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The notation is misleading: CC does not corresponds to genuine complex numbers (unlike for NN, ZZ, QQ or AA), but only refers to floating-point approximation of complex numbers.

In your case, if you want exact computations, you should work on the algebraic field:

sage: QQbar
Algebraic Field
sage: realpoly.<x> = PolynomialRing(QQbar)
sage: factor(x^2-2,x)
(x - 1.414213562373095?) * (x + 1.414213562373095?)

The notation is misleading: CC does not corresponds to genuine complex numbers (unlike for NN, ZZ, QQ or AA), but only refers to floating-point approximation of complex numbers.

In your case, if you want exact computations, you should work on the algebraic field:

sage: QQbar
Algebraic Field
sage: realpoly.<x> = PolynomialRing(QQbar)
sage: factor(x^2-2,x)
(x - 1.414213562373095?) * (x + 1.414213562373095?)

sage: s2 = x-factor(x^2-2,x)[0][0]
sage: s2
1.414213562373095?
sage: s2 == sqrt(QQbar(2))
True

The notation is misleading: CC does not corresponds to genuine complex numbers (unlike for NN, ZZ, QQ or AA), but only refers to floating-point approximation of complex numbers.

In your case, if you want exact computations, you should work on the algebraic field:

sage: QQbar
Algebraic Field
sage: realpoly.<x> = PolynomialRing(QQbar)
sage: factor(x^2-2,x)
(x - 1.414213562373095?) * (x + 1.414213562373095?)

This looks the same, but you should notice the question mark after 1.414213562373095, which means that 1.414213562373095? is only a representation of some algebraic number. You can check that it is really sqrt(2) as follows:

sage: s2 = x-factor(x^2-2,x)[0][0]
sage: s2
1.414213562373095?
sage: s2 == sqrt(QQbar(2))
True

sage: QQbar(s2).as_number_field_element()
(Number Field in a with defining polynomial y^2 - 2,
 a,
 Ring morphism:
  From: Number Field in a with defining polynomial y^2 - 2
  To:   Algebraic Real Field
  Defn: a |--> 1.414213562373095?)

The notation CC (as well for RR) is misleading: CC does not corresponds to genuine complex numbers (unlike for NN, ZZ, QQ or , AA, QQbar), but only refers to floating-point approximation of complex numbers.

In your case, if you want exact computations, you should work on the algebraic field:field QQbar instead of CC:

sage: QQbar
Algebraic Field
sage: realpoly.<x> = PolynomialRing(QQbar)
sage: factor(x^2-2,x)
(x - 1.414213562373095?) * (x + 1.414213562373095?)

This looks the same, but you should notice the question mark after 1.414213562373095, which means that 1.414213562373095? is only a representation of some algebraic number. You can check that it is really sqrt(2) as follows:

sage: s2 = x-factor(x^2-2,x)[0][0]
sage: s2
1.414213562373095?
sage: s2 == sqrt(QQbar(2))
True

sage: QQbar(s2).as_number_field_element()
(Number Field in a with defining polynomial y^2 - 2,
 a,
 Ring morphism:
  From: Number Field in a with defining polynomial y^2 - 2
  To:   Algebraic Real Field
  Defn: a |--> 1.414213562373095?)