What is the typing.Final type?

Question

Accepted Answer

**Final** indicates a variable **cannot be reassigned** (constant).

```python
from typing import Final

PI: Final = 3.14159
MAX_SIZE: Final = 1000

# PI = 3.14  # Type error: cannot reassign

CLASS_CONSTANT: Final[str] = "constant"

# Final with class attributes
class Config:
    API_VERSION: Final = "v1"
    MAX_RETRIES: Final = 3

Config.API_VERSION  # OK to read
# Config.API_VERSION = "v2"  # Type error!

# Final in function
def get_config():
    DEBUG: Final = True  # Local constant
    return DEBUG

# Final with Literal
from typing import Literal

STATUS_ACTIVE: Final = "active"
```

**Final vs ClassVar:**
- `Final`: cannot reassign.
- `ClassVar`: class-level, not instance attribute.
- Can combine: `ClassVar[Final[int]] = 42`.

**Runtime:** `Final` is ignored — no enforcement.

Python: What is the typing.Final type?