Pythonic way to check if a dataclass field has a default value

Question

I've been using python 3.7 lately and was looking for ways to leverage the new dataclasses. Basically I had a method that iterates over the dataclass fields and checks if they have a default value:

from dataclasses import fields, MISSING

@classmethod
def from_json(cls)
    datacls_fields = fields(cls)
    for field in datacls_fields:
        if  (field.default != MISSING):
            #...

However in the official documentation, it says:

MISSING value is a sentinel object used to detect if the default and default_factory parameters are provided. This sentinel is used because None is a valid value for default. No code should directly use the MISSING value.

Anyone knows a better/more pythonic way to do it?

Answer 1

MISSING这是python 源代码中的定义dataclasses.py：

# A sentinel object to detect if a parameter is supplied or not.  Use
# a class to give it a better repr.
class _MISSING_TYPE:
    pass
MISSING = _MISSING_TYPE()

定义非常清楚，它的用例只是检查是否已提供参数，并区分 的值None和未提供的值：

def my_func(a=MISSING):
    if a is not MISSING:
        # a value has been supplied, whatever his value

所以在代码中使用它来进行值比较是完全可以的。通过告诉“No code should direct use the MISSING value”，他们只是警告我们该变量没有特定用途（除了用于比较），并且不应在代码中使用以避免意外行为。

您应该更新代码以使用更Pythonic 的语法is not MISSING：

from dataclasses import fields, MISSING

@classmethod
def from_json(cls)
    datacls_fields = fields(cls)
    for field in datacls_fields:
        if field.default is not MISSING: