Merge pull request #377 from realpython/solid-principles-python

Sample code for the SOLID principles article

Author: KateFinegan

solid-principles-python/README.md

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+# SOLID Principles: Improve Object-Oriented Design in Python
+
+This folder provides the code examples for the Real Python tutorial [SOLID Principles: Improve Object-Oriented Design in Python](https://realpython.com/solid-principles-python/).

solid-principles-python/app_dip.py

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+from abc import ABC, abstractmethod
+
+# Bad example
+# class FrontEnd:
+#     def __init__(self, back_end):
+#         self.back_end = back_end
+
+#     def display_data(self):
+#         data = self.back_end.get_data_from_database()
+#         print("Display data:", data)
+
+
+# class BackEnd:
+#     def get_data_from_database(self):
+#         """Return data from the database."""
+#         return "Data from the database"
+
+
+# Good example
+class FrontEnd:
+    def __init__(self, data_source):
+        self.data_source = data_source
+
+    def display_data(self):
+        data = self.data_source.get_data()
+        print("Display data:", data)
+
+
+class DataSource(ABC):
+    @abstractmethod
+    def get_data(self):
+        pass
+
+
+class Database(DataSource):
+    def get_data(self):
+        return "Data from the database"
+
+
+class API(DataSource):
+    def get_data(self):
+        return "Data from the API"

solid-principles-python/file_manager_srp.py

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+from pathlib import Path
+from zipfile import ZipFile
+
+# Bad example
+# class FileManager:
+#     def __init__(self, filename):
+#         self.filename = Path(filename)
+
+#     def read(self):
+#         with self.filename.open(mode="r") as file:
+#             data = file.read()
+#         return data
+
+#     def write(self, data):
+#         with self.filename.open(mode="w") as file:
+#             file.write(data)
+
+#     def compress(self):
+#         with ZipFile(self.filename.stem + ".zip", mode="w") as archive:
+#             archive.write(self.filename)
+
+#     def decompress(self, archive_name):
+#         with ZipFile(archive_name, mode="r") as archive:
+#             archive.extractall()
+
+
+# Good example
+class FileManager:
+    def __init__(self, filename):
+        self.filename = Path(filename)
+
+    def read(self):
+        with self.filename.open(mode="r") as file:
+            data = file.read()
+        return data
+
+    def write(self, data):
+        with self.filename.open(mode="w") as file:
+            file.write(data)
+
+
+class ZipFileManager:
+    def __init__(self, filename):
+        self.filename = Path(filename)
+
+    def compress(self):
+        with ZipFile(self.filename.stem + ".zip", mode="w") as archive:
+            archive.write(self.filename)
+
+    def decompress(self, archive_name):
+        with ZipFile(archive_name, mode="r") as archive:
+            archive.extractall()

solid-principles-python/printers_isp.py

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+from abc import ABC, abstractmethod
+
+# Bad example
+# class Printer(ABC):
+#     @abstractmethod
+#     def print(self, document):
+#         pass
+
+#     @abstractmethod
+#     def fax(self, document):
+#         pass
+
+#     @abstractmethod
+#     def scan(self, document):
+#         pass
+
+
+# class OldPrinter(Printer):
+#     def print(self, document):
+#         print(f"Printing {document} in black and white...")
+
+#     def fax(self, document):
+#         raise NotImplementedError("Fax functionality not supported")
+
+#     def scan(self, document):
+#         raise NotImplementedError("Scan functionality not supported")
+
+
+# class ModernPrinter(Printer):
+#     def print(self, document):
+#         print(f"Printing {document} in color...")
+
+#     def fax(self, document):
+#         print(f"Faxing {document}...")
+
+#     def scan(self, document):
+#         print(f"Scanning {document}...")
+
+
+# Good example
+class Printer(ABC):
+    @abstractmethod
+    def print(self, document):
+        pass
+
+
+class Fax(ABC):
+    @abstractmethod
+    def fax(self, document):
+        pass
+
+
+class Scanner(ABC):
+    @abstractmethod
+    def scan(self, document):
+        pass
+
+
+class OldPrinter(Printer):
+    def print(self, document):
+        print(f"Printing {document} in black and white...")
+
+
+class NewPrinter(Printer, Fax, Scanner):
+    def print(self, document):
+        print(f"Printing {document} in color...")
+
+    def fax(self, document):
+        print(f"Faxing {document}...")
+
+    def scan(self, document):
+        print(f"Scanning {document}...")

solid-principles-python/shapes_lsp.py

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+class Rectangle:
+    def __init__(self, width, height):
+        self.width = width
+        self.height = height
+
+    def area(self):
+        return self.width * self.height
+
+    def perimeter(self):
+        return 2 * (self.width + self.height)
+
+
+class Square(Rectangle):
+    def __init__(self, side):
+        super().__init__(side, side)
+        self.side = side
+
+    @property
+    def side(self):
+        return self._side
+
+    @side.setter
+    def side(self, value):
+        self._side = value
+        self.width = value
+        self.height = value

solid-principles-python/shapes_ocp.py

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+from abc import ABC, abstractmethod
+from math import pi
+
+# Bad example
+# class Shape:
+#     def __init__(self, shape_type, **kwargs):
+#         self.shape_type = shape_type
+#         if self.shape_type == "rectangle":
+#             self.width = kwargs["width"]
+#             self.height = kwargs["height"]
+#         elif self.shape_type == "circle":
+#             self.radius = kwargs["radius"]
+
+#     def calculate_area(self):
+#         if self.shape_type == "rectangle":
+#             return self.width * self.height
+#         elif self.shape_type == "circle":
+#             return pi * self.radius**2
+
+
+# Good example
+class Shape(ABC):
+    def __init__(self, shape_type):
+        self.shape_type = shape_type
+
+    @abstractmethod
+    def calculate_area(self):
+        pass
+
+
+class Circle(Shape):
+    def __init__(self, radius):
+        super().__init__("circle")
+        self.radius = radius
+
+    def calculate_area(self):
+        return pi * self.radius**2
+
+
+class Rectangle(Shape):
+    def __init__(self, width, height):
+        super().__init__("rectangle")
+        self.width = width
+        self.height = height
+
+    def calculate_area(self):
+        return self.width * self.height
+
+
+class Square(Shape):
+    def __init__(self, side):
+        super().__init__("square")
+        self.side = side
+
+    def calculate_area(self):
+        return self.side**2