why does TemporaryDirectory change type within a "with" block?

In python3 console

>>> x1=tempfile.TemporaryDirectory()>>> print(type(x1))<class 'tempfile.TemporaryDirectory'>>>> with tempfile.TemporaryDirectory() as x2:...   print(type(x2))... <class 'str'>

Why is x1 a TemporaryDirectory and x2 a str?