What is the typing.TypeGuard?

Question

Accepted Answer

**TypeGuard** narrows types in **type checking** with custom functions.

```python
from typing import TypeGuard

def is_string_list(val: list[any]) -> TypeGuard[list[str]]:
    return all(isinstance(x, str) for x in val)

def process(data: list[any]):
    if is_string_list(data):
        # Type checker knows data is list[str] here
        print(" ".join(data))  # No warning!
    else:
        # data is still list[any]
        print(data)

# With Pydantic
from pydantic import BaseModel

class Success(BaseModel):
    status: str = "ok"
    data: dict

class Error(BaseModel):
    status: str = "error"
    message: str

def is_success(response: Success | Error) -> TypeGuard[Success]:
    return response.status == "ok"

def handle_response(response: Success | Error):
    if is_success(response):
        print(response.data)  # Type checker knows it's Success
    else:
        print(response.message)  # Type checker knows it's Error
```

**Benefits:**
- Custom type narrowing.
- Better type safety.
- Clear code intent.

Python: What is the typing.TypeGuard?