Implementing an Interface in Python

Interfaces play an important role in software engineering. As an application grows, updates and changes to the code base become more difficult to manage. More often than not, you wind up having classes that look very similar but are unrelated, which can lead to some confusion. In this tutorial, you’ll see how you can use a Python interface to help determine what class you should use to tackle the current problem.

In this tutorial, you’ll be able to:

Understand how interfaces work and the caveats of Python interface creation
Comprehend how useful interfaces are in a dynamic language like Python
Implement an informal Python interface
Use abc.ABCMeta and @abc.abstractmethod to implement a formal Python interface

Interfaces in Python are handled differently than in most other languages, and they can vary in their design complexity. By the end of this tutorial, you’ll have a better understanding of some aspects of Python’s data model, as well as how interfaces in Python compare to those in languages like Java, C++, and Go.

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Python Interface Overview

At a high level, an interface acts as a blueprint for designing classes. Like classes, interfaces define methods. Unlike classes, these methods are abstract. An abstract method is one that the interface simply defines. It doesn’t implement the methods. This is done by classes, which then implement the interface and give concrete meaning to the interface’s abstract methods.

Python’s approach to interface design is somewhat different when compared to languages like Java, Go, and C++. These languages all have an interface keyword, while Python does not. Python further deviates from other languages in one other aspect. It doesn’t require the class that’s implementing the interface to define all of the interface’s abstract methods.

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Informal Interfaces

In certain circumstances, you may not need the strict rules of a formal Python interface. Python’s dynamic nature allows you to implement an informal interface. An informal Python interface is a class that defines methods that can be overridden, but there’s no strict enforcement.

In the following example, you’ll take the perspective of a data engineer who needs to extract text from various different unstructured file types, like PDFs and emails. You’ll create an informal interface that defines the methods that will be in both the PdfParser and EmlParser concrete classes:

Python
      
    
class InformalParserInterface:
    def load_data_source(self, path: str, file_name: str) -> str:
        """Load in the file for extracting text."""
        pass

    def extract_text(self, full_file_name: str) -> dict:
        """Extract text from the currently loaded file."""
        pass

InformalParserInterface defines the two methods .load_data_source() and .extract_text(). These methods are defined but not implemented. The implementation will occur once you create concrete classes that inherit from InformalParserInterface.

As you can see, InformalParserInterface looks identical to a standard Python class. You rely on duck typing to inform users that this is an interface and should be used accordingly.

Note: Haven’t heard of duck typing? This term says that if you have an object that looks like a duck, walks like a duck, and quacks like a duck, then it must be a duck! To learn more, check out Duck Typing.

With duck typing in mind, you define two classes that implement the InformalParserInterface. To use your interface, you must create a concrete class. A concrete class is a subclass of the interface that provides an implementation of the interface’s methods. You’ll create two concrete classes to implement your interface. The first is PdfParser, which you’ll use to parse the text from PDF files:

Python
      
    
class PdfParser(InformalParserInterface):
    """Extract text from a PDF"""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides InformalParserInterface.load_data_source()"""
        pass

    def extract_text(self, full_file_path: str) -> dict:
        """Overrides InformalParserInterface.extract_text()"""
        pass

The concrete implementation of InformalParserInterface now allows you to extract text from PDF files.

The second concrete class is EmlParser, which you’ll use to parse the text from emails:

Python
      
    
class EmlParser(InformalParserInterface):
    """Extract text from an email"""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides InformalParserInterface.load_data_source()"""
        pass

    def extract_text_from_email(self, full_file_path: str) -> dict:
        """A method defined only in EmlParser.
        Does not override InformalParserInterface.extract_text()
        """
        pass

The concrete implementation of InformalParserInterface now allows you to extract text from email files.

So far, you’ve defined two concrete implementations of the InformalPythonInterface. However, note that EmlParser fails to properly define .extract_text(). If you were to check whether EmlParser implements InformalParserInterface, then you’d get the following result:

Python
      
>>> # Check if both PdfParser and EmlParser implement InformalParserInterface
>>> issubclass(PdfParser, InformalParserInterface)
True

>>> issubclass(EmlParser, InformalParserInterface)
True

This would return True, which poses a bit of a problem since it violates the definition of an interface!

Now check the method resolution order (MRO) of PdfParser and EmlParser. This tells you the superclasses of the class in question, as well as the order in which they’re searched for executing a method. You can view a class’s MRO by using the dunder method cls.__mro__:

Python
      
>>> PdfParser.__mro__
(__main__.PdfParser, __main__.InformalParserInterface, object)

>>> EmlParser.__mro__
(__main__.EmlParser, __main__.InformalParserInterface, object)

Such informal interfaces are fine for small projects where only a few developers are working on the source code. However, as projects get larger and teams grow, this could lead to developers spending countless hours looking for hard-to-find logic errors in the codebase!

Using Metaclasses

Ideally, you would want issubclass(EmlParser, InformalParserInterface) to return False when the implementing class doesn’t define all of the interface’s abstract methods. To do this, you’ll create a metaclass called ParserMeta. You’ll be overriding two dunder methods:

.__instancecheck__()
.__subclasscheck__()

In the code block below, you create a class called UpdatedInformalParserInterface that builds from the ParserMeta metaclass:

Python
      
    
class ParserMeta(type):
    """A Parser metaclass that will be used for parser class creation.
    """
    def __instancecheck__(cls, instance):
        return cls.__subclasscheck__(type(instance))

    def __subclasscheck__(cls, subclass):
        return (hasattr(subclass, 'load_data_source') and 
                callable(subclass.load_data_source) and 
                hasattr(subclass, 'extract_text') and 
                callable(subclass.extract_text))

class UpdatedInformalParserInterface(metaclass=ParserMeta):
    """This interface is used for concrete classes to inherit from.
    There is no need to define the ParserMeta methods as any class
    as they are implicitly made available via .__subclasscheck__().
    """
    pass

Now that ParserMeta and UpdatedInformalParserInterface have been created, you can create your concrete implementations.

First, create a new class for parsing PDFs called PdfParserNew:

Python
      
    
class PdfParserNew:
    """Extract text from a PDF."""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides UpdatedInformalParserInterface.load_data_source()"""
        pass

    def extract_text(self, full_file_path: str) -> dict:
        """Overrides UpdatedInformalParserInterface.extract_text()"""
        pass

Here, PdfParserNew overrides .load_data_source() and .extract_text(), so issubclass(PdfParserNew, UpdatedInformalParserInterface) should return True.

In this next code block, you have a new implementation of the email parser called EmlParserNew:

Python
      
    
class EmlParserNew:
    """Extract text from an email."""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides UpdatedInformalParserInterface.load_data_source()"""
        pass

    def extract_text_from_email(self, full_file_path: str) -> dict:
        """A method defined only in EmlParser.
        Does not override UpdatedInformalParserInterface.extract_text()
        """
        pass

Here, you have a metaclass that’s used to create UpdatedInformalParserInterface. By using a metaclass, you don’t need to explicitly define the subclasses. Instead, the subclass must define the required methods. If it doesn’t, then issubclass(EmlParserNew, UpdatedInformalParserInterface) will return False.

Running issubclass() on your concrete classes will produce the following:

Python
      
>>> issubclass(PdfParserNew, UpdatedInformalParserInterface)
True

>>> issubclass(EmlParserNew, UpdatedInformalParserInterface)
False

As expected, EmlParserNew is not a subclass of UpdatedInformalParserInterface since .extract_text() wasn’t defined in EmlParserNew.

Now, let’s have a look at the MRO:

Python
      
>>> PdfParserNew.__mro__
(<class '__main__.PdfParserNew'>, <class 'object'>)

As you can see, UpdatedInformalParserInterface is a superclass of PdfParserNew, but it doesn’t appear in the MRO. This unusual behavior is caused by the fact that UpdatedInformalParserInterface is a virtual base class of PdfParserNew.

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Using Virtual Base Classes

In the previous example, issubclass(EmlParserNew, UpdatedInformalParserInterface) returned True, even though UpdatedInformalParserInterface did not appear in the EmlParserNew MRO. That’s because UpdatedInformalParserInterface is a virtual base class of EmlParserNew.

The key difference between these and standard subclasses is that virtual base classes use the .__subclasscheck__() dunder method to implicitly check if a class is a virtual subclass of the superclass. Additionally, virtual base classes don’t appear in the subclass MRO.

Take a look at this code block:

Python
      
    
class PersonMeta(type):
    """A person metaclass"""
    def __instancecheck__(cls, instance):
        return cls.__subclasscheck__(type(instance))

    def __subclasscheck__(cls, subclass):
        return (hasattr(subclass, 'name') and 
                callable(subclass.name) and 
                hasattr(subclass, 'age') and 
                callable(subclass.age))

class PersonSuper:
    """A person superclass"""
    def name(self) -> str:
        pass

    def age(self) -> int:
        pass

class Person(metaclass=PersonMeta):
    """Person interface built from PersonMeta metaclass."""
    pass

Here, you have the setup for creating your virtual base classes:

The metaclass PersonMeta
The base class PersonSuper
The Python interface Person

Now that the setup for creating virtual base classes is done you’ll define two concrete classes, Employee and Friend. The Employee class inherits from PersonSuper, while Friend implicitly inherits from Person:

Python
      
    
# Inheriting subclasses
class Employee(PersonSuper):
    """Inherits from PersonSuper
    PersonSuper will appear in Employee.__mro__
    """
    pass

class Friend:
    """Built implicitly from Person
    Friend is a virtual subclass of Person since
    both required methods exist.
    Person not in Friend.__mro__
    """
    def name(self):
        pass

    def age(self):
        pass

Although Friend does not explicitly inherit from Person, it implements .name() and .age(), so Person becomes a virtual base class of Friend. When you run issubclass(Friend, Person) it should return True, meaning that Friend is a subclass of Person.

The following UML diagram shows what happens when you call issubclass() on the Friend class:

Taking a look at PersonMeta, you’ll notice that there’s another dunder method called .__instancecheck__(). This method is used to check if instances of Friend are created from the Person interface. Your code will call .__instancecheck__() when you use isinstance(Friend, Person).

Formal Interfaces

Informal interfaces can be useful for projects with a small code base and a limited number of programmers. However, informal interfaces would be the wrong approach for larger applications. In order to create a formal Python interface, you’ll need a few more tools from Python’s abc module.

Using `abc.ABCMeta`

To enforce the subclass instantiation of abstract methods, you’ll utilize Python’s builtin ABCMeta from the abc module. Going back to your UpdatedInformalParserInterface interface, you created your own metaclass, ParserMeta, with the overridden dunder methods .__instancecheck__() and .__subclasscheck__().

Rather than create your own metaclass, you’ll use abc.ABCMeta as the metaclass. Then, you’ll overwrite .__subclasshook__() in place of .__instancecheck__() and .__subclasscheck__(), as it creates a more reliable implementation of these dunder methods.

Using `.subclasshook()`

Here’s the implementation of FormalParserInterface using abc.ABCMeta as your metaclass:

Python
      
    
import abc

class FormalParserInterface(metaclass=abc.ABCMeta):
    @classmethod
    def __subclasshook__(cls, subclass):
        return (hasattr(subclass, 'load_data_source') and 
                callable(subclass.load_data_source) and 
                hasattr(subclass, 'extract_text') and 
                callable(subclass.extract_text))

class PdfParserNew:
    """Extract text from a PDF."""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides FormalParserInterface.load_data_source()"""
        pass

    def extract_text(self, full_file_path: str) -> dict:
        """Overrides FormalParserInterface.extract_text()"""
        pass

class EmlParserNew:
    """Extract text from an email."""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides FormalParserInterface.load_data_source()"""
        pass

    def extract_text_from_email(self, full_file_path: str) -> dict:
        """A method defined only in EmlParser.
        Does not override FormalParserInterface.extract_text()
        """
        pass

If you run issubclass() on PdfParserNew and EmlParserNew, then issubclass() will return True and False, respectively.

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Using `abc` to Register a Virtual Subclass

Once you’ve imported the abc module, you can directly register a virtual subclass by using the .register() metamethod. In the next example, you register the interface Double as a virtual base class of the built-in __float__ class:

Python
      
    
class Double(metaclass=abc.ABCMeta):
    """Double precision floating point number."""
    pass

Double.register(float)

You can check out the effect of using .register():

Python
      
>>> issubclass(float, Double)
True

>>> isinstance(1.2345, Double)
True

By using the .register() meta method, you’ve successfully registered Double as a virtual subclass of float.

Once you’ve registered Double, you can use it as class decorator to set the decorated class as a virtual subclass:

Python
      
    
@Double.register
class Double64:
    """A 64-bit double-precision floating-point number."""
    pass

print(issubclass(Double64, Double))  # True

The decorator register method helps you to create a hierarchy of custom virtual class inheritance.

Using Subclass Detection With Registration

You must be careful when you’re combining .__subclasshook__() with .register(), as .__subclasshook__() takes precedence over virtual subclass registration. To ensure that the registered virtual subclasses are taken into consideration, you must add NotImplemented to the .__subclasshook__() dunder method. The FormalParserInterface would be updated to the following:

Python
      
    
class FormalParserInterface(metaclass=abc.ABCMeta):
    @classmethod
    def __subclasshook__(cls, subclass):
        return (hasattr(subclass, 'load_data_source') and 
                callable(subclass.load_data_source) and 
                hasattr(subclass, 'extract_text') and 
                callable(subclass.extract_text) or 
                NotImplemented)

class PdfParserNew:
    """Extract text from a PDF."""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides FormalParserInterface.load_data_source()"""
        pass

    def extract_text(self, full_file_path: str) -> dict:
        """Overrides FormalParserInterface.extract_text()"""
        pass

@FormalParserInterface.register
class EmlParserNew:
    """Extract text from an email."""
    def load_data_source(self, path: str, file_name: str) -> str:
        """Overrides FormalParserInterface.load_data_source()"""
        pass

    def extract_text_from_email(self, full_file_path: str) -> dict:
        """A method defined only in EmlParser.
        Does not override FormalParserInterface.extract_text()
        """
        pass

print(issubclass(PdfParserNew, FormalParserInterface))  # True
print(issubclass(EmlParserNew, FormalParserInterface))  # True

Implementing an Interface in Python

Python Interface Overview

Informal Interfaces

Using Metaclasses

Using Virtual Base Classes

Formal Interfaces

Using abc.ABCMeta

Using .__subclasshook__()

Using abc to Register a Virtual Subclass

Using Subclass Detection With Registration

Using `abc.ABCMeta`

Using `.subclasshook()`

Using `abc` to Register a Virtual Subclass