I have a class Animal and Dog which inherits Animal. Why is it that I get an error if I try to give my Dog class a breed field?
TypeError: non-default argument 'breed' follows default argument
This is my code
from dataclasses import dataclass
@dataclass
class Animal:
species: str
arms: int
legs: int
@dataclass
class Dog(Animal):
breed: str
species: str = "Dog"
arms: int = 0
legs: int = 4
if __name__ == '__main__':
jake = Dog(breed="Bulldog")
print(jake)
I did find that if I add a breed field to Animal I wouldn't get the error.
The way dataclasses combines attributes prevents you from being able to use attributes with defaults in a base class and then use attributes without a default (positional attributes) in a subclass.
That's because the attributes are combined by starting from the bottom of the MRO, and building up an ordered list of the attributes in first-seen order; overrides are kept in their original location. So Parent starts out with ['name', 'age', 'ugly'], where ugly has a default, and then Child adds ['school'] to the end of that list (with ugly already in the list). This means you end up with ['name', 'age', 'ugly', 'school'] and because school doesn't have a default, this results in an invalid argument listing for __init__.
This is documented in PEP-557 Dataclasses, under inheritance:
When the Data Class is being created by the
@dataclassdecorator, it looks through all of the class's base classes in reverse MRO (that is, starting atobject) and, for each Data Class that it finds, adds the fields from that base class to an ordered mapping of fields. After all of the base class fields are added, it adds its own fields to the ordered mapping. All of the generated methods will use this combined, calculated ordered mapping of fields. Because the fields are in insertion order, derived classes override base classes.
and under Specification:
TypeErrorwill be raised if a field without a default value follows a field with a default value. This is true either when this occurs in a single class, or as a result of class inheritance.
You do have a few options here to avoid this issue.
The first option is to use separate base classes to force fields with defaults into a later position in the MRO order. At all cost, avoid setting fields directly on classes that are to be used as base classes, such as Parent.
The following class hierarchy works:
# base classes with fields; fields without defaults separate from fields with.
@dataclass
class _ParentBase:
name: str
age: int
@dataclass
class _ParentDefaultsBase:
ugly: bool = False
@dataclass
class _ChildBase(_ParentBase):
school: str
@dataclass
class _ChildDefaultsBase(_ParentDefaultsBase):
ugly: bool = True
# public classes, deriving from base-with, base-without field classes
# subclasses of public classes should put the public base class up front.
@dataclass
class Parent(_ParentDefaultsBase, _ParentBase):
def print_name(self):
print(self.name)
def print_age(self):
print(self.age)
def print_id(self):
print(f"The Name is {self.name} and {self.name} is {self.age} year old")
@dataclass
class Child(_ChildDefaultsBase, Parent, _ChildBase):
pass
By pulling out fields into separate base classes with fields without defaults and fields with defaults, and a carefully selected inheritance order, you can produce an MRO that puts all fields without defaults before those with defaults. The reversed MRO (ignoring object) for Child is:
_ParentBase
_ChildBase
_ParentDefaultsBase
Parent
_ChildDefaultsBase
Note that while Parent doesn't set any new fields, it does inherit the fields from _ParentDefaultsBase and should not end up 'last' in the field listing order; the above order puts _ChildDefaultsBase last so its fields 'win'. The dataclass rules are also satisfied; the classes with fields without defaults (_ParentBase and _ChildBase) precede the classes with fields with defaults (_ParentDefaultsBase and _ChildDefaultsBase).
The result is Parent and Child classes with a sane field older, while Child is still a subclass of Parent:
>>> from inspect import signature
>>> signature(Parent)
<Signature (name: str, age: int, ugly: bool = False) -> None>
>>> signature(Child)
<Signature (name: str, age: int, school: str, ugly: bool = True) -> None>
>>> issubclass(Child, Parent)
True
and so you can create instances of both classes:
>>> jack = Parent('jack snr', 32, ugly=True)
>>> jack_son = Child('jack jnr', 12, school='havard', ugly=True)
>>> jack
Parent(name='jack snr', age=32, ugly=True)
>>> jack_son
Child(name='jack jnr', age=12, school='havard', ugly=True)
Another option is to only use fields with defaults; you can still make in an error to not supply a school value, by raising one in __post_init__:
_no_default = object()
@dataclass
class Child(Parent):
school: str = _no_default
ugly: bool = True
def __post_init__(self):
if self.school is _no_default:
raise TypeError("__init__ missing 1 required argument: 'school'")
but this does alter the field order; school ends up after ugly:
<Signature (name: str, age: int, ugly: bool = True, school: str = <object object at 0x1101d1210>) -> None>
and a type hint checker will complain about _no_default not being a string.
You can also use the attrs project, which was the project that inspired dataclasses. It uses a different inheritance merging strategy; it pulls overridden fields in a subclass to the end of the fields list, so ['name', 'age', 'ugly'] in the Parent class becomes ['name', 'age', 'school', 'ugly'] in the Child class; by overriding the field with a default, attrs allows the override without needing to do a MRO dance.
attrs supports defining fields without type hints, but lets stick to the supported type hinting mode by setting auto_attribs=True:
import attr
@attr.s(auto_attribs=True)
class Parent:
name: str
age: int
ugly: bool = False
def print_name(self):
print(self.name)
def print_age(self):
print(self.age)
def print_id(self):
print(f"The Name is {self.name} and {self.name} is {self.age} year old")
@attr.s(auto_attribs=True)
class Child(Parent):
school: str
ugly: bool = True
Note that with Python 3.10, it is now possible to do it natively with dataclasses.
Dataclasses 3.10 added the kw_only attribute (similar to attrs).
It allows you to specify which fields are keyword_only, thus will be set at the end of the init, not causing an inheritance problem.
Taking directly from Eric Smith's blog post on the subject:
There are two reasons people [were asking for] this feature:
- When a dataclass has many fields, specifying them by position can become unreadable. It also requires that for backward compatibility, all new fields are added to the end of the dataclass. This isn't always desirable.
- When a dataclass inherits from another dataclass, and the base class has fields with default values, then all of the fields in the derived class must also have defaults.
What follows is the simplest way to do it with this new argument, but there are multiple ways you can use it to use inheritance with default values in the parent class:
from dataclasses import dataclass
@dataclass(kw_only=True)
class Parent:
name: str
age: int
ugly: bool = False
@dataclass(kw_only=True)
class Child(Parent):
school: str
ch = Child(name="Kevin", age=17, school="42")
print(ch.ugly)
Take a look at the blogpost linked above for a more thorough explanation of kw_only.
Cheers !
PS: As it is fairly new, note that your IDE might still raise a possible error, but it works at runtime
Define another method to initialize u and d, so that you can override that part of B without overriding how qu and qd are defined.
@dataclass
class B(A):
u: float = field(init=False)
d: float = field(init=False)
qu: float = field(init=False)
qd: float = field(init=False)
def __post_init__(self):
super().__post_init__()
self._define_u_and_d()
self.qu = (math.exp((self.r - self.div) * self.dt) - self.d)/(self.u - self.d)
self.qd = 1 - self.qu
def _define_u_and_d(self):
self.u = 1 + self.pu
self.d = 1 - self.pd
@dataclass
class C(B):
def _define_u_and_d(self):
self.u = math.exp(self.sigma * math.sqrt(self.dt))
self.d = 1/self.u
Python supports multiple inheritance. You can inherit from A before B, which means any overlapping methods will be taken from A (such as __post_init__). Any code you write in class C will overwrite what's inherited from A and B. If you need to have more control over which methods come from which class, you can always define the method in C and make a function call to A or B (like A.dt(self)).
class C(A, B):
...
ANOTHER EDIT:
I just saw that A initializes some stuff you want in C. Because C's parent is now A (if you used my code above), you can add back in the super().__post_init__() line to C's __post_init__ so that it calls A's __post_init__.
If this doesn't work, you can always just put A.__post_init__(self) in the __post_init__ of C.
» pip install field-properties
I would argue that #1 is the most correct method. For the example you showed, it appears to be irrelevant which method you use, but if you add a second variable, the differences become apparent. This is implicitly confirmed by the Inheritance section in the documentation.
@dataclass
class ParentClass:
a: str
b: str = "parent-b"
# This works smoothly
@dataclass
class ChildClass1(ParentClass):
a: str = "child-a"
# This works, but is a maintenance nightmare
@dataclass
class ChildClass2(ParentClass):
def __init__(self, a="child-a", b="parent-b"):
super().__init__(a, b)
# This works, but it changes the signature and only works if a is first
@dataclass
class ChildClass3(ParentClass):
def __init__(self, a="child-a", **kwargs):
super().__init__(a, **kwargs)
Right now, the dataclass decorator is adding default methods, including __init__ to your class. That means that if you wanted to use option #2 or #3, you would have to know and copy the function signature for all the parameters. At the same time, option #1 allows you to change the default for just a.
The other way to do what you're doing is to create a __post_init__ method for your child classes, which can then override the parent default value:
@dataclass
class ParentClass:
a: str = '' # Or pick some other universally acceptable marker
@dataclass
class ChildClass(ParentClass):
def __post_init__(self):
if self.a == '':
self.a = "child-a"
This is also needlessly complex for most scenarios, but may be useful for a more complex situation. Normally __post_init__ is meant to be used to initialize derived fields, as in the example in the linked documentation.
OK, so first thing, I came by here looking for just field-level inheritance on dataclasses (mostly to allow for isinstance() testing (yes,yes, with all caveats about that approach).
I then found a blog post Python dataclass inheritance, finally ! | by Anis Campos at Medium (wonders of wonders, an open Medium url too...) that relies on kw_only on the dataclass decorator, available from Python 3.10 on.
I got things to work for myself so here's a stab at a simple minimal proof of concept for what I think your use case looks like.
I am not sure it does exactly what you had in mind and this approach does constrain you to calling the constructors with keywords only, but that fine for me since I typically do things like inst = cls(**data) anyway.
(Note: you could also use Pydantic instead, which does support this straight out of the box)
@dataclass(kw_only=True) #
class ParentClass:
a_variable: str
def a_function(self) -> None:
return f" {self.a_variable=} on {type(self).__name__}"
# ONE
@dataclass(kw_only=True) #
class DataclassChild1(ParentClass):
a_variable: str = "default DataclassChild1"
@dataclass(kw_only=True) #
class DataclassChild2(ParentClass):
a_variable: str = "default DataclassChild2"
child2_var : str = "?"
def a_function(self) -> None:
return f" Dataclass2!{self.a_variable=} on {type(self).__name__}"
for inst in [
ParentClass(a_variable="parent"),
DataclassChild1(a_variable="child1!"),
DataclassChild2(a_variable="child2!"),
DataclassChild1(),
DataclassChild2(child2_var = "x"),
]:
print(inst)
print(inst.a_function())
and the output
test.<locals>.ParentClass(a_variable='parent')
self.a_variable='parent' on ParentClass
test.<locals>.DataclassChild1(a_variable='child1!')
self.a_variable='child1!' on DataclassChild1
test.<locals>.DataclassChild2(a_variable='child2!', child2_var='?')
Dataclass2!self.a_variable='child2!' on DataclassChild2
test.<locals>.DataclassChild1(a_variable='default DataclassChild1')
self.a_variable='default DataclassChild1' on DataclassChild1
test.<locals>.DataclassChild2(a_variable='default DataclassChild2', child2_var='x')
Dataclass2!self.a_variable='default DataclassChild2' on DataclassChild2