Or you can write it all out on multiple lines

    if cond1:
        if cond2:
            x = 12
        else:
            x = 11
    else:
        x = 13

That way you can debug it more easily (because you can trace execution rather than printing out the values that cond1 and cond2 depend on and figuring out in your head which way the ternary will go).

This also removes ambiguity in the case where you might want to assign to x some falsey thing like None or [] or zero:

    >>> y = None
    >>> x = False or ( y or False )
    >>> x
    False

The above code depends on the value of y for control flow. Clearer is to use syntax for control flow and not abuse falsey-ness or truthiness.

    >>> print [bool(f) for f in [None, 0, 0.0, [], ()]]
    [False, False, False, False, False]

If you really do want to test the falsey behavior, IMHO it's much, much clearer to branch on bool(the_maybe_falsey_thing) rather than just the_maybe_falsey_thing.