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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def export_csv(obj, file_name, point_type='evalpts', **kwargs):
 """ Exports control points or evaluated points as a CSV file. :param obj: a spline geometry object :type obj: abstract.SplineGeometry :param file_name: output file name :type file_name: str :param point_type: ``ctrlpts`` for control points or ``evalpts`` for evaluated points :type point_type: str :raises GeomdlException: an error occurred writing the file """

    if not 0 < obj.pdimension < 3:
        raise exch.GeomdlException("Input object should be a curve or a surface")

    # Pick correct points from the object
    if point_type == 'ctrlpts':
        points = obj.ctrlptsw if obj.rational else obj.ctrlpts
    elif point_type == 'evalpts':
        points = obj.evalpts
    else:
        raise exch.GeomdlException("Please choose a valid point type option. Possible types: ctrlpts, evalpts")

    # Prepare CSV header
    dim = len(points[0])
    line = "dim "
    for i in range(dim-1):
        line += str(i + 1) + ", dim "
    line += str(dim) + "\n"

    # Prepare values
    for pt in points:
        line += ",".join([str(p) for p in pt]) + "\n"

    # Write to file
    return exch.write_file(file_name, line)