dict

@dataclass
class User:
    name: str
    age: int
    is_active: bool

data = {
    'name': 'john',
    'age': 30,
    'is_active': True,
}

user = from_dict(data_class=User, data=data)

assert user == User(name='john', age=30, is_active=True)

Moreover dacite supports following features:

• nested structures
• (basic) types checking
• optional fields (i.e. typing.Optional)
• unions
• collections
• values casting and transformation
• remapping of fields names

... and it's well tested - 100% code coverage!

To install dacite, simply use pip (or pipenv):

$ pip install dacite

Below is the CPython implementation of asdict – or specifically, the internal recursive helper function _asdict_inner that it uses:

# Source: https://github.com/python/cpython/blob/master/Lib/dataclasses.py

def _asdict_inner(obj, dict_factory):
    if _is_dataclass_instance(obj):
        result = []
        for f in fields(obj):
            value = _asdict_inner(getattr(obj, f.name), dict_factory)
            result.append((f.name, value))
        return dict_factory(result)
    elif isinstance(obj, tuple) and hasattr(obj, '_fields'):
        # [large block of author comments]
        return type(obj)(*[_asdict_inner(v, dict_factory) for v in obj])
    elif isinstance(obj, (list, tuple)):
        # [ditto]
        return type(obj)(_asdict_inner(v, dict_factory) for v in obj)
    elif isinstance(obj, dict):
        return type(obj)((_asdict_inner(k, dict_factory),
                          _asdict_inner(v, dict_factory))
                         for k, v in obj.items())
    else:
        return copy.deepcopy(obj)

asdict simply calls the above with some assertions, and dict_factory=dict by default.

How can this be adapted to create an output dictionary with the required type-tagging, as mentioned in the comments?

---

1. Adding type information

My attempt involved creating a custom return wrapper inheriting from dict:

class TypeDict(dict):
    def __init__(self, t, *args, **kwargs):
        super(TypeDict, self).__init__(*args, **kwargs)

        if not isinstance(t, type):
            raise TypeError("t must be a type")

        self._type = t

    @property
    def type(self):
        return self._type

Looking at the original code, only the first clause needs to be modified to use this wrapper, as the other clauses only handle containers of dataclass-es:

# only use dict for now; easy to add back later
def _todict_inner(obj):
    if is_dataclass_instance(obj):
        result = []
        for f in fields(obj):
            value = _todict_inner(getattr(obj, f.name))
            result.append((f.name, value))
        return TypeDict(type(obj), result)

    elif isinstance(obj, tuple) and hasattr(obj, '_fields'):
        return type(obj)(*[_todict_inner(v) for v in obj])
    elif isinstance(obj, (list, tuple)):
        return type(obj)(_todict_inner(v) for v in obj)
    elif isinstance(obj, dict):
        return type(obj)((_todict_inner(k), _todict_inner(v))
                         for k, v in obj.items())
    else:
        return copy.deepcopy(obj)

Imports:

from dataclasses import dataclass, fields, is_dataclass

# thanks to Patrick Haugh
from typing import *

# deepcopy 
import copy

Functions used:

# copy of the internal function _is_dataclass_instance
def is_dataclass_instance(obj):
    return is_dataclass(obj) and not is_dataclass(obj.type)

# the adapted version of asdict
def todict(obj):
    if not is_dataclass_instance(obj):
         raise TypeError("todict() should be called on dataclass instances")
    return _todict_inner(obj)

Tests with the example dataclasses:

c = C([Point(0, 0), Point(10, 4)])

print(c)
cd = todict(c)

print(cd)
# {'mylist': [{'x': 0, 'y': 0}, {'x': 10, 'y': 4}]}

print(cd.type)
# <class '__main__.C'>

Results are as expected.

---

2. Converting back to a dataclass

The recursive routine used by asdict can be re-used for the reverse process, with some relatively minor changes:

def _fromdict_inner(obj):
    # reconstruct the dataclass using the type tag
    if is_dataclass_dict(obj):
        result = {}
        for name, data in obj.items():
            result[name] = _fromdict_inner(data)
        return obj.type(**result)

    # exactly the same as before (without the tuple clause)
    elif isinstance(obj, (list, tuple)):
        return type(obj)(_fromdict_inner(v) for v in obj)
    elif isinstance(obj, dict):
        return type(obj)((_fromdict_inner(k), _fromdict_inner(v))
                         for k, v in obj.items())
    else:
        return copy.deepcopy(obj)

Functions used:

def is_dataclass_dict(obj):
    return isinstance(obj, TypeDict)

def fromdict(obj):
    if not is_dataclass_dict(obj):
        raise TypeError("fromdict() should be called on TypeDict instances")
    return _fromdict_inner(obj)

Test:

c = C([Point(0, 0), Point(10, 4)])
cd = todict(c)
cf = fromdict(cd)

print(c)
# C(mylist=[Point(x=0, y=0), Point(x=10, y=4)])

print(cf)
# C(mylist=[Point(x=0, y=0), Point(x=10, y=4)])

Again as expected.

All it takes is a five-liner:

def dataclass_from_dict(klass, d):
    try:
        fieldtypes = {f.name:f.type for f in dataclasses.fields(klass)}
        return klass(**{f:dataclass_from_dict(fieldtypes[f],d[f]) for f in d})
    except:
        return d # Not a dataclass field

Sample usage:

from dataclasses import dataclass, asdict

@dataclass
class Point:
    x: float
    y: float

@dataclass
class Line:
    a: Point
    b: Point

line = Line(Point(1,2), Point(3,4))
assert line == dataclass_from_dict(Line, asdict(line))

Full code, including to/from json, here at gist: https://gist.github.com/gatopeich/1efd3e1e4269e1e98fae9983bb914f22