NotImplementedError: How to resolve Python unimplemented method error?

In Python programming, when we want to call a method that has not yet been implemented, a NotImplementedError error message will appear. This error can confuse us because it doesn't clearly tell us how to fix it. In this article, we will explore the causes of NotImplementedError and provide some solutions to help you overcome this error.

1. What is NotImplementedError?

NotImplementedError is one of Python's built-in exceptions that represents an unimplemented method or operation. Usually, this kind of error message will appear in unimplemented methods in abstract classes or interfaces, reminding programmers that they need to implement related functions.

The abstract class in Python is a special class that cannot be instantiated and can only be inherited. Methods in abstract classes usually only have names and parameters, but no implementation. These methods need to be overridden by subclasses to obtain actual functionality.

In Python, we can use the ABC library to define abstract classes and interfaces. If we do not implement all the abstract methods in the parent class in the child class, then when we try to call these methods, a NotImplementedError exception will be triggered.

2. How to solve NotImplementedError?

(1) Implement unimplemented methods

When a NotImplementedError exception occurs, the simplest solution is to implement the relevant method according to the error prompt. For example, if we use a method foo() that is not implemented, then we can implement this method in the code and rerun the program.

(2) Use placeholders

When we design a class or interface, we may encounter situations where we need to reserve methods but do not know the specific implementation. At this time, we can use placeholders to temporarily replace the implementation of the method. In Python, we can use the pass keyword to implement placeholders.

For example, suppose we have an abstract class Animal, which contains an eat() method:

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def eat(self):
        pass

In this example, we use pass to temporarily replace the implementation of the eat method. When we implement a subclass and need to override the eat method, we can delete pass and implement the relevant code.

(3) Overloading base class methods

If we cannot implement the abstract method or interface in the parent class in the subclass, we can consider overloading the base class method. Overloading refers to a method that is redefined in a subclass and has the same name as the method in the base class but has different behavior.

For example, we have an abstract class named Shape, which contains an abstract method named area(). We can overload the area method in subclasses and implement related logic.

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass
    
class Circle(Shape):
    def __init__(self, r):
        self.radius = r
        
    def area(self):
        return 3.14 * self.radius * self.radius

In this example, we create a subclass named Circle and overload the area method in the Shape class. Therefore, even if the area method in the Shape class does not have a concrete implementation, we can call the method in the Circle object and it will run normally.

(4) Use raise NotImplementedError

In some cases, we may not want to implement a specific method in the subclass, but want to throw a NotImplementedError exception when calling. In this case, we can use the raise keyword to raise an exception.

For example, we have a Database class, which contains a connect method. We hope that when instantiating the object, if no parameters are passed, a NotImplementedError exception will be thrown:

class Database:
    def __init__(self, host=None, port=None):
        if host is None and port is None:
            raise NotImplementedError("Database connection details not provided")
        self.host = host
        self.port = port
        
    def connect(self):
        # connect to database using host and port
        pass

In this In the example, if we try to instantiate the Database object without providing any parameters, a NotImplementedError exception will be raised.

In summary, NotImplementedError is a common exception in Python programming, and is usually related to unimplemented methods in abstract classes or interfaces. According to Python's philosophy, "use error messages to remind programmers that they have violated a contract", therefore, we should learn to resolve NotImplementedError exceptions and try to avoid this situation in our programs.

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