Two important Python notes: 1. `is` really shouldn't be used for string comparison. It checks object identities rather than string equality, which is broken he

Two important Python notes: 1. `is` really shouldn't be used for string comparison. It checks object identities rather than string equality, which is broken here for two reasons: first, because it depends on the interpreter to keep one true copy of `''` (which CPython seems to do, but doesn't have to by the spec), and, second, it doesn't even work consistently in CPython anyway if `expr` is a Unicode string, since `'' is not u''`. (Try replacing `expr` with `u'28+32+++32++39'` and you'll get an error.) 2. If an object is known to be a string, test non-emptiness by testing the string itself: `bool('') == False`, whereas `bool('any other string') == True`. It's more concise and is equally clear to someone comfortable with Python. I agree that the given example of functional programming is pretty ugly (though it might've been chosen just to illustrate the author's specific points), but I think the version below is really quite nice and won't break on the string comparison. expr = '28+32+++32++39' print sum(int(i) for i in expr.split('+') if i)