Replacements based on advanced regular expressions.\ We also use a temporary file thanks to the 'tempfile' library.

regex_replace.py

r'''
The script uses regular expressions (regex) to perform replacements on a file.
The regex replacement is applied once by line.
The search pattern is built like this:
  (designator)-(pin name)space((prefixes, optional)(component names))
Example:
  ([A-Z]+[0-9]+)-([^ ]+) ((NC_|DNP_)?(2000UF|SS2040FL|DMP1046UFDB|TMP1075DSGR))
  ^              ^       ^^          ^ match group 5
  ^              ^       ^^ match group 4
  ^              ^       ^ match group 3
  ^              ^ match group 2
  ^ match group 1

Once a matching line is found, the script checks it has a replacement recipe for
this particular pin of this particular component. If yes, the pin identifier is
replaced.

Need help on regular expressions?
https://docs.python.org/3/library/re.html
'''

import re
import tempfile, os

optionalNamePrefixes = (
  'NC_',
  'DNP_',
)

with open('file.txt', 'r') as iFile:
  with open('mappings.json', 'r') as rFile:
    replacementsList = json.load(rFile)['clientKey']
    
    with tempfile.NamedTemporaryFile(mode='w', delete=False) as oFile:
      tmpNetlistFilepath = oFile.name
      
      # build the lookup pattern (the part names we're looking after)
      # the pipe character '|' is a OR operator
      prefixes = '|'.join(re.escape(prefix) for prefix in optionalNamePrefixes)
      partNames = '|'.join(re.escape(partName) for partName in replacementsList)
      lookupPattern = r'^([A-Z]+[0-9]+)-([^ ]+) (({})?({}))'.format(prefixes, partNames)
      
      # walk through the file content
      while True:
        line = iFile.readline() 
        if not line:
          break # end of file
        
        if re.search(lookupPattern, line):
          print(f'Match found: {line.rstrip()}\n')
        
        try:
          # rewrite the line by replacing some of the matched groups
          line = re.sub(lookupPattern, lambda match: f'{match.group(1)}-{replacementsList[match.group(5)][match.group(2)]} {match.group(3)}', line)
          # inspiration taken from: https://stackoverflow.com/a/14157778
        except KeyError:
          print(f'Can\'t find a replacement instruction\n')
        finally:
          oFile.write(line)

# move the temporary file to the final location; overwrites an existing file if any
os.replace(tmpNetlistFilepath, 'newfile.txt')