Creo que deberíamos de comer mas sano CC @nehnemini

pizzas.py

#!/usr/bin/env python
#	= ^ . ^ =

import math
pi = math.pi

a = 2 * pi
print "a = 2 * pi => " , a

b = pi ** a
print "b = pi ** a => " , b

c = math.sqrt(b)
print "c = math.sqrt(b) => " , c

# https://stackoverflow.com/a/6681808
d,e = math.modf(c)
print "d,e = math.modf(c) => (" , d , e , ")"

f = e - 15
print "f = e - 15 => " , f

Si da 21, pero sobra cambio para la propina xD

$ ./pizzas.py 
a = 2 * pi =>  6.28318530718
b = pi ** a =>  1329.48908322
c = math.sqrt(b) =>  36.4621596072
d = c - 15 =>  21.4621596072

Siempre podemos "aplicar la ñera" y forzar el resultado xD

$ ./pizzas.py 
a = 2 * pi =>  6.28318530718
b = pi ** a =>  1329.48908322
c = math.sqrt(b) =>  36.4621596072
d,e = math.modf(c) => ( 0.462159607208 36.0 )
f = e - 15 =>  21.0