To add more options, I would go with `kwargs.popitem()[0]`.

Also, the default Python error message for this is slightly different, and even if we don't adhere strictly to it, mentioning the function that fails is probably a good idea:

```
>>> def f(a=5):
...     pass
...
>>> f(b=6)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() got an unexpected keyword argument 'b'
```
Lastly, I would use the `.format` method to create the error message, but that's probably just me...