Defining characters in SymPy that contain numeric values

I am trying to define a constant in SymPy with the same functionality as constants like pi, e and i. When accurate answers are given, they remain as their symbols, and are not evaluated by their meanings. For example, if I run

2*pi

he will return 2⋅π. If I wanted a rough answer, I could run

N(2*pi)

which returns 6.28318530717959.

I want to define the constant tau, where

tau

returns τ, and

N(tau)

returns 6.28318530717959.

Basically, I'm just trying to define a new character, τ, equal to 2⋅π.