Generalised hashable types in Python

halfmutable.py

from dataclasses import dataclass, field, fields, MISSING

def immutable_field(*, default=MISSING, default_factory=MISSING):
    return field(default=default, default_factory=default_factory, metadata={'frozen': True})

def mutable_field(*, default=MISSING, default_factory=MISSING):
    return field(default=default, default_factory=default_factory, compare=False)

def halfmutable(_cls=None, *, init=True, repr=True, order=False):
    def wrap(cls):
        cls = dataclass(cls, init=init, repr=repr, order=order, frozen=True)
        namespace = {}
        immutable_fields = tuple(f.name for f in fields(cls) if f.metadata.get('frozen'))
        exec(f'''
def __setattr__(self, key, value):
    if key in {immutable_fields}:
        raise ValueError(f"immutable field: {{key}}")
    object.__setattr__(self, key, value)
def __delattr__(self, key):
    if key in {immutable_fields}:
        raise ValueError(f"immutable field: {{key}}")
    object.__delattr__(self, key)
''', globals(), namespace)
        cls.__setattr__ = namespace['__setattr__']
        cls.__delattr__ = namespace['__delattr__']
        return cls

    # See if we're being called as @halfmutable or @halfmutable().
    if _cls is None:
        # We're called with parens.
        return wrap

    # We're called as @halfmutable without parens.
    return wrap(_cls)

@halfmutable
class MyHalfMutable:
    spam: int = immutable_field()
    bar: str = mutable_field()
    ham: str = immutable_field(default='my hovercraft is full of eels')
    foo: int = mutable_field(default=42)

a = MyHalfMutable(spam=1, bar='hotdog')
b = MyHalfMutable(spam=1, bar='nothing')
some_dict = {a: a}
print(b in some_dict)
a.foo = 'look ma, i can be modified'
print(a in some_dict)
a.spam = 10 # error: parts of an object that contribute to equality and hash value cannot be modified