Unlock normals on a mesh in Maya, but detect and preserve soft edges.

unlock_soft_edge_normals.py

import pymel.core as pm

def is_soft_edge(edge):
    vtx_faces = pm.polyListComponentConversion(edge, fromEdge=True, toVertexFace=True)
    expanded_vtx_faces = pm.filterExpand(vtx_faces, expand=True, selectionMask=70)
    
    vtx_faces_by_vtx = dict()
    for vtx_face in expanded_vtx_faces:
        vtx = pm.polyListComponentConversion(vtx_face, fromVertexFace=True, toVertex=True)
        vtx_str = str(vtx)
        if vtx_str not in vtx_faces_by_vtx:
            vtx_faces_by_vtx[vtx_str] = []
        vtx_faces_by_vtx[vtx_str].append(vtx_face)
    
    for vtx, vtx_faces in vtx_faces_by_vtx.items():
        first_normal = None
        for vtx_face in vtx_faces:
            normal = pm.polyNormalPerVertex(vtx_face, query=True, normalXYZ=True)
            if first_normal is None:
                first_normal = normal
            elif normal != first_normal:
                return False
    return True


def unlock_normals_keep_soft_edges(shape):
    soft_edges = [edge for edge in shape.edges if is_soft_edge(edge)]
    pm.polyNormalPerVertex(shape, unFreezeNormal=True)
    pm.polySoftEdge(soft_edges, angle=180)


# run unlock for all selected nodes
for s in pm.selected():
    unlock_normals_keep_soft_edges(s.getShape())