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.
Related
from typing_extensions import Protocol
class IFoo(Protocol):
value: int
class Foo(IFoo):
#property
def value(self) -> int:
return 2
_value: int
#value.setter
def value(self, value: int):
self._value = value
Pylance in strict mode(basic mode doesn't) is giving an error at the getter and the setter saying that:
"value" overrides symbol of the same name in class "IFoo"
"property" is incompatible with "int".
I could make this work by changing the Protocol to:
class IFoo(Protocol):
#property
def value(self) -> int:
raise NotImplemented
But this now makes this invalid:
class Foo(IFoo):
value: int
This doesn't makes sense, the Foo would still have the property value that is an int, why being a getter should makes it different (in typescript this doesn't make a difference)?
How can I fix this?
Reading the Defining a protocol section of the relevant pep (PEP 544), the example implementation (in their case, class Resource) does not directly inherit from the protocol - their class SupportsClose functions as a reference type for type hinting validators.
Your example is also reminiscent of the long established zope.interface package, which this PEP also referenced. Note that the example usage the PEP have cited the following example (irrelevant lines trimmed):
from zope.interface import Interface, implementer
class IEmployee(Interface):
...
#implementer(IEmployee)
class Employee:
...
The Employee class does not directly subclass from IEmployee (a common mistake for newbie Zope/Plone developers back in the days), it's simply decorated with the zope.interface.implementer(IEmployee) class decorator to denote that the class Employee implements from the interface IEmployee.
Likewise, reading further down under the section Protocol members, we have an example of the template and concrete classes (again, example trimmed):
from typing import Protocol
class Template(Protocol):
name: str # This is a protocol member
value: int = 0 # This one too (with default)
class Concrete:
def __init__(self, name: str, value: int) -> None:
self.name = name
self.value = value
var: Template = Concrete('value', 42) # OK
Again, note that the Concrete implementation does not inherit from the Template, yet, the variable var is denoted to have an expected type of Template. Note that the Concrete instance can be assigned to it as it matches the expected protocol as defined for Template.
So all that being said, given your example, you may wish to define class Foo: as is rather than having it inherit from IFoo as you had originally, and fill in the type information such that things that expect IFoo be type hinted as appropriately in the relevant context (e.g. some_foo: IFoo = Foo(...) or def some_func(foo: IFoo):).
As an addendum, you may wish to define Foo as such:
class Foo:
_value: int
#property
def value(self) -> int:
return 2
#value.setter
def value(self, value: int):
self._value = value
Having the _value definition in between the property and its setter seems to confuse mypy due to this issue.
I have something like this:
from typing import TypeVar, Generic, Tuple
T = TypeVar('T')
S = TypeVar('S')
U = TypeVar('U')
class Foo(Generic[T, S]):
def get_type_vars(self) -> Tuple[TypeVar]:
return #TODO how do I return T and S here?
assert Foo().get_type_vars() == (T, S)
Is there any way to get this behavior? I need a way to find out, that S and T are the TypeVars of the generic Class Foo. Any ideas?
I should mention that I write some class decorators and the method get_type_vars() will be added to the class by a decorator. So all I have is the instance self in the method:
def get_type_vars(self) -> Tuple[TypeVar]:
return #TODO how do I return T and S here in case that self is an instance of Foo?
You can use get_args combined with __orig_bases__ to inspect generic types of the base class:
class Foo(Generic[T, S]):
def get_type_vars(self) -> Tuple[TypeVar]:
return get_args(type(self).__orig_bases__[0])
This would get a bit more complicated for more complex inheritance chains though.
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.