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Oh my god. I have used Jupyter notebooks for 7 years and never knew this. I just had to fire one up to verify -- and yep; exactly as described. How amazing! Thank you!

I believe the _5 syntax is specific to Jupyter, but in Python, the output of the last statement you execute is bound to _

For example, if you simply enter 4 into a Python REPL, printing _ will also print 4

It's really handy if you call a long statement in the REPL and forget to save the output to a variable

I wrote about it once if you'd like to see some better examples: https://utf9k.net/blog/lost-python-results/

I knew this, which may be handy when developing (assigning variables is superior IMO though)

But in final output like this? I don't like it.

No, you should not use output-references in code you share with people, but in my opinion it is one of the main reasons why jupyter notebooks in general put a large emphasis on displaying cell numbers

You just blew my mind.