Python interpreter: Underscore

I noticed that I used this trick in a previous post, and I feel like I should post about it since it's so unspoken of, and useful at the same time.
When in a CPython interpreter session (your plain old python session), the value given to "_" is always the last value automatically printed by the interpreter as the result of an expression.

    >>> 1+1
    2
    >>> _
    2
    >>> x = 3+5
    >>> _
    2
    >>> print "another value"
    another value
    >>> _
    2

As you can see, "_" is always the value that the interpreter automatically prints. If you use variable assignment, you can prevent the shortcut from gaining a value when you want to preserve it for later.
This can be very useful when you don't want to type again the full expression you used to obtain a certain value when you want to perform operations on such value. For example, in a Django shell, performing regex searches, testing generators and lists( list(_) ), etc.
Of course you can always use the Up arrow on your keyboard to repeat the last line of code and then edit it, if your shell supports history. But sometimes you really can't, and it's more practical to do it this way.

    >>> [1,2,3] + [4,5,6]
    [1, 2, 3, 4, 5, 6]
    >>> len([1,2,3] + [4,5,6])
    6
    >>> [1,2,3] + [4,5,6]
    [1, 2, 3, 4, 5, 6]
    >>> len(_)
    6

But don't worry about using "_" as a variable name inside the interpreter shell. It will simply override the behaviour described above.

    >>> _=100
    >>> _
    100
    >>> 4
    4
    >>> _
    100
    >>>

Trivia: the "underscore shortcut" doesn't show up in locals() or dir()

    >>> locals()
    {'__builtins__': , '__name__': '__main__', '__d
    oc__': None, '__package__': None}
    >>> dir()
    ['__builtins__', '__doc__', '__name__', '__package__']
    >>>