Guido van Rossum <[email protected]> added the comment:
Raising for order=True if one of the ordering dunders exists sounds fine.
I am confused by the corner case for hash. Your table:
"""
eq=? frozen=? __hash__
False False do not generate __hash__
False True do not generate __hash__
True False set __hash__ to None unless it already exists
True True generate __hash__ unless it already exists
and is None
"""
Then you write at the end of that message:
"""
One special case to recognize is if the class defines a __eq__. In this case,
Python will assign __hash__=None before the dataclass decorator is called. The
decorator cannot distinguish between these two cases (except possibly by using
the order of __dict__ keys, but that seems overly fragile):
@dataclass
class A:
def __eq__(self, other): pass
@dataclass
class B:
def __eq__(self, other): pass
__hash__ = None
This is the source of the last line in the above table: for a dataclass where
eq=True, frozen=True, and hash=None, if __hash__ is None it will still be
overwritten. The assumption is that this is what the user wants, but it's a
tricky corner case. It also occurs if setting hash=True and defining __eq__.
Again, it's not expected to come up in normal usage.
"""
I think I understand what you are saying there -- the two cases are treated the
same, and a __hash__ is created (assuming the decorator is really
"@dataclass(eq=True, frozen=True)"), overwriting the "__hash__ = None" for
class B.
However the table's last line says "generate __hash__ unless it already exists
and is None". Perhaps that was a typo and you meant to write "and is *not*
None"?
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<https://bugs.python.org/issue32513>
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