Previously defined variables like _3 are silently overwritten

[phaunt]

I was working on a little program in the IPython shell (invoked like ipython3 -i example.py) and got stumped for a while when a variable named _3 defined in example.py produced unexpected behaviour, but not immediately.

It took a while before I noticed that this problem didn't occur in the regular python shell, and even then the penny didn't drop immediately, because I wasn't yet familiar with the _n automatic variables for the output cache – I freely admit I'm still something of an IPython newbie.

Possible suggestions to prevent this kind of surprise:

• present the user with a warning when assigning to a variable that may be silently overwritten in the future
• or even an info or debug notice instead of a warning (e.g. when the user passes the --debug command-line flag to ipython)
• or perhaps present this warning at the point that a previously assigned variable gets overwritten
• or more fancily: make it configurable whether automatic variables are used at all (for the case of _3, setting --cache-size=0 would have worked, but this still doesn't prevent the likes of _i3 getting overwritten)
• or yet more fancily: make it configurable whether automatic variables are allowed to overwrite previously set values

Finally, I suggest mentioning _4 in the %quickref, like _i4 already is. It's a strange omission at this point. For symmetry,

_oh              : Output history

could be replaced by

_4, _oh[2:5]     : Output history line 4, lines 2-4