On 2/1/2018 8:17 PM, Eric V. Smith wrote:
On 2/1/2018 7:34 PM, Elvis Pranskevichus wrote:
There appears to be a critical omission from the current dataclass
implementation: it does not make hash=True fields immutable.
Per Python spec:
"the implementation of hashable collections requires that a key’s hash
value is immutable (if the object’s hash value changes, it will be in
the wrong hash bucket)"
Yet:
import dataclasses
@dataclasses.dataclass(hash=True)
class A:
foo: int = dataclasses.field(hash=True, compare=True)
a = A(foo=1)
s = set()
s.add(a) # s == {a}
a.foo = 2
print(a in s)
print({a} == s}
print(s == s)
# prints False False True
This looks to me like a clearly wrong behavior.
Elvis
Data classes do not protect you from doing the wrong thing. This is the
same as writing:
class A:
def __init__(self, foo):
self.foo = foo
def __hash__(self):
return hash((self.foo,))
You're allowed to override the parameters to dataclasses.dataclass for
cases where you know what you're doing. Consenting adults, and all.
I should add: This is why you shouldn't override the default (hash=None)
unless you know what the consequences are. Can I ask why you want to
specify hash=True?
Eric
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