core-xy with stepper truncate

#from sys import argv

#functions
def deltaX(a,b):
    return .5*(a+b)
def deltaY(a,b):
    return .5*(a-b)
def deltaA(x,y):
    return x+y
def deltaB(x,y):
    return x-y
def trunc(f,n):
    '''Truncates/pads a float f to n decimal places without rounding'''
    slen = len('%.*f' % (n, f))
    return str(f)[:slen]
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#constants
diametre = int(raw_input('What is the pully diametre? '))
steps = int(raw_input('How many steps (including micro)? '))
pi = 3.14159
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#math
circ = diametre*pi
steps_per_deg = steps/360
mm_per_deg = circ/360
mm_step = mm_per_deg/steps_per_deg
step_mm = steps_per_deg/mm_per_deg
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a = int(raw_input('What is new position Delta A? '))
b = int(raw_input('What is new position Delta B? '))
#math with functions
pos_x = deltaX(a,b)
pos_y = deltaY(a,b)
pos_a = deltaA(pos_x,pos_y)
pos_b = deltaB(pos_x,pos_y)
motora = step_mm * pos_a
print("""
New Position dX = %s
New Position dY = %s
New Position dA = %s
Motor moves %s steps
New Position dB = %s
""" % (pos_x,pos_y,pos_a,trunc(motora,0),pos_b))