From the official Python 3 documentation for __init__subclass__:
class Philosopher:
def __init_subclass__(cls, /, default_name: str, **kwargs):
super().__init_subclass__(**kwargs)
cls.default_name = default_name
class AustralianPhilosopher(Philosopher, default_name="Bruce"):
pass
The problem is, mypy raises "Type[Philosopher]" has no attribute "default_name". What is the solution for this? How can I make mypy take these values?
Just like the style of other statically-typed languages, you simply declare the variable as an attribute in the class body:
from typing import ClassVar
class Philosopher:
default_name: ClassVar[str]
def __init_subclass__(cls, /, default_name: str, **kwargs: object) -> None:
super().__init_subclass__(**kwargs)
cls.default_name = default_name
Related
I have a set of unrelated classes (some imported) which all have a common attribute (or property) a of type dict[str, Any].
Within a there should be another dict under the key "b", which I would like to expose on any of these classes as an attribute b to simplify inst.a.get("b", {})[some_key] to inst.b[some_key].
I have made the following subclass factory to work as a class decorator for local classes and a function for imported classes.
But so far I'm failing to type hint its cls argument and return value correctly.
from functools import wraps
def access_b(cls):
#wraps(cls, updated=())
class Wrapper(cls):
#property
def b(self) -> dict[str, bool]:
return self.a.get("b", {})
return Wrapper
MRE of my latest typing attemp (with mypy 0.971 errors):
from functools import wraps
from typing import Any, Protocol, TypeVar
class AProtocol(Protocol):
a: dict[str, Any]
class BProtocol(AProtocol, Protocol):
b: dict[str, bool]
T_a = TypeVar("T_a", bound=AProtocol)
T_b = TypeVar("T_b", bound=BProtocol)
def access_b(cls: type[T_a]) -> type[T_b]:
#wraps(cls, updated=())
class Wrapper(cls): # Variable "cls" is not valid as a type & Invalid base class "cls"
#property
def b(self) -> dict[str, bool]:
return self.a.get("b", {})
return Wrapper
#access_b
class Demo1:
"""Local class."""
def __init__(self, a: dict[str, Any]):
self.a = a.copy()
demo1 = Demo1({"b": {"allow_X": True}})
demo1.b["allow_X"] # "Demo1" has no attribute "b"
class Demo2:
"""Consider me an imported class."""
def __init__(self, a: dict[str, Any]):
self.a = a.copy()
demo2 = access_b(Demo2)({"b": {"allow_X": True}}) # Cannot instantiate type "Type[<nothing>]"
demo2.b["allow_X"]
I do not understand why cls is not valid as a type, even after reading https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases.
I understand I should probably not return a Protocol (I suspect that is the source of Type[<nothing>]), but I don't see how I could specify "returns the original type with an extension".
PS1. I have also tried with a decorator which adds b dynamically, still failed to type it...
PS2. ...and with a decorator which uses a mixin as per #DaniilFajnberg's answer, still failing.
References:
functools.wraps(cls, update=()) from https://stackoverflow.com/a/65470430/17676984
(Type) Variables as base classes?
This is actually a really interesting question and I am curious about what solutions other people come up with.
I read up a little on these two errors:
Variable "cls" is not valid as a type / Invalid base class "cls"
There seems to be an issue here with mypy that has been open for a long time now. There doesn't seem to be a workaround yet.
The problem, as I understand it, is that no matter how you annotate it, the function argument cls will always be a type variable and that is considered invalid as a base class. The reasoning is apparently that there is no way to make sure that the value of that variable isn't overwritten somewhere.
I honestly don't understand the intricacies well enough, but it is really strange to me that mypy seems to treat a class A defined via class A: ... different than a variable of Type[A] since the former should essentially just be syntactic sugar for this:
A = type('A', (object,), {})
There was also a related discussion in the mypy issue tracker. Again, hoping someone can shine some light onto this.
Adding a convenience property
In any case, from your example I gather that you are not dealing with foreign classes, but that you define them yourself. If that is the case, a Mix-in would be the simplest solution:
from typing import Any, Protocol
class AProtocol(Protocol):
a: dict[str, Any]
class MixinAccessB:
#property
def b(self: AProtocol) -> dict[str, bool]:
return self.a.get("b", {})
class SomeBase:
...
class OwnClass(MixinAccessB, SomeBase):
def __init__(self, a: dict[str, Any]):
self.a = a.copy()
demo1 = OwnClass({"b": {"allow_X": True}})
print(demo1.b["allow_X"])
Output: True
No mypy issues in --strict mode.
Mixin with a foreign class
If you are dealing with foreign classes, you could still use the Mix-in and then use functools.update_wrapper like this:
from functools import update_wrapper
from typing import Any, Protocol
class AProtocol(Protocol):
a: dict[str, Any]
class MixinAccessB:
"""My mixin"""
#property
def b(self: AProtocol) -> dict[str, bool]:
return self.a.get("b", {})
class Foreign:
"""Foreign class documentation"""
def __init__(self, a: dict[str, Any]):
self.a = a.copy()
class MixedForeign(MixinAccessB, Foreign):
"""foo"""
pass
update_wrapper(MixedForeign, Foreign, updated=())
demo2 = MixedForeign({"b": {"allow_X": True}})
print(demo2.b["allow_X"])
print(f'{MixedForeign.__name__=} {MixedForeign.__doc__=}')
Output:
True
MixedForeign.__name__='Foreign' MixedForeign.__doc__='Foreign class documentation'
Also no mypy issues in --strict mode.
Note that you still need the AProtocol to make it clear that whatever self will be in that property follows that protocol, i.e. has an attribute a with the type dict[str, Any].
I hope I understood your requirements correctly and this at least provides a solution for your particular situation, even though I could not enlighten you on the type variable issue.
Is it possible to have an abstract class require some specific arguments for a method of a Python class, while still leaving the possibility for concrete classes to add more arguments?
from abc import ABC, abstractmethod
class FooBase(ABC):
#abstractmethod
def __init__(self, required: str, also_required: int):
pass
And then the concrete class would go, conceptually:
class Foo(FooBase):
def __init__(self, required: str, also_required: int, something_else: float):
do_stuff()
Context
This is in the context of a package/library intended to be imported by client. I'd like to provide the FooBase abstract class, that has a particular contract regarding other parts of the library. Clients would be free to implement their concrete Foo, but in this use case, the method arguments of __init__ are to be considered as "mandatory minimum", not as being exhaustive.
Solution: using **kwargs ?
One "solution" could be to use **kwargs in both abstract and concrete methods to accept any other keyword argument, but the issue is precisely that: the class now accepts any other keyword argument, instead of enforcing specific ones, which brings at least two issues:
argument names validation can now happen only at runtime (e.g. no mypy)
can't know what argument names to use based on method signature alone
class UglyBase:
#abstractmethod
def __init__(self, required: str, **kwargs):
pass
class Ugly(UglyBase):
def __init__(self, required: str, **kwargs):
# yurk
self.something_else = kwargs.get("something_else", "default")
# small typo in the last argument's name goes undetected
# neither IDE nor MyPy can detect it statically
ugly_object = Ugly(required="hello", someting_else="blabla")
# mistyped argument was silently ignored
ugly_object.something_else
>>> "default"
I think this will give you a lead on how to (possibly) solve your issue. I don't know what to think about inspect, but it exists. So why not use it?
from abc import ABC, abstractmethod
import inspect
class FooBase(ABC):
def __new__(cls, *args, **kwargs):
# args = ('test', 1, 2.0)
# cls == Foo
# inspect.signature(cls.__init__) = <Signature (self, required: str, also_required: int, something_else: float)>
# You could compare the signature of cls.__init__ with the signature of FooBase.__init__
# for example
# set(inspect.signature(FooBase.__init__).parameters.keys()).issubset(set(inspect.signature(cls.__init__).parameters.keys()))
# this is a very long oneliner so you might want to use intermediate variables
return super(FooBase, cls).__new__(cls)
#abstractmethod
def __init__(self, required: str, also_required: int):
print("init")
pass
class Foo(FooBase):
def __init__(self, required: str, also_required: int, something_else: float):
pass
f = Foo("test", 1, 2.0)
Is there any way to type an abstract parent class method such that the child class method is known to return itself, instead of the abstract parent.
class Parent(ABC):
#abstractmethod
def method(self) -> [what to hint here]:
pass
class Child1(Parent)
def method(self):
pass
def other_method(self):
pass
class GrandChild1(Child1)
def other_method_2(self):
pass
This is more to improve autocompletes for IDEs like PyCharm or VScode's python plugin.
So, the general approach is described in the docs here
import typing
from abc import ABC, abstractmethod
T = typing.TypeVar('T', bound='Parent') # use string
class Parent(ABC):
#abstractmethod
def method(self: T) -> T:
...
class Child1(Parent):
def method(self: T) -> T:
return self
def other_method(self):
pass
class GrandChild1(Child1):
def other_method_2(self):
pass
reveal_type(Child1().method())
reveal_type(GrandChild1().method())
And mypy gives us:
test_typing.py:22: note: Revealed type is 'test_typing.Child1*'
test_typing.py:23: note: Revealed type is 'test_typing.GrandChild1*'
Note, I had to keep using type-variables to get this to work, so when I originally tried to use the actual child class in the child class annotation, it (erroneously?) inherited the type in the grandchild:
class Child1(Parent):
def method(self) -> Child1:
return self
I'd get with mypy:
test_typing.py:22: note: Revealed type is 'test_typing.Child1'
test_typing.py:23: note: Revealed type is 'test_typing.Child1'
Again, I am not sure if this is expected/correct behavior. The mypy documentation currently has a warning:
This feature is experimental. Checking code with type annotations for
self arguments is still not fully implemented. Mypy may disallow valid
code or allow unsafe code.
Python 3.11 introduced more elegant solution based on PEP-0673, predefined type Self (official docs). Example:
from typing import Self
class Parent(ABC):
#abstractmethod
def method(self) -> Self:
pass
class Child1(Parent)
def method(self) -> Self:
pass
def other_method(self) -> Self:
pass
class GrandChild1(Child1)
def other_method_2(self) -> Self:
pass
It covers classmethods too:
class Shape:
#classmethod
def from_config(cls, config: dict[str, float]) -> Self:
return cls(config["scale"])
NOTE: for pre-python-3.11 one can use:
1 - Quoted type, e.g.
class Parent:
def method(self) -> "Parent":
pass
2 - or postoponed type hint evaluation (PEP-0563, or other SO answer, python 3.7+):
from __future__ import annotations
class Parent:
def method(self) -> Parent:
pass
I'm using class decorators in Python and cannot figure out which type annotation to give to my class to make mypy happy.
My code is the following:
from typing import Type
from pprint import pformat
def betterrepr(cls:Type[object]):
"""Improve representation of a class"""
class improved_class(cls): # line 12
def __repr__(self) -> str:
return f"Instance of {cls.__name__}, vars = {pformat(vars(self))}"
return improved_class
I'm currently having the 2 following errors:
myprog.py:12: error: Invalid type "cls"
myprog.py:12: error: Invalid base class
What shall I use for the type of cls (and by the way, is it Pythonic to use this keyword for a class used as argument?)?
Thanks
Using function arguments as base classes is currently not supported by mypy. Your only option is to silence the error, either with a type: ignore comment or a dummy alias like base: Any = cls.
Even without annotating cls, mypy will correctly infer the type of a class decorated with betterrepr. To document that your decorator returns a class similar to the decorated class, use a TypeVar.
from typing import Type, TypeVar
from pprint import pformat
T = TypeVar('T')
def betterrepr(cls: Type[T]) -> Type[T]:
"""Improve representation of a class"""
class IClass(cls): # type: ignore
def __repr__(self) -> str:
return f"Instance of {cls.__name__}, vars = {pformat(vars(self))}"
return IClass
Given a class with a helper method for initialization:
class TrivialClass:
def __init__(self, str_arg: str):
self.string_attribute = str_arg
#classmethod
def from_int(cls, int_arg: int) -> ?:
str_arg = str(int_arg)
return cls(str_arg)
Is it possible to annotate the return type of the from_int method?
I'v tried both cls and TrivialClass but PyCharm flags them as unresolved references which sounds reasonable at that point in time.
Starting with Python 3.11 you can use the new typing.Self object. For older Python versions you can get the same object by using the typing-extensions project:
try:
from typing import Self
except ImportError:
from typing_extensions import Self
class TrivialClass:
# ...
#classmethod
def from_int(cls, int_arg: int) -> Self:
# ...
return cls(...)
Note that you don't need to annotate cls in this case.
Warning: mypy support for the Self type has not yet been released; you'll need to wait for the next version after 0.991. Pyright already supports it.
If you can't wait for Mypy support, then you can use a generic type to indicate that you'll be returning an instance of cls:
from typing import Type, TypeVar
T = TypeVar('T', bound='TrivialClass')
class TrivialClass:
# ...
#classmethod
def from_int(cls: Type[T], int_arg: int) -> T:
# ...
return cls(...)
Any subclass overriding the class method but then returning an instance of a parent class (TrivialClass or a subclass that is still an ancestor) would be detected as an error, because the factory method is defined as returning an instance of the type of cls.
The bound argument specifies that T has to be a (subclass of) TrivialClass; because the class doesn't yet exist when you define the generic, you need to use a forward reference (a string with the name).
See the Annotating instance and class methods section of PEP 484.
Note: The first revision of this answer advocated using a forward reference
naming the class itself as the return value, but issue 1212 made it possible to use generics instead, a better solution.
As of Python 3.7, you can avoid having to use forward references in annotations when you start your module with from __future__ import annotations, but creating a TypeVar() object at module level is not an annotation. This is still true even in Python 3.10, which defers all type hint resolution in annotations.
From Python 3.7 you can use __future__.annotations:
from __future__ import annotations
class TrivialClass:
# ...
#classmethod
def from_int(cls, int_arg: int) -> TrivialClass:
# ...
return cls(...)
Edit: you can't subclass TrivialClass without overriding the classmethod, but if you don't require this then I think it's neater than a forward reference.
A simple way to annotate the return type is to use a string as the annotation for the return value of the class method:
# test.py
class TrivialClass:
def __init__(self, str_arg: str) -> None:
self.string_attribute = str_arg
#classmethod
def from_int(cls, int_arg: int) -> 'TrivialClass':
str_arg = str(int_arg)
return cls(str_arg)
This passes mypy 0.560 and no errors from python:
$ mypy test.py --disallow-untyped-defs --disallow-untyped-calls
$ python test.py
In Python 3.11 there is a nicer way to do this using the new Self type:
from typing import Self
class TrivialClass:
def __init__(self, str_arg: str):
self.string_attribute = str_arg
#classmethod
def from_int(cls, int_arg: int) -> Self:
str_arg = str(int_arg)
return cls(str_arg)
This also works correctly with sub classes as well.
class TrivialSubClass(TrivialClasss):
...
TrivialSubclass.from_int(42)
The IDE shows return type TrivialSubClass and not TrivialClass.
This is described in PEP 673.