Arcpy: programmatically adding labels to layers using fields values

ApLabels.py

# Import arcpy
import arcpy as ap

# Load the reference to the current map document.
# Caveat: not sure it will work if the toolbox is running in background.
# This is because background execution is performed within a separate
# kernel.
mxd = ap.mapping.MapDocument("CURRENT")  # (mxd_path)

# Get list of dataframes. There might be more than one dataframe in a map
# document. You can use the wildcard to specify which one is of interest.
# Here we are "blindly" extracting the first element in the returned list
dfs = ap.mapping.ListDataFrames(mxd, "")  # (map_document, {wildcard})
df = dfs[0]

# Get list of layers. Once again there might be more than one layer within
# the selected dataframe. Wildcard can be used to specify the subset or
# specific one. Once again we are "blindly" using the first one. 
lyrs = ap.mapping.ListLayers(mxd, "", df)  # (map_document_or_layer, {wildcard}, {data_frame})
lyr = lyrs[0]

# Check if the layer support label classes
if lyr.supports("LABELCLASSES"):
    # Get list of labelClasses. There might be more than one label class. 
    # We are selecting/defining the first.
    lcs = lyr.labelClasses
    lc = lcs[0]
    
    # Define the expression to use. Here we are using the attributes
    # [HEIGHT] and [VEL] from the display data table.
    # Notes:
    # - switching ' with " didn't work
    # - inserting escaped characters such as '\n' did not work
    lc.expression = '[HEIGHT] + "m, " + [VEL] + "mm/yr"'
    
    # Turn the labels on for the entire layer.
    lyr.showLabels = True
    
    # Refresh the current view
    ap.RefreshActiveView()