dictionary changed size during iteration字典类型在迭代时不能改变大小的

d={
    1:{'a':123},
    2:'mnnn'
    }
for k in d:
    if not 'b' in d[k]:
        d['b'] = 12
for k in d.keys():
    if not 'b' in d[k]:
        d['b'] = 12