What are type aliases and NewType?

Question

Accepted Answer

**Type aliases** create readable names for complex types. **NewType** creates distinct types.

```python
from typing import TypeAlias, NewType

# TypeAlias — readable name
UserID: TypeAlias = int
JSON: TypeAlias = dict[str, any]
Handler: TypeAlias = callable[[str, int], bool]

def get_user(user_id: UserID) -> JSON:
    return {"id": user_id}

# NewType — distinct type (checked at compile time)
UserID = NewType("UserID", int)
OrderID = NewType("OrderID", int)

def get_user(user_id: UserID) -> dict:
    return {"id": user_id}

def get_order(order_id: OrderID) -> dict:
    return {"id": order_id}

user_id = UserID(123)
order_id = OrderID(456)

get_user(user_id)    # OK
get_user(order_id)   # Error! Wrong type
get_user(123)        # Error! Plain int

# Runtime: NewType is just identity function
print(UserID(123))  # 123
```

**When to use:**
- `TypeAlias` — complex types that need a name.
- `NewType` — prevent mixing semantically different types.

Python: What are type aliases and NewType?