Sample code for: Implementing Interfaces in Python: ABCs and Protocols

python-interface/README.md

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+# Implementing Interfaces in Python: ABCs and Protocols
+
+This folder provides the code examples for the Real Python tutorial [Implementing Interfaces in Python: ABCs and Protocols](https://realpython.com/python-interface/)

python-interface/check_protocols.py

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+from readers_protocols import FileReaderProtocol, PdfReader, EmailReader
+
+
+def read(reader: FileReaderProtocol, path: str) -> str:
+    reader.load_file(path)
+    return reader.extract_text()
+
+
+# Accepted by the type checker
+read(PdfReader(), "/reports/report.pdf")
+read(EmailReader(), "/mail/message.eml")

python-interface/check_virtual_classes.py

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+from meta_classes import EmailReader, PdfReader, VirtualFileReader
+
+print(isinstance(PdfReader(), VirtualFileReader))
+print(issubclass(PdfReader, VirtualFileReader))
+print(issubclass(EmailReader, VirtualFileReader))

python-interface/create_readers.py

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+from readers_abc import EmailReader, FileReaderInterface, PdfReader
+
+pdf_reader = PdfReader()
+email_reader = EmailReader()
+
+print(isinstance(PdfReader(), FileReaderInterface))
+print(issubclass(PdfReader, FileReaderInterface))

python-interface/meta_classes.py

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+class ReaderMeta(type):
+    """A Reader metaclass used for reader class creation."""
+
+    def __instancecheck__(cls, instance):
+        return cls.__subclasscheck__(type(instance))
+
+    def __subclasscheck__(cls, subclass):
+        return (
+            hasattr(subclass, "load_file")
+            and callable(subclass.load_file)
+            and hasattr(subclass, "extract_text")
+            and callable(subclass.extract_text)
+        )
+
+
+class VirtualFileReader(metaclass=ReaderMeta):
+    """Virtual class."""
+
+
+class PdfReader:
+    """Extract text from a PDF."""
+
+    def load_file(self, path: str) -> None:
+        print(f"Loading PDF from {path}")
+
+    def extract_text(self) -> str:
+        return "Extracted PDF text"
+
+
+class EmailReader:
+    """Extract text from an Email."""
+
+    def load_file(self, path: str) -> None:
+        print(f"Loading Email from {path}")
+
+    def extract_email_text(self) -> str:
+        return "Extracted Email text"

python-interface/readers.py

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+class PdfReader:
+    """Extract text from a PDF."""
+
+    def load_file(self, path: str) -> None:
+        print(f"Loading PDF from {path}")
+
+    def extract_text(self) -> str:
+        return "Extracted PDF text"
+
+
+class EmailReader:
+    """Extract text from an Email."""
+
+    def load_file(self, path: str) -> None:
+        print(f"Loading Email from {path}")
+
+    def extract_text(self) -> str:
+        return "Extracted Email text"
+
+
+def read(reader, path: str) -> str:
+    reader.load_file(path)
+    return reader.extract_text()
+
+
+print(read(PdfReader(), "/reports/report.pdf"))
+print(read(EmailReader(), "/mail/message.eml"))
+
+# class EmailReader:
+#     """Extract text from an Email."""
+
+#     def load_file(self, path: str) -> None:
+#         print(f"Loading Email from {path}")
+
+#     def extract_email_text(self) -> str:
+#         return "Extracted Email text"

python-interface/readers_abc.py

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+from abc import ABC, abstractmethod
+
+
+class FileReaderInterface(ABC):
+    """Interface for file readers."""
+
+    @abstractmethod
+    def load_file(self, path: str) -> None:
+        """Load a file for text extraction."""
+
+    @abstractmethod
+    def extract_text(self) -> str:
+        """Return text extracted from the loaded file."""
+
+
+class PdfReader(FileReaderInterface):
+    """Extract text from a PDF."""
+
+    def load_file(self, path: str) -> None:
+        """Load a PDF file for text extraction."""
+        print(f"Loading PDF from {path}")
+
+    def extract_text(self) -> str:
+        """Return text extracted from the loaded PDF."""
+        return "Extracted PDF text"
+
+
+class EmailReader(FileReaderInterface):
+    """Extract text from an Email."""
+
+    def load_file(self, path: str) -> None:
+        """Load an EML file for text extraction."""
+        print(f"Loading Email from {path}")
+
+    def extract_email_text(self) -> str:
+        """Return text extracted from the loaded Email."""
+        return "Extracted Email text"

python-interface/readers_protocol.py

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+from typing import Protocol
+
+
+# @runtime_checkable # Uncomment for runtime checks of protocol adherence
+class FileReaderProtocol(Protocol):
+    """Protocol for file readers."""
+
+    def load_file(self, path: str) -> None:
+        """Load a file for text extraction."""
+        ...
+
+    def extract_text(self) -> str:
+        """Return text extracted from the loaded file."""
+        ...
+
+
+class PdfReader:
+    """Extract text from a PDF."""
+
+    def load_file(self, path: str) -> None:
+        """Load a PDF file for text extraction."""
+        print("Loading your PDF...")
+
+    def extract_text(self) -> str:
+        """Return text extracted from the loaded PDF."""
+        return "Your PDF content"
+
+
+class EmailReader:
+    """Extract text from an Email."""
+
+    def load_file(self, path: str) -> None:
+        """Load an EML file for text extraction."""
+        print("Loading your Email...")
+
+    def extract_text(self) -> str:
+        """Return text extracted from the loaded Email."""
+        return "Your Email content"

python-interface/registered_subclass.py

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+from virtual_subclass import Double
+
+print(issubclass(float, Double))
+print(isinstance(1.2345, Double))
+
+
+@Double.register
+class Double64:
+    """A 64-bit double-precision floating-point number."""
+
+
+print(issubclass(Double64, Double))

python-interface/virtual_subclass.py

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+from abc import ABC
+
+
+class Double(ABC):
+    """Double precision floating point number."""
+
+
+Double.register(float)