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Abstract class in Python

When we want to avoid creating an instance from a class, we define the class as an abstract because this class is base for another class and we don't want to create an instance from this class.

We implement the method of the base class as an abstract method because this method is common among all child class and each child class want to custom implementation of the method.

python

from abc import abstractmethod, ABC
class InvalidOperationError(Exception):  # new exception
    pass
class Stream(ABC):
    def __init__(self):
        self.streamopen = False
    def open(self):
        if self.streamopen:
            raise InvalidOperationError("The File is already open")
        self.streamopen = True
    def close(self):
        if not self.streamopen:
            raise InvalidOperationError("The File is already close")
        self.streamopen = False
    @abstractmethod
    def read(self):
        pass
class FileStream(Stream):
    def read(self):
        print("reading from File")
class NetworkStream(Stream):
    def read(self):
        print("reading from Network")

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