This was discussed on this thread:
https://mail.python.org/archives/list/[email protected]/message/I3RKK4VINZUBCGF2TBJN6HTDV3PVUEUQ/
Ultimately the thing that doomed it for me was the repr that would be
required for eval(repr(dataclass_instance)) to work. You'd have to drop
the field name, combined with re-ordering the parameters. It just seemed
ugly, and it's not a battle I'm willing to fight while trying to
keyword-only in to 3.10.
But I believe this current proposal is a subset of that one, and it
could be added in the future if demand is high.
Also, note that attrs doesn't support this, and I don't think we need to
be blazing a trail here.
Eric
On 3/16/2021 12:17 PM, Abdulla Al Kathiri wrote:
Why don’t we also add dataclasses.POS_ONLY as well? Anything before it
is positional argument.
@dataclasses.dataclass
class LotsOfFields:
x: Any
y: Any
z: Any
_:dataclasses.POS_ONLY
a: Any
__: dataclasses.KW_ONLY
b: Any = 0
c: Any = 'foo'
d: Any
e: Any = 0.0
f: Any
g: Any = ()
h: Any = 'bar'
i: Any = 3+4j
j: Any = 10
k: Any
The generated __init__ would look like:
def __init__(self, x, y, z, /, a, *, b=0, c='foo', d, e=0.0, f, g=(),
h='bar', i=3+4j, j=10, k):
Similar to dataclasses.KW_ONLY, you can use dataclasses.POS_ONLY only
once. It should always come before dataclasses.KW_ONLY. Anything else,
it should raise an error. If dataclasses.POS_ONLY came right after the
class declaration (no instance attribute preceding it), an error is
generated similar to what we have right now. Inheritance is build up
(from super class to sub class) of three groups of arguments: pos_only
arguments together, then mixed arguments, and then kw_only arguments.
repr should work the same way. We would like to reconstruct the object
from repr. We don’t want too much change from current behavior.
Lets create a class that inherits from the above class..
@dataclasses.dataclass
class LessFields(LotsOfFields):
x2: Any
_: dataclasses.POS_ONLY
a2: Any
__:dataclasses.KW_ONLY
b2: Any
The generated __init__ would look like:
def __init__(self, x, y, z, x2, /, a, a2, *, b=0, c='foo', d, e=0.0,
f, g=(), h='bar', i=3+4j, j=10, k, b2):
Another class..
@dataclasses.dataclass
Class EvenLessFields(LessFields):
a3: Any
_: dataclasses.KW_ONLY
b3: Any = 9
The generated __init__ would look like:
def __init__(self, x, y, z, x2, /, a, a2, a3, *, b=0, c='foo', d,
e=0.0, f, g=(), h='bar', i=3+4j, j=10, k, b2, b3=9):
All arguments are ordered from the super super class until
the subclass all in their respective path or group of arguments.
Abdulla
On 16 Mar 2021, at 1:18 AM, Eric V. Smith <[email protected]
<mailto:[email protected]>> wrote:
@dataclasses.dataclass
class LotsOfFields:
a: Any
_: dataclasses.KW_ONLY
b: Any = 0
c: Any = 'foo'
d: Any
e: Any = 0.0
f: Any
g: Any = ()
h: Any = 'bar'
i: Any = 3+4j
j: Any = 10
k: Any
Which I think is a lot clearer.
The generated __init__ would look like:
def __init__(self, a, *, b=0, c='foo', d, e=0.0, f, g=(), h='bar',
i=3+4j, j=10, k):
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Eric V. Smith
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