Give this a try:
# get a list of all the values of fieldA
# this ONLY works for shapefiles
NextValues = [row[0] for row in arcpy.da.SeachCursor(copyFile,'FieldA')]
with arcpy.da.UpdateCursor(copyFile, ['OID@', 'FieldA', 'FieldB']) as cursor:
counter = 1
for row in cursor:
NextFID = row[0] + 1 # the index of the next value
row[2] = counter
fieldA = row[1]
# get next fieldA
# make sure this isn't the last record or there will be no next
if NextFID < len(NextValues):
if fieldA == NextValues[NextFID]:
counter += 1
else:
counter = 1
cursor.updateRow(row)
else:
# do something here for the last record
If you're using a geodatabase feature class you will need to keep a track of the index as you can't use the OID - it's not guaranteed to be 0 based, unique and contiguous - only 0 based and unique (not contiguous) so calling the next OID value may not retrieve a record:
# get a list of all the values of fieldA
NextValues = [row[0] for row in arcpy.da.SeachCursor(copyFile,'FieldA')]
with arcpy.da.UpdateCursor(copyFile, ['OID@', 'FieldA', 'FieldB']) as cursor:
counter = 1
NextRow = 1
for row in cursor:
row[2] = counter
fieldA = row[1]
# get next fieldA
# make sure this isn't the last record or there will be no next
if NextRow < len(NextValues):
if fieldA == NextValues[NextRow]:
counter += 1
else:
counter = 1
cursor.updateRow(row)
else:
# do something here for the last record
NextRow += 1
