如何使用Type Hint确保两个变量始终为相同类型并让Mypy在类型不匹配时抛出错误？

The problem with your original code is that Union[int, str] lets each parameter independently be either type—Mypy has no way of knowing they need to match. To fix this, you’ll want to use a bound TypeVar to tie both parameters to the same type.

Here’s the corrected code that triggers a Mypy error when parameter types don’t match:

from typing import TypeVar

# Define a TypeVar restricted to only int or str
T = TypeVar("T", int, str)

def add(a: T, b: T) -> str:
    return str(a) + str(b)

# Valid cases (no Mypy errors)
a: int = 1
b: int = 2
out = add(a, b)

a: str = 'one'
b: str = 'two'
out = add(a, b)

# Invalid case (Mypy will throw an error)
a: int = 1
b: str = 'two'
out = add(a, b)  # Error: Argument 2 has incompatible type "str"; expected "int"

How this works: #

• TypeVar("T", int, str) creates a type variable that can only resolve to either int or str.
• By annotating both a and b with T, you’re telling Mypy that both parameters must be the same type—either both int or both str.
• When you mix types (like int and str), Mypy detects the mismatch and throws a clear error.

Bonus: Flexible unbound TypeVar #

If you want the function to accept any type as long as both parameters match (not just int/str), you can define an unbound TypeVar instead:

from typing import TypeVar

T = TypeVar("T")

def add(a: T, b: T) -> str:
    return str(a) + str(b)

# This will also work (both floats, no error)
x: float = 3.14
y: float = 2.71
out = add(x, y)

# Still errors if types don't match
x: float = 3.14
y: str = "pi"
out = add(x, y)  # Mypy error

Just make sure your Mypy setup has strict type checking enabled (e.g., set strict = true in your mypy.ini or run mypy --strict your_file.py) to ensure all type violations are caught.

内容的提问来源于stack exchange，提问作者Jingnan Jia