add builder pattern to python patterns plus associated tests

Author: wael-jaber

python/src/creational/builder/__init__.py

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+from .builder import Builder, ConcreteBuilder
+from .product import Product
+from .director import Director
+
+__all__ = ["Builder", "ConcreteBuilder", "Product", "Director"]

python/src/creational/builder/builder.py

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+from abc import ABC, abstractmethod
+from .product import Product
+
+
+class Builder(ABC):
+    """
+    Abstract Builder: Declares product creation steps.
+    """
+
+    @abstractmethod
+    def reset(self):
+        pass
+
+    @abstractmethod
+    def build_part_a(self):
+        pass
+
+    @abstractmethod
+    def build_part_b(self):
+        pass
+
+    @abstractmethod
+    def get_result(self) -> Product:
+        pass
+
+
+class ConcreteBuilder(Builder):
+    """
+    Concrete Builder: Implements the building steps.
+    """
+
+    def __init__(self):
+        self.product = Product()
+
+    def reset(self):
+        self.product = Product()
+
+    def build_part_a(self):
+        self.product.add_part("Part A")
+
+    def build_part_b(self):
+        self.product.add_part("Part B")
+
+    def get_result(self) -> Product:
+        return self.product

python/src/creational/builder/director.py

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+from .builder import Builder
+
+
+class Director:
+    """
+    Director: Defines the order in which steps are executed.
+    """
+
+    def __init__(self, builder: Builder):
+        self.builder = builder
+
+    def construct_minimal_product(self):
+        """
+        Constructs a minimal product with only part A.
+        """
+        self.builder.reset()
+        self.builder.build_part_a()
+
+    def construct_full_product(self):
+        """
+        Constructs a full product with both part A and part B.
+        """
+        self.builder.reset()
+        self.builder.build_part_a()
+        self.builder.build_part_b()
+
+# Example Usage:
+#
+# from src.creational.builder.builder import ConcreteBuilder
+# from src.creational.builder.director import Director
+#
+# builder = ConcreteBuilder()
+# director = Director(builder)
+#
+# director.construct_full_product()
+# product = builder.get_result()
+#
+# print(product.list_parts())  # Output: "Part A, Part B"
+#
+# Common Mistakes to Avoid:
+#
+# 1. Forgetting to Call `reset()`:
+#    - Mistake: Not resetting the builder before constructing a new product.
+#    - Fix: Always call `reset()` before starting a new build process.
+#
+# 2. Using Builder When Simple Instantiation Works:
+#    - Mistake: Using Builder Pattern when the product can be created easily
+#    via a constructor.
+#    - Fix: Only use Builder when constructing the product is complex.
+#
+# 3. Tightly Coupling Director to Concrete Builders:
+#    - Mistake: The Director should use the abstract `Builder` interface.
+#    - Fix: Use dependency injection to allow different builders.

python/src/creational/builder/product.py

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+class Product:
+    """
+    Product: The complex object being built.
+    """
+
+    def __init__(self):
+        self.parts = []
+
+    def add_part(self, part: str):
+        """
+        Adds a new part to the product.
+        """
+        self.parts.append(part)
+
+    def list_parts(self) -> str:
+        """
+        Lists all parts of the product.
+        """
+        return ", ".join(self.parts)

python/tests/creational/builder/test_builder.py

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+from src.creational.builder.builder import ConcreteBuilder
+from src.creational.builder.product import Product
+
+
+def test_concrete_builder_initialization():
+    """
+    Test that a ConcreteBuilder is initialized with an empty product.
+    """
+    builder = ConcreteBuilder()
+    assert isinstance(builder.get_result(), Product)
+    assert builder.get_result().list_parts() == ""
+
+
+def test_concrete_builder_build_part_a():
+    """
+    Test that ConcreteBuilder correctly adds Part A.
+    """
+    builder = ConcreteBuilder()
+    builder.build_part_a()
+    product = builder.get_result()
+    assert product.list_parts() == "Part A"
+
+
+def test_concrete_builder_build_part_b():
+    """
+    Test that ConcreteBuilder correctly adds Part B.
+    """
+    builder = ConcreteBuilder()
+    builder.build_part_b()
+    product = builder.get_result()
+    assert product.list_parts() == "Part B"
+
+
+def test_concrete_builder_reset():
+    """
+    Test that calling reset() clears the product.
+    """
+    builder = ConcreteBuilder()
+    builder.build_part_a()
+    builder.reset()
+    assert builder.get_result().list_parts() == ""

python/tests/creational/builder/test_director.py

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+from src.creational.builder.director import Director
+from src.creational.builder.builder import ConcreteBuilder
+
+
+def test_director_construct_minimal_product():
+    """
+    Test that the director correctly constructs a minimal product
+    with only Part A.
+    """
+    builder = ConcreteBuilder()
+    director = Director(builder)
+
+    director.construct_minimal_product()
+    product = builder.get_result()
+
+    assert product.list_parts() == "Part A"
+
+
+def test_director_construct_full_product():
+    """
+    Test that the director correctly constructs a full product
+    with Part A and Part B.
+    """
+    builder = ConcreteBuilder()
+    director = Director(builder)
+
+    director.construct_full_product()
+    product = builder.get_result()
+
+    assert product.list_parts() == "Part A, Part B"

python/tests/creational/builder/test_product.py

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+from src.creational.builder.product import Product
+
+
+def test_product_initialization():
+    """
+    Test that a new product is initialized with an empty list.
+    """
+    product = Product()
+    assert product.list_parts() == ""
+
+
+def test_product_add_part():
+    """
+    Test that parts can be added to the product.
+    """
+    product = Product()
+    product.add_part("Part A")
+    assert product.list_parts() == "Part A"
+
+    product.add_part("Part B")
+    assert product.list_parts() == "Part A, Part B"