test_pickle_rewrite.py

from dataclasses import dataclass
import pickle
import io

@dataclass(frozen=True)
class Foo:
    x: int

@dataclass(frozen=True)
class Bar:
    y: int

# initial write
with open('test.pickle', 'wb') as file:
    foo = Foo(10)
    pickle.dump(foo, file)

# assert write correct
with open('test.pickle', 'rb') as file:
    foo = pickle.load(file)
    assert isinstance(foo, Foo)
    assert foo.x == 10

# rewrite with single descriptor. Watch here
with open('test.pickle', 'r+b') as file:
    foo = pickle.load(file)
    assert foo.x == 10

    file.seek(0)

    bar = Bar(25)
    pickle.dump(bar, file)

# assert write correct
with open('test.pickle', 'rb') as file:

    bar = pickle.load(file)
    assert isinstance(bar, Bar)
    assert bar.y == 25