recursion in sys.displayhook in Python

Question

I was checking the sys.displayhook()'s document, there is a pseudo-code there:

def displayhook(value):
    if value is None:
        return

    # Set '_' to None to avoid recursion
    builtins._ = None

    text = repr(value)
    try:
        sys.stdout.write(text)
    except UnicodeEncodeError:
        bytes = text.encode(sys.stdout.encoding, 'backslashreplace')
        if hasattr(sys.stdout, 'buffer'):
            sys.stdout.buffer.write(bytes)
        else:
            text = bytes.decode(sys.stdout.encoding, 'strict')
            sys.stdout.write(text)
    sys.stdout.write("\n")
    builtins._ = value

I understood how it works but there is a comment there that is ambiguous for me. It says # Set '_' to None to avoid recursion. After I delete builtins._ = None line, I didn't see any difference:

>>> import sys, builtins
>>> 
>>> def displayhook(value):
...     print("displayhook called")
...     if value is None:
...         return
...     text = repr(value)
...     try:
...         sys.stdout.write(text)
...     except UnicodeEncodeError:
...         bytes = text.encode(sys.stdout.encoding, 'backslashreplace')
...         if hasattr(sys.stdout, 'buffer'):
...             sys.stdout.buffer.write(bytes)
...         else:
...             text = bytes.decode(sys.stdout.encoding, 'strict')
...             sys.stdout.write(text)
...     sys.stdout.write("\n")
...     builtins._ = value
... 
>>> sys.displayhook = displayhook
>>>
>>>
>>> a = 10
>>> a
displayhook called
10
>>> _
displayhook called
10
>>> builtins._
displayhook called
10
>>> b = None
>>> b
displayhook called
>>> _
displayhook called
10
>>> repr(a)
displayhook called
'10'

I was not be able to reproduce recursion and I have no idea about what recursion is it talking about. Could you tell me in which situation does the recursion occur?