title: Edit prototxt file from python
body: <p>I would like to edit a prototxt file using python.  I simply need to change the source from one directory to another.  Is there a simple way to go about this, to not have to redefine the net and rewrite the entire file?</p>
  
  <p>For example, in my prototxt file I have</p>
  
  <pre><code>layer {
    name: "data"
    type: "Data"
    top: "data"
    top: "label"
    include {
      phase: TRAIN
    }
    transform_param {
      mirror: true
      crop_size: 227
      mean_file: "data/ilsvrc12/imagenet_mean.binaryproto"
    }
    data_param {
      source: "examples/imagenet/ilsvrc12_train_lmdb"
      batch_size: 256
      backend: LMDB
    }
  }
  </code></pre>
  
  <p>and I ONLY want to change the field 'source'.  This is from a relatively long net def (posted here is just a typical example, I have my own net def), so not having to manually enter every field like in the caffe examples would save me lots of time</p>
  
  <p>Thanks</p>