Interactive "_"

This is a really useful feature that surprisingly few people know.

In the interactive interpreter, whenever you evaluate an expression or call a function, the result is bound to a temporary name, _ (an underscore):

>>> 1 + 1
2
>>> _
2

_ stores the last printed expression.

When a result is None, nothing is printed, so _ doesn't change. That's convenient!

This only works in the interactive interpreter, not within a module.

It is especially useful when you're working out a problem interactively, and you want to store the result for a later step:

>>> import math
>>> math.pi / 3
1.0471975511965976
>>> angle = _
>>> math.cos(angle)
0.50000000000000011
>>> _
0.50000000000000011

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