This is a simple example of how to add callbacks to any Caffe solver

net.prototxt

name: "DemoNet"
layer {
  name: "data"
  type: "Input"
  top: "data"
  input_param { shape: { dim: 1 dim: 1 dim: 1 dim: 1 } }
}
layer {
  name: "labels"
  type: "Input"
  top: "labels"
  input_param { shape: { dim: 1 dim: 1 dim: 1 dim: 1 } }
}
layer {
  name: "multiply"
  type: "InnerProduct"
  bottom: "data"
  top: "output"
  inner_product_param {
    num_output: 1
  }
}
layer {
  name: "loss"
  type: "EuclideanLoss"
  bottom: "output"
  bottom: "labels"
  top: "loss"
}

solver.prototxt

net: "net.prototxt"
max_iter: 10

lr_policy: "fixed"
base_lr: 1
momentum: 0
weight_decay: 0

solver_mode: CPU

test_callbacks.py

"""
This simple example shows, how to add  callback functions to
any Caffe solver, using the Python `add_callback` function.
The `net.prototxt` and `solver.prototxt` are simply to make
this a runnable example.

>> python test_callbacks.py
    [...]
    Hello World!
    Bye Bye!
    Hello World!
    Bye Bye!
    [...]
    
This is to illustrate my stackoverflow answer to the following question:
http://stackoverflow.com/q/41490526/4221706

01/15/2017 - Hannes Badertscher    
License: CC0

"""

import caffe

def hello():
    print "Hello World!"

def bye():
    print "Bye Bye!"


solver = caffe.SGDSolver('solver.prototxt')
solver.add_callback(hello, bye)
solver.solve()