import bpy
import random


def main(context,group):
    for v in bpy.context.active_object.data.vertices:
        for g in v.groups:
            if g.group == group.index:
                g.weight = random.random()


class RandomVertexWeight(bpy.types.Operator):
    '''Tooltip'''
    bl_idname = "object.random_vertex_weight"
    bl_label = "Randomize Vertex Weights"
    bl_options = {'REGISTER', 'UNDO'}
    
    GroupName = bpy.props.StringProperty(
            name="Group Name",
            description="name of group to randomize",
            default="Group",
            )

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def execute(self, context):
        group = context.active_object.vertex_groups[self.GroupName]
        main(context,group)
        return {'FINISHED'}

def menu_func(self, context):
    self.layout.operator(RandomVertexWeight.bl_idname)

def register():
    bpy.utils.register_class(RandomVertexWeight)


def unregister():
    bpy.utils.unregister_class(RandomVertexWeight)


if __name__ == "__main__":
    register()