import bpy
import bmesh


def findMinMax(listlist):
    minU=None
    minV=None
    maxU=None
    maxV = None
    for list in listlist:
        for uv in list:
            if minU is None or uv[0]>maxU:
                maxU = uv[0]
            if minU is None or uv[0]<minU:
                minU = uv[0]
            if minV is None or uv[1]>maxV:
                maxV = uv[1]
            if minV is None or uv[1]<minV:
                minV = uv[1]
    return [ minU, maxU, minV, maxV ]

class Normalizer:
    def __init__(self, min, max):
        self.b = -min;
        self.a = 1/(max-min)

    def transform(self, val):

        return (val+self.b) *self.a;

def expandUVToFit(mesh):

    bm = bmesh.from_edit_mesh(mesh)

    uvl = bm.loops.layers.uv[mesh.uv_layers.active_index]
    minMax = findMinMax( [ [ le[uvl].uv for le in face.loops if le[uvl].select] for face in bm.faces if face.select])
    print(minMax)
    xn = Normalizer(minMax[0], minMax[1])
    yn = Normalizer(minMax[2], minMax[3])

    for face in bm.faces:
        if not face.select:
            continue
        for le in face.loops:
            if not le[uvl].select:
                continue
            le[uvl].uv[0] = xn.transform(le[uvl].uv[0])
            le[uvl].uv[1] = yn.transform(le[uvl].uv[1])

    bmesh.update_edit_mesh(mesh)

obj = bpy.context.active_object
expandUVToFit(obj.data)