What is the __set_name__ method in descriptors?

Question

Accepted Answer

**__set_name__** is called when a descriptor is **defined in a class**. Receives the attribute name.

```python
class Validated:
    def __set_name__(self, owner, name):
        self.name = name
        self.private_name = f"_{name}"

def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        return getattr(obj, self.private_name, None)

def __set__(self, obj, value):
        if not isinstance(value, str):
            raise TypeError(f"{self.name} must be a string")
        setattr(obj, self.private_name, value)

class User:
    name = Validated()   # __set_name__ called with "name"
    email = Validated()  # __set_name__ called with "email"

u = User()
u.name = "Alice"   # Works
# u.name = 123     # TypeError: name must be a string

# Without __set_name__ — need manual naming
class OldValidated:
    def __init__(self, name):
        self.name = name
        self.private_name = f"_{name}"

class OldUser:
    name = OldValidated("name")  # Manual!
    email = OldValidated("email")
```

**Benefits:**
- Automatic name detection.
- Cleaner descriptor API.
- No need to pass name manually.

Python: What is the __set_name__ method in descriptors?