Replace multiple regex string matches in a file

Question

I am trying to replace multiple strings in a file. But in the following code, only my last key value gets replaced. How can I replace all the key,value in the file?

fp1 = open(final,"w")
data = open(initial).read()
for key, value in mydict.items():
    fp1.write(re.sub(key,value, data)
fp1.close()

Answer 1

This is one task for which regular expressions can really help:

import re

def replacemany(adict, astring):
  pat = '|'.join(re.escape(s) for s in adict)
  there = re.compile(pat)
  def onerepl(mo): return adict[mo.group()]
  return there.sub(onerepl, astring)

if __name__ == '__main__':
  d = {'k1': 'zap', 'k2': 'flup'}
  print replacemany(d, 'a k1, a k2 and one more k1')

Run as the main script, this prints a zap, a flup and one more zap as desired.

This focuses on strings, not files, of course -- the replacement, per se, occurs in a string-to-string transformation. The advantage of the RE-based approach is that looping is reduced: all strings to be replaced are matched in a single pass, thanks to the regular expression engine. The re.escape calls ensure that strings containing special characters are treated just as literals (no weird meanings;-), the vertical bars mean "or" in the RE pattern language, and the sub method calls the nested onerepl function for each match, passing the match-object so the .group() call easily retrieves the specific string that was just matched and needs to be replaced.

To work at file level,

with open(final, 'w') as fin:
  with open(initial, 'r') as ini:
    fin.write(replacemany(mydict, ini.read()))

The with statement is recommended, to ensure proper closure of the files; if you're stuck with Python 2.5, use from __future__ import with_statement at the start of your module or script to gain use of the with statement.

Answer 2

fp1 = open("final","w")
fp2 = open("file", 'r')
for line in fp2:
    sline=line.rstrip().split()
    for n,item in enumerate(sline):
        if item in d:
            sline[n]=d[item]
    fp1.write(' '.join(sline) +"\n")

Answer 3

This should be better.

fp1 = open(final,"w")
fp2 = open(initial, 'r')
data = fp2.read()
fp2.close()
for key, value in mydict.items():
    data = data.replace(key, value)
fp1.write(data)
fp1.close()