Reentrant Context Manager
Reentrant context managers can not only be used in multiple with statements,
but may also be used inside a with statement that is already using the same context manager.
Such as threading.RLock，contextlib.suppres()，contextlib.redirect_stdout().
Here’s a very simple example of reentrant use:
>>> from contextlib import redirect_stdout
>>> from io import StringIO
>>> stream = StringIO()
>>> write_to_stream = redirect_stdout(stream)
>>> with write_to_stream:
... print("This is written to the stream rather than stdout")
... with write_to_stream:
... print("This is also written to the stream")
...
>>> print("This is written directly to stdout")
This is written directly to stdout
>>> print(stream.getvalue())
This is written to the stream rather than stdout
This is also written to the stream
Note: Being reentrant is not the same thing as being thread safe.