Issue
Please check the code below
import dataclasses
@dataclasses.dataclass
class A:
a: int = 10
def __init_subclass__(cls, **kwargs):
for f in dataclasses.fields(cls):
print(" ", cls, f.name)
print("Defining Subclass B")
@dataclasses.dataclass
class B(A):
b: int = 100
print("Defining Subclass C")
@dataclasses.dataclass
class C(B):
c: int = 1000
The output is
Defining Subclass B
<class '__main__.B'> a
Defining Subclass C
<class '__main__.C'> a
<class '__main__.C'> b
I was expecting
Defining Subclass B
<class '__main__.B'> a
<class '__main__.B'> b
Defining Subclass C
<class '__main__.C'> a
<class '__main__.C'> b
<class '__main__.C'> c
Clearly the dunder method __init_subclass__
does not have the idea of fields in subclass yet as it is not loaded. Please let me know how can I get the expected output.
Solution
So, a decorator works the following way:
@some_decorator
class A:
pass
Is equivalent to:
class A:
pass
A = some_decorator(A)
So, the dataclass
decorator which adds the __dataclass_fields__
attribute to the class object hasn't run when __init_subclass__
runs, because the entire class objects has been created already.
You could manually inspect the __annotations__
(which is what dataclass
relies on to create field
objects anyway). Note, you will have to manually inspect all the classes in the MRO, taking care of doing it in reverse order and guarding against potential classes that lack annotations, so something like:
import dataclasses
@dataclasses.dataclass
class A:
a: int = 10
def __init_subclass__(cls, **kwargs):
for klass in reversed(cls.mro()):
# in case a base class lacks annotations, e.g. object
annotations = getattr(klass, '__annotations__', {})
for name, value in annotations.items():
print(" ", cls, name, value)
print("Defining Subclass B")
@dataclasses.dataclass
class B(A):
b: int = 100
print("Defining Subclass C")
@dataclasses.dataclass
class C(B):
c: int = 1000
Could work for you.
The above prints the following output for me:
Defining Subclass B
<class '__main__.B'> a <class 'int'>
<class '__main__.B'> b <class 'int'>
Defining Subclass C
<class '__main__.C'> a <class 'int'>
<class '__main__.C'> b <class 'int'>
<class '__main__.C'> c <class 'int'>
Answered By - juanpa.arrivillaga
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