Hi,
Well, I’m not sure to understand what you want. If you just want to apply a curve as bevel of an other curve, you just have to modify the property bevel_object of the guide curve. For example, the following code should work :
# Adding the guide
bpy.ops.curve.primitive_bezier_curve_add()
# Adding the bevel
bpy.ops.curve.primitive_bezier_circle_add()
# Setting the circle as the guide's bevel
bpy.data.objects['BezierCurve'].data.bevel_object=bpy.data.objects['BezierCircle']
bpy.data.objects['BezierCurve'].data.splines[0].bezier_points[0].co.x
bpy.data.objects['BezierCurve'].data.splines[0].bezier_points[0].co.y
bpy.data.objects['BezierCurve'].data.splines[0].bezier_points[0].co.z
# To set the tangent, you use :
bpy.data.objects['BezierCurve'].data.splines[0].bezier_points[0].handle_left
bpy.data.objects['BezierCurve'].data.splines[0].bezier_points[0].handle_right
# those are vectors too, so you should assign to it a three numbers list or add .x.y or .z at the end