Snippet to truncate feature class field to new length

snippet-fieldtruncate.py

import arcpy

# get parameters
fc = "c:/geodatabase.gdb/featureclass"  ## feature class
field = "field"                         ## name of field to truncate
newLength = 100                         ## new length: whatever is needed

# create new name, length, expression to truncate existing data
newField = field + "tmp"
newLengthSlice = newLength - 1

# the expression slices up to the new field length
# uses string formatting to insert field name and new length
exp = "!{0}![0:{1}]".format(field, newLengthSlice)

# get list of field names using list comprehension
fieldNames = [f.name for f in arcpy.ListFields(fc)]

# if the "new" field already exists, delete it
if newField in fieldNames:
    arcpy.DeleteField_management(fc, newField)

# add new field specifying target length
arcpy.AddField_management(fc,newField,"TEXT","","",newLength)

# calculate the new field using the expression which brings in only
# the left-most characters up to the character limit you specified (the new length)
arcpy.CalculateField_management(fc,newField,exp,"PYTHON_9.3")

# delete the old, too-big field
arcpy.DeleteField_management(fc,field)

# add a new field, named the same as the old field, with the new length
arcpy.AddField_management(fc,field,"TEXT","","",newLength)

# create second expression to populate the new/old field
exp2 = "!{0}!".format(newField)

# calculate field using second expression
arcpy.CalculateField_management(fc,field,exp2,"PYTHON_9.3")

# delete the field you used to make the transfer
arcpy.DeleteField_management(fc,newField)