Both typing.NamedTuple and dataclasses.dataclass use the somewhat beautiful PEP
526 variable notations at the class level:
@dataclasses.dataclass
class Color:
hue: int
saturation: float
lightness: float = 0.5
and
class Color(typing.NamedTuple):
hue: int
saturation: float
lightness: float = 0.5
I'm looking for guidance or workarounds for two issues that have arisen.
First, the use of default values seems to completely preclude the use of
__slots__. For example, this raises a ValueError:
class A:
__slots__ = ['x', 'y']
x: int = 10
y: int = 20
The second issue is that the different annotations give different signatures
than would produced for manually written classes. It is unclear what the best
practice is for where to put the annotations and their associated docstrings.
In Pydoc for example, this class:
class A:
'Class docstring. x is distance in miles'
x: int
y: int
gives a different signature and docstring than for this class:
class A:
'Class docstring'
def __init__(self, x: int, y: int):
'x is distance in kilometers'
pass
or for this class:
class A:
'Class docstring'
def __new__(cls, x: int, y: int) -> A:
'''x is distance in inches
A is a singleton (once instance per x,y)
'''
if (x, y) in cache:
return cache[x, y]
return object.__new__(cls, x, y)
The distinction is important because the dataclass decorator allows you to
suppress the generation of __init__ when you need more control than dataclass
offers or when you need a __new__ method. I'm unclear on where the docstring
and signature for the class is supposed to go so that we get useful signatures
and matching docstrings.
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