Python typing.TypeVar详解 - 类型变量与泛型编程完全指南

from typing import TypeVar

# 无约束类型变量
T = TypeVar('T')

# 多个类型变量
K = TypeVar('K')
V = TypeVar('V')

# 带约束的类型变量
Number = TypeVar('Number', int, float)

# 带上界的类型变量
T_bound = TypeVar('T_bound', bound=BaseClass)

# 协变和逆变
T_co = TypeVar('T_co', covariant=True)
T_contra = TypeVar('T_contra', contravariant=True)

from typing import TypeVar, List

T = TypeVar('T')

def reverse_list(items: List[T]) -> List[T]:
    """反转列表，保持元素类型不变"""
    return items[::-1]

def get_middle(items: List[T]) -> T:
    """获取列表中间元素"""
    if not items:
        raise ValueError("列表为空")
    return items[len(items) // 2]

# 整数列表
nums = [1, 2, 3, 4, 5]
print(f"反转: {reverse_list(nums)}")
print(f"中间: {get_middle(nums)}")

# 字符串列表
words = ["alpha", "beta", "gamma", "delta"]
print(f"反转: {reverse_list(words)}")
print(f"中间: {get_middle(words)}")

反转: [5, 4, 3, 2, 1]
中间: 3
反转: ['delta', 'gamma', 'beta', 'alpha']
中间: beta

from typing import TypeVar, List

# 约束为 int 或 float
Number = TypeVar('Number', int, float)

def calculate_stats(values: List[Number]) -> dict:
    """计算数值列表的统计信息"""
    if not values:
        return {"count": 0}

    total: Number = 0
    for v in values:
        total += v

    return {
        "count": len(values),
        "sum": total,
        "min": min(values),
        "max": max(values),
        "avg": total / len(values)
    }

# 整数统计
int_stats = calculate_stats([85, 90, 78, 92, 88])
print("整数统计:")
for key, value in int_stats.items():
    print(f"  {key}: {value}")

# 浮点数统计
float_stats = calculate_stats([3.5, 4.0, 3.8, 4.2, 3.9])
print("\n浮点数统计:")
for key, value in float_stats.items():
    print(f"  {key}: {value}")

整数统计:
  count: 5
  sum: 433
  min: 78
  max: 92
  avg: 86.6

浮点数统计:
  count: 5
  sum: 19.4
  min: 3.5
  max: 4.2
  avg: 3.88

from typing import TypeVar, List

class Shape:
    """形状基类"""
    def area(self) -> float:
        return 0.0

    def describe(self) -> str:
        return f"{self.__class__.__name__} (面积: {self.area():.2f})"

class Circle(Shape):
    def __init__(self, radius: float) -> None:
        self.radius = radius

    def area(self) -> float:
        return 3.14159 * self.radius ** 2

class Rectangle(Shape):
    def __init__(self, width: float, height: float) -> None:
        self.width = width
        self.height = height

    def area(self) -> float:
        return self.width * self.height

class Triangle(Shape):
    def __init__(self, base: float, height: float) -> None:
        self.base = base
        self.height = height

    def area(self) -> float:
        return 0.5 * self.base * self.height

# 带上界的TypeVar：必须是Shape的子类
S = TypeVar('S', bound=Shape)

def find_largest(shapes: List[S]) -> S:
    """找到面积最大的形状"""
    if not shapes:
        raise ValueError("形状列表为空")
    return max(shapes, key=lambda s: s.area())

def print_all(shapes: List[S]) -> None:
    """打印所有形状的信息"""
    for shape in shapes:
        print(f"  {shape.describe()}")

shapes: List[Shape] = [
    Circle(5),
    Rectangle(4, 6),
    Triangle(3, 8)
]

print("所有形状:")
print_all(shapes)

largest = find_largest(shapes)
print(f"\n最大形状: {largest.describe()}")

所有形状:
  Circle (面积: 78.54)
  Rectangle (面积: 24.00)
  Triangle (面积: 12.00)

最大形状: Circle (面积: 78.54)