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Infinity is not signed There also +Infinity and -Infinity .

example :

sage: limit(1/x, x=0)
Infinity
sage: limit(1/x, x=0, dir='+')
+Infinity
sage: limit(1/x, x=0, dir='-')
-Infinity

Look at here : http://www.sagemath.org/doc/reference/calculus/sage/calculus/calculus.html