type — Intermediate Examples

Declares a type alias (3.12+)

EasyIntermediateAdvancedExpert

Generic type aliases

Creating parameterized type aliases.

python
from typing import TypeVar, Generic, TypeAlias

T = TypeVar("T")

# Type aliases with generics
Matrix: TypeAlias = list[list[float]]
Result: TypeAlias = tuple[bool, str]

def transpose(m: Matrix) -> Matrix:
    return [list(row) for row in zip(*m)]

m = [[1, 2, 3], [4, 5, 6]]
print(f"Original: {m}")
print(f"Transposed: {transpose(m)}")

# Callable type alias
from typing import Callable
Predicate: TypeAlias = Callable[[int], bool]

def filter_with(data: list[int], pred: Predicate) -> list[int]:
    return [x for x in data if pred(x)]

evens = filter_with([1,2,3,4,5], lambda x: x % 2 == 0)
print(f"Evens: {evens}")

# Type checking at runtime
print(f"\nMatrix is: {Matrix}")
print(f"Result is: {Result}")

Type aliases give readable names to complex types. They're especially useful for callbacks (Callable), nested generics, and domain-specific types.

type() for dynamic class creation

Using type() as a class factory.

python
# type(name, bases, dict) creates a new class
Dog = type("Dog", (), {
    "__init__": lambda self, name: setattr(self, "name", name),
    "bark": lambda self: f"{self.name} says Woof!",
    "species": "Canis familiaris",
})

d = Dog("Rex")
print(d.bark())
print(f"Species: {d.species}")
print(f"Type: {type(d)}")

# Dynamic class with inheritance
Animal = type("Animal", (), {
    "breathe": lambda self: "breathing..."
})

Cat = type("Cat", (Animal,), {
    "__init__": lambda self, name: setattr(self, "name", name),
    "meow": lambda self: f"{self.name} says Meow!",
})

c = Cat("Whiskers")
print(c.meow())
print(c.breathe())
print(f"Is Animal: {isinstance(c, Animal)}")
Expected Output
Rex says Woof!
Species: Canis familiaris
Type: <class '__main__.Dog'>
Whiskers says Meow!
breathing...
Is Animal: True

type() with three arguments creates a class dynamically. This is what Python does behind the scenes for every class statement.

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