如何提取abaqus中的最大位移值？

Question

square_odb_Path='squaremodalanalysis.odb'
squareodb=session.openodb(name=square_odb_Path)
max_displacement=0
for x in range(len(squareodb.steps['Loading Step'].frames[-1]\.fieldoutputs['U'].values[0].instance.nodes)):

第4行出错！