Implementing F-bounded polymorphism in Python type prompts: the precise application of typing.Self

This article discusses the method to implement F-bounded polymorphism in Python type prompts, that is, how to make the base class method return the type of its specific subclass. In view of the limitations encountered by traditional TypeVar when referencing its own subclasses, the article details the typing.Self type, shows how it gracefully solves this problem, ensures the accurate delivery of type information, and provides two implementation examples based on instance methods and class methods.

Understanding F-bounded polymorphism and its challenges in Python

F-bounded polymorphism is an advanced type system concept that allows a type variable to be limited by itself as a boundary. In object-oriented programming, this usually manifests itself as a method in the base class needs to return the type of its caller (i.e., the actual subclass instance). For example, when a Copy method is defined in a base class, we want it to return a new instance of the subclass itself when it is called in the subclass, rather than a generalized base class instance. This is essential for maintaining type accuracy, especially when making chain calls or relying on specific subclass methods.

Consider a Copyable base class whose Copy method is designed to create a copy of itself. If Example is a subclass of Copyable, we expect the return type of Example().Copy() to be Example, not a simple Copyable.

The initial attempt might look like this, trying to use TypeVar to represent this self-reference type:

 from typing import TypeVar, Generic
from abc import abstractmethod, ABC

# Try to bind itself with TypeVar, but this is not allowed # T = TypeVar( "T", bound="Copyable['T']" ) # Pylance error: TypeVar bound type cannot be generic

class Copyable( Generic[T]): # MyPy error: Type variable "T" is unbound
    @abstractmethod
    def Copy( self ) -> T:
        pass

class Example( Copyable[ 'Example' ] ):
    def Copy( self ) -> 'Example':
        return Example()

The above code tries to use TypeVar T to represent the subclass type of Copyable and to use itself as a boundary. However, this approach throws an error in Python type checkers such as Pylance and MyPy. Pylance complains about "TypeVar bound type cannot be generic", while MyPy points out "Type variable 'T' is unbound". This shows that TypeVar's design does not directly support this recursive, self-referenced generic boundary definition.

If you just let the Copy method return the base class type, although type errors can be avoided, important type information will be lost. For example:

 class Copyable(ABC):
    @abstractmethod
    def Copy(self) -> 'Copyable': # The return type is the base class pass

class Example(Copyable):
    def Copy(self) -> 'Copyable': # Actually return the Example instance, but the type is "degraded"
        return Example()

# When used, the type checker thinks that copy_of_example is a Copyable type copy_of_example: Example = Example().Copy() # The type checker will report an error or issue a warning because Copy() returns Copyable

In this case, the return value of Example().Copy() is a Example instance at runtime, but the type checker treats it as a Copyable type. This means we cannot directly access Example-specific methods or properties unless explicit type conversion is performed, which reduces the utility of the type system.

Introducing typing.Self to solve the problem of F-bounded polymorphism

Python 3.11 introduces the typing.Self type, which is designed to solve the above F-bounded polymorphism problem. The Self type describes a method that returns the type of its current class instance, even if the class is inherited or subclassed by multiple inheritance.

Using Self, we can gracefully refactor the Copyable class so that its Copy method can correctly return the type of the subclass:

 from typing import Self
from abc import abstractmethod, ABC

class Copyable(ABC):
    @abstractmethod
    def Copy(self) -> Self: # Use Self type """
        Create a copy of the current instance and return its specific subclass type.
        """
        pass

class Example(Copyable):
    def Copy(self) -> Self: # The return type is still Self, in the Example context, that is, Example
        """
        Implement the copy method of the Example class.
        """
        return self.__class__() # Create an instance of the current class using self.__class__() # Type checker now correctly recognizes the return type example_instance = Example()
copy_of_example: Example = example_instance.Copy() # The type checker recognizes as Example type print(f"Original type: {type(example_instance)}")
print(f"Copied type: {type(copy_of_example)}")

In this example:

• The Copy method in the Copyable base class is annotated to return Self.
• When the Example class inherits Copyable and implements the Copy method, self.__class__() will dynamically create a new instance of the Example class.
• typing.Self is parsed into the Example type in the context of Example, so the type checker can correctly infer that the type of copy_of_example is Example. This perfectly solves the problem of type information loss.

Use Self in combination with class methods

In some scenarios, Copy operations may be more suitable as classmethods, for example, when we need to create a new instance through the class itself rather than copying based on an existing instance. The Self type also applies to class methods to ensure that the return type is the concrete class that calls the class method.

 from typing import Self
from abc import abstractmethod, ABC

class Copyable(ABC):
    @abstractmethod
    @classmethod
    def Copy(cls) -> Self: # Use Self type """ in class methods
        Create an instance of the current class through the class method and return its specific subclass type.
        """
        pass

class Example(Copyable):
    @classmethod
    def Copy(cls) -> Self: # The return type is still Self, in the Example context, that is, Example
        """
        Implement the class copy method of the Example class.
        """
        return cls() # Create an instance of the current class using cls() # Type checker now correctly identifies the instance type created by class methods new_example_instance: Example = Example.Copy() # Type checker recognizes as Example type print(f"New instance type created via class method: {type(new_example_instance)}")

In this class method example:

• The Copy method is decorated as @classmethod, and its first argument is the class itself (usually named cls).
• The return type is still Self. When Example.Copy() is called, Self is parsed to the Example type.
• cls() directly calls the constructor of the current class Example to create a new instance.

This pattern is useful in factory methods or scenarios where instances need to be created from the class level, while maintaining precise type hints.

Summary and precautions

• Advantages of typing.Self : typing.Self is the standard and recommended way to implement F-bounded polymorphism in Python. It enables methods defined in base classes to return the types of their specific subclasses, thereby avoiding the problem of type information loss and improving the readability of code and the accuracy of type checks.
• Version requirements : typing.Self was introduced in Python 3.11. For older versions of Python, you may need to use from typing_extensions import Self.
• Applicable scenarios : The Self type is especially suitable for methods that need to return the current instance (or class) type, such as copy method, factory method, chain calls in the builder pattern, etc.
• Difference with TypeVar : Although TypeVar is also used for generics, it is mainly used to define parameterized types, while Self is designed specifically for methods to return the type of the class they belong to, especially in inheritance systems.
• Abstract base class : In the above example, Copyable is defined as ABC (Abstract Base Class) and the Copy method is implemented through the @abstractmethod decorator, which is a good practice.

By understanding and proper use of typing.Self, developers can write more robust and more precise Python code, especially when dealing with complex class inheritance and polymorphism scenarios.

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