Concatenate multiple string / text fields some of which may have NULL values ArcGIS

I am trying to repeat the behavior of this code from the ESRI website.

I want to replicate the output example in ALL_Type field, so that it concatenates a field but it deals (ignores) any that have NULL values.

Unfortunately this example no longer works at 10.2.2 because all example of NULL return a "None" text. Instead I just want a blank to show.

And here is the code.

   "*args" allows this routine to accept any number of field values.
# the values are passed as a Python tuple, essentially a
# non-editable list
#
def concat(*args):

  # Initialize the return value to an empty string,
  # then set the separator character

  #
  retval = ""
  sep = "_"

  # For each value passed in...
  #
  for t in args:
    # Convert to a string (this catches any numbers),
    # then remove leading and trailing blanks
    #

    s = str(t).strip()

    # Add the field value to the return value, using the separator
    # defined above
    #
    if s <> '':
      retval += sep + s

  # Strip of any leading separators before returning the value
  #

  return retval.lstrip(sep)

Answer

I think this should do what you want:

def concat(*args):
    sep = "_"

    nonnull_args = [str(arg).strip() for arg in args if arg]  # Filter NULLs
    good_args = [arg for arg in nonnull_args if arg]          # Filter blanks

    retval = sep.join(good_args)


    return retval

The args just get passed through a couple filters and joined by sep at the end.