Invokes the Protocol Compiler to generate a _pb2.py from the given
  .proto file.  Does nothing if the output already exists and is newer than
  the input.

def generate_proto(source, require = True):
  """Invokes the Protocol Compiler to generate a _pb2.py from the given
  .proto file.  Does nothing if the output already exists and is newer than
  the input."""

  if not require and not os.path.exists(source):
    return

  output = source.replace(".proto", "_pb2.py").replace("../src/", "")

  if (not os.path.exists(output) or
      (os.path.exists(source) and
       os.path.getmtime(source) > os.path.getmtime(output))):
    print("Generating %s..." % output)

    if not os.path.exists(source):
      sys.stderr.write("Can't find required file: %s\n" % source)
      sys.exit(-1)

    if protoc is None:
      sys.stderr.write(
          "protoc is not installed nor found in ../src.  Please compile it "
          "or install the binary package.\n")
      sys.exit(-1)

    protoc_command = [ protoc, "-I../src", "-I.", "--python_out=.", source ]
    if subprocess.call(protoc_command) != 0:
      sys.exit(-1)