Python Factory Method: Create Shapes by Type String

A factory method that maps a type string to a concrete shape class and returns an instance, with runtime error handling.

Easy Python 3.9+ Aug 9, 2026 OOP & classes 12 views 0 copies

factory-pattern oop polymorphism design-patterns shapes

Python code

43 lines

Python 3.9+

class Shape:
    def draw(self):
        raise NotImplementedError


class Circle(Shape):
    def draw(self):
        return "Drawing a circle"


class Square(Shape):
    def draw(self):
        return "Drawing a square"


class Triangle(Shape):
    def draw(self):
        return "Drawing a triangle"


class ShapeFactory:
    @staticmethod
    def create(shape_type):
        shape_type = shape_type.strip().lower()
        shapes = {
            "circle": Circle,
            "square": Square,
            "triangle": Triangle,
        }
        try:
            return shapes[shape_type]()
        except KeyError:
            raise ValueError(f"Unknown shape type: {shape_type}")


if __name__ == "__main__":
    factory = ShapeFactory()
    for name in ["circle", "square", "triangle", "hexagon"]:
        try:
            shape = factory.create(name)
            print(shape.draw())
        except ValueError as e:
            print(e)

Output

stdout

Drawing a circle
Drawing a square
Drawing a triangle
Unknown shape type: hexagon

How it works

The factory pattern centralizes object creation logic in one place. ShapeFactory.create normalizes the input by stripping whitespace and lowercasing, then looks up the class in a dictionary. This avoids long if-elif chains and makes adding new shapes as simple as adding a new class and a dictionary entry. Raising a ValueError for unknown types gives clear feedback at runtime, and the NotImplementedError in the base Shape enforces that subclasses implement draw.

Common mistakes

Forgetting to normalize input (stem casing or whitespace) leading to KeyError
Raising KeyError instead of converting it to a more descriptive exception

Variations

Use a class-level registry dict and `__init_subclass__` to auto-register shapes.
Create a module-level function `create_shape(shape_type)` instead of a static method.

Real-world use cases

Game engines instantiate enemies or items based on a string from level data.
Serialization libraries deserialize different object types from JSON type tags.
Plugin systems load modules by name from configuration files at runtime.

Python Factory Method: Create Shapes by Type String

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from OOP & classes

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Tutorials

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