属性的 Python 数据类后初始化

Question

In the documentation for a dataclass the following example is given when doing some additional calculations at the initialisation of the instance:

@dataclass
class C:
    a: float
    b: float
    c: float = field(init=False)

    def __post_init__(self):
        self.c = self.a + self.b

In the example, the field function is called with init=False . I'm wondering why this is required, because it doesn't seem to be needed. Have I missed something important?

For example, the following code also seems to work fine:

@dataclass
class C:
    a: float
    b: float

    def __post_init__(self):
        self.c = self.a + self.b
        self.d = self.a**2 + self.b**2

Are there likely to be any problems caused by the second example where the attributes are not defined before __post_init__ is called? I'm wondering because I'm doing something like the second example, but with many more attributes and with the addition that b is an InitVar , and I've not noticed any problems yet.

Answer 1

In this case

@dataclass
class C:
    a: float
    b: float

    def __post_init__(self):
        self.c = self.a + self.b
        self.d = self.a**2 + self.b**2

c and d are available in instance of C , but repr of said instance does not include that, for example:

somec = C(3.0,5.0)
print(somec)

output

C(a=3.0, b=5.0)