Easiest way to copy all fields from one dataclass instance to another?

Question

Let’s assume you have defined a Python dataclass:

@dataclass
class Marker:
    a: float
    b: float = 1.0

What’s the easiest way to copy the values from an instance marker_a to another instance marker_b?

Here’s an example of what I try to achieve:

marker_a = Marker(1.0, 2.0)
marker_b = Marker(11.0, 12.0)
# now some magic happens which you hopefully can fill in
print(marker_b)
# result: Marker(a=1.0, b=2.0)

As a boundary condition, I do not want to create and assign a new instance to marker_b.

OK, I could loop through all defined fields and copy the values one by one, but there has to be a simpler way, I guess.

Asked By: Tom Pohl

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Source

Answer 1

@dataclass
class Marker:
    a: float
    b: float = 1.0

marker_a = Marker(0.5)

marker_b = Marker(**marker_a.__dict__)

marker_b

# Marker(a=0.5, b=1.0)

If you didn’t want to create a new instance, try this:

marker_a = Marker(1.0, 2.0)
marker_b = Marker(11.0, 12.0)

marker_b.__dict__ = marker_a.__dict__.copy()

# result: Marker(a=1.0, b=2.0)

Not sure whether that’s considered a bad hack though…

Answered By: r.ook

Answer 2

I think that looping over the fields probably is the easiest way. All the other options I can think of involve creating a new object.

from dataclasses import fields

marker_a = Marker(5)
marker_b = Marker(0, 99)

for field in fields(Marker):
    setattr(marker_b, field.name, getattr(marker_a, field.name))

print(marker_b)  # Marker(a=5, b=1.0)

Answered By: Patrick Haugh

Answer 3

The dataclasses.replace function returns a new copy of the object.
Without passing in any changes, it will return a copy with no modification:

>>> import dataclasses
>>> @dataclasses.dataclass
... class Dummy:
...     foo: int
...     bar: int
... 
>>> dummy = Dummy(1, 2)
>>> dummy_copy = dataclasses.replace(dummy)
>>> dummy_copy.foo = 5
>>> dummy
Dummy(foo=1, bar=2)
>>> dummy_copy
Dummy(foo=5, bar=2)

Note that this is a shallow copy.

Edit to address comments:

If a copy is undesirable, I would probably go with the following:

for key, value in dataclasses.asdict(dummy).items():
    setattr(some_obj, key, value)

Answered By: Daniel Perez

Answer 4

Another option which may be more elegant:

import dataclasses

marker_a = Marker(1.0, 2.0)
marker_b = Marker(**dataclasses.asdict(marker_a))

Answered By: Nur L

Answer 5

Here’s a version that also lets you choose the result dataclass type and override attributes:

dataclassWith(Y(x=2, z=5), y=3)      # > Y(x=3, y=3, z=5)
dataclassWith(Y(x=2, z=5), X, x=99)  # > X(z=5, x=99)  # There is no z

MISSING = object()
def dataclassWith(other, clz=None, **kw):
    if clz is None: clz = other.__class__

    k = other.__dict__.copy()
    k.update(kw)
    return clz(**{k:v for k,v in k.items()
                  if getattr(clz, k, MISSING) is not MISSING})


class TestDataclassUtil(unittest.TestCase):
    def test_dataclassWith(self):
        @dataclasses.dataclass
        class X():
            x:int = 1
            z:int = 99

        @dataclasses.dataclass
        class Y(X):
            y:int = 2

        r = dataclassWith(Y(x=2), y=3)
        self.assertTrue(isinstance(r, Y))
        self.assertTrue(r.x==2)
        self.assertTrue(r.y==3)
        self.assertTrue(r.z==99)

        r = dataclassWith(Y(x=2), X, z=100)
        self.assertTrue(isinstance(r, X))
        self.assertTrue(r.x==2)
        self.assertTrue(r.z==100)

Answered By: user48956

Easiest way to copy all fields from one dataclass instance to another?

Question:

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Edit to address comments: