What is the Union and Optional types?

Question

Accepted Answer

**Union** allows a variable to hold **multiple types**. **Optional** is shorthand for Union with None.

```python
from typing import Union, Optional

# Union — multiple types
def process(value: Union[str, int]) -> str:
    if isinstance(value, int):
        return str(value)
    return value

process("hello")  # "hello"
process(42)       # "42"

# Python 3.10+ syntax
def process(value: str | int) -> str:
    return str(value)

# Optional — Union with None
def find_user(user_id: int) -> Optional[dict]:
    if user_id in users:
        return users[user_id]
    return None  # Optional allows None

# Python 3.10+ syntax
def find_user(user_id: int) -> dict | None:
    return users.get(user_id)

# Complex unions
from typing import Literal

Status = Literal["pending", "active", "deleted"]

def update_status(status: Status) -> None:
    # Only accepts literal strings
    pass

update_status("active")  # OK
# update_status("unknown")  # Error!
```

**When to use:**
- `Optional` — function may return None.
- `Union` — argument accepts multiple types.
- `Literal` — argument must be specific values.

Python: What is the Union and Optional types?