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/
