Python: Refactoring to Patterns

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This concise guide maps Python code smells to their corresponding design pattern solutions.

<code class="language-python">class CodeSmellSolutions:
    DUPLICATED_CODE = [
        "form_template_method",
        "introduce_polymorphic_creation_with_factory_method",
        "chain_constructors",
        "replace_one__many_distinctions_with_composite",
        "extract_composite",
        "unify_interfaces_with_adapter",
        "introduce_null_object",
    ]
    LONG_METHOD = [
        "compose_method",
        "move_accumulation_to_collecting_parameter",
        "replace_conditional_dispatcher_with_command",
        "move_accumulation_to_visitor",
        "replace_conditional_logic_with_strategy",
    ]
    CONDITIONAL_COMPLEXITY = [  # Complex conditional logic
        "replace_conditional_logic_with_strategy",
        "move_emblishment_to_decorator",
        "replace_state_altering_conditionals_with_state",
        "introduce_null_object",
    ]
    PRIMITIVE_OBSESSION = [
        "replace_type_code_with_class",
        "replace_state_altering_conditionals_with_state",
        "replace_conditional_logic_with_strategy",
        "replace_implict_tree_with_composite",
        "replace_implicit_language_with_interpreter",
        "move_emblishment_to_decorator",
        "encapsulate_composite_with_builder",
    ]
    INDECENT_EXPOSURE = [  # Lack of information hiding
        "encapsulate_classes_with_factory"
    ]
    SOLUTION_SPRAWL = [  # Scattered logic/responsibility
        "move_creation_knowledge_to_factory"
    ]
    ALTERNATIVE_CLASSES_WITH_DIFFERENT_INTERFACES = [  # Similar classes, different interfaces
        "unify_interfaces_with_adapter"
    ]
    LAZY_CLASS = [  # Insufficient functionality
        "inline_singleton"
    ]
    LARGE_CLASS = [
        "replace_conditional_dispatcher_with_command",
        "replace_state_altering_conditionals_with_state",
        "replace_implict_tree_with_composite",
    ]
    SWITCH_STATEMENTS = [  # Complex switch statements
        "replace_conditional_dispatcher_with_command",
        "move_accumulation_to_visitor",
    ]
    COMBINATION_EXPLOSION = [  # Similar code for varying data
        "replace_implicit_language_with_interpreter"
    ]
    ODDBALL_SOLUTIONS = [  # Multiple solutions for same problem
        "unify_interfaces_with_adapter"
    ]</code>

Refactoring Examples in Python

This project translates refactoring examples from Refactoring to Patterns (Joshua Kerievsky) into Python. Each example shows original and refactored code, highlighting improvements. The refactoring process involved interpreting UML diagrams and adapting Java code to Python's nuances (handling cyclic imports and interfaces).

Example: Compose Method

The "Compose Method" refactoring simplifies complex code by extracting smaller, more meaningful methods.

<code class="language-python"># Original (complex) code
def add(element):
    readonly = False
    size = 0
    elements = []
    if not readonly:
        new_size = size + 1
        if new_size > len(elements):
            new_elements = []
            for i in range(size):
                new_elements[i] = elements[i]  # Potential IndexError
            elements = new_elements
        size += 1
        elements[size] = element # Potential IndexError

# Refactored (simplified) code
def is_at_capacity(new_size, elements):
    return new_size > len(elements)

def grow_array(size, elements):
    new_elements = [elements[i] for i in range(size)] # List comprehension for clarity
    return new_elements

def add_element(elements, element, size):
    elements.append(element) # More Pythonic approach
    return len(elements) -1

def add_refactored(element):
    readonly = False
    if readonly:
        return
    size = len(elements)
    new_size = size + 1
    if is_at_capacity(new_size, elements):
        elements = grow_array(size, elements)
    size = add_element(elements, element, size)

</code>

Example: Polymorphism (Test Automation)

This example demonstrates polymorphism in test automation, abstracting test setup for reusability.

<code class="language-python"># Original code (duplicate setup)
class TestCase:
    pass

class DOMBuilder:
    def __init__(self, orders): pass
    def calc(self): return 42

class XMLBuilder:
    def __init__(self, orders): pass
    def calc(self): return 42

class DOMTest(TestCase):
    def run_dom_test(self):
        expected = 42
        builder = DOMBuilder("orders")
        assert builder.calc() == expected

class XMLTest(TestCase):
    def run_xml_test(self):
        expected = 42
        builder = XMLBuilder("orders")
        assert builder.calc() == expected

# Refactored code (polymorphic setup)
class OutputBuilder:
    def calc(self): raise NotImplementedError

class DOMBuilderRefac(OutputBuilder):
    def calc(self): return 42

class XMLBuilderRefac(OutputBuilder):
    def calc(self): return 42

class TestCaseRefac:
    def create_builder(self): raise NotImplementedError
    def run_test(self):
        expected = 42
        builder = self.create_builder()
        assert builder.calc() == expected

class DOMTestRefac(TestCaseRefac):
    def create_builder(self): return DOMBuilderRefac()

class XMLTestRefac(TestCaseRefac):
    def create_builder(self): return XMLBuilderRefac()
</code>

Example: Visitor Pattern

The Visitor pattern decouples classes from their methods.

<code class="language-python"># Original code (conditional logic in TextExtractor)
class Node: pass
class LinkTag(Node): pass
class Tag(Node): pass
class StringNode(Node): pass

class TextExtractor:
    def extract_text(self, nodes):
        result = []
        for node in nodes:
            if isinstance(node, StringNode): result.append("string")
            elif isinstance(node, LinkTag): result.append("linktag")
            elif isinstance(node, Tag): result.append("tag")
            else: result.append("other")
        return result

# Refactored code (using Visitor)
class NodeVisitor:
    def visit_link_tag(self, node): return "linktag"
    def visit_tag(self, node): return "tag"
    def visit_string_node(self, node): return "string"

class Node:
    def accept(self, visitor): pass

class LinkTagRefac(Node):
    def accept(self, visitor): return visitor.visit_link_tag(self)

class TagRefac(Node):
    def accept(self, visitor): return visitor.visit_tag(self)

class StringNodeRefac(Node):
    def accept(self, visitor): return visitor.visit_string_node(self)

class TextExtractorVisitor(NodeVisitor):
    def extract_text(self, nodes):
        result = [node.accept(self) for node in nodes]
        return result
</code>

Conclusion

This practical, hands-on approach to learning design patterns through refactoring significantly enhances understanding. The challenges encountered while translating the code solidify theoretical knowledge.

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