Python function for calculating a planar inner angle of three points

inner_angle_plane.py

def inner_angle_plane(point1,point2,point3):
	"""Given three points,                     p1
		calculate the smaller of the two angles   \    a
		formed by the sequence.                    \  
		Returns degrees.                            p2------p3
    Input is three (x,y) tuples
	"""
	# be sure there IS an angle to measure
	assert point1 != point2 and point2 != point3
	# get coordinate values
	p1x,p1y = point1
	p2x,p2y = point2
	p3x,p3y = point3
	# the rounding is for rare precision issues with acos() at ~ 180 degrees
	radians = math.acos( round( 
		(
			( (p2x-p1x)**2 + (p2y-p1y)**2 ) +
			( (p2x-p3x)**2 + (p2y-p3y)**2 ) -
			( (p3x-p1x)**2 + (p3y-p1y)**2 )
		) /
		math.sqrt(
			4 * 
			( (p2x-p1x)**2 + (p2y-p1y)**2 ) *
			( (p2x-p3x)**2 + (p2y-p3y)**2 )
		)
	, 10 ) )
	degrees = math.degrees(radians)
	assert degrees <= 180
	return degrees

Well, my image works with 3-space tabs but not here :-/