replaces whatever is in the 856 tags in the MARC record with the value provided in the CSV file after matching the 001 with the first column.

replace856.py

import csv
from pymarc import *

marcFile = 'C:\\Users\\Fenichele\\Desktop\\fasdocl.mrc'
resultsMarcFile = 'C:\\Users\\Fenichele\\Desktop\\x_fasdocl.mrc'
urlFile = 'C:\\Users\\Fenichele\\Desktop\\em.csv'


def readCSV():
    urlList = []
    with open(urlFile, 'r') as c:
        reader = csv.reader(c)
        for row in reader:
            urlList.append(row)

    return urlList

def returnURL(ocn):
    urlList = readCSV()

    url = None
    for o in urlList:
        if ocn == o[0]:
            url = o[1]

    return url


def remove856(record):
    listof856 = record.get_fields('856')
    for l in listof856:
        record.remove_field(l)
    return record

def add856(record,urlUse):
    add856 = Field(tag='856', indicators='40', subfields=['u', urlUse])
    record.add_field(add856)
    return record

def updateMarc(record):
    with open(resultsMarcFile, 'ab') as x:
        try:
            marc = record.as_marc()
            marc = marc.decode('utf-8')
            x.write(bytes(marc, 'utf-8'))
        except UnicodeEncodeError:
            ocn = record['001'].value()
            ocn = ocn.strip('ocn')
            ocn = ocn.strip('ocm')
            ocn = ocn.strip(' ')
            print("couldn't write "+ocn)

def readMarc():
    with open(marcFile, 'rb') as fh:
        reader = MARCReader(fh)

        for record in reader:
            record.force_utf8 = True
            # (print['001'], record['856'])
            ocn = record['001'].value()
            ocn = ocn.strip('ocn')
            ocn = ocn.strip('ocm')
            ocn = ocn.strip(' ')
            urlUse = returnURL(ocn)
            if urlUse is None:
                print ("no url found for ", ocn)
                return record


            record = remove856(record)

            record = add856(record, urlUse)

            updateMarc(record)

        return record

readMarc()