Should I use numpy and cython for complex numbers?

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It depends on what kind of complex number you want. Sage provide tons, you do not need numpy, cython or whatever. Here are some examples (these are not the only ones):

With

sage: z = 1+2*I
sage: z.parent()
Symbolic Ring

You indeed get symbolic representation of a complex number, so you can do symboloc things such as:

sage: exp(pi*z).simplify()
e^pi

WIth

sage: z = 1+2*CDF.0
sage: z.parent()
Complex Double Field

You get a fast numerical representation of your complex number (you should prefer CDF over CC since both have the same precision, but CDF takes the advantage of the CPU floating-point arithmetics, and the functions you will call from it will use optimized libraries).

With

sage: z = 1+2*ComplexIntervalField(1000).0
sage: z.parent()
Complex Interval Field with 1000 bits of precision

You get a certified numerical representation of your complex number with high precision (interval arithmetics).

With

sage: z = 1+2*QQbar.0
sage: z.parent()
Algebraic Field

You get an algebraic (hence exact) representation of your complex number (but pi does not exist here).

With

sage: z = 1+2*ZZ[i].1
sage: z.parent()
Gaussian Integers in Number Field in I with defining polynomial x^2 + 1

You get a Gaussian integer.

And so on...

Now, you just have to decide what do you want to do with z to select an appropriate representation.