#!/usr/bin/env python3
from os import rename,remove
from os.path import exists
from subprocess import run
from sys import argv,stderr,stdin,stdout
from time import time as secs
dne = 0
abort = False
if len(argv)<3:
    exit('string substition needs old string in arg 1 and new string in arg2 followed by an optional list of file names.')
exe = argv.pop(0)
old = argv.pop(0)
new = argv.pop(0)
fns = argv if argv else ['-']
for fn in fns:
    if fns.count(fn)>1:
        exit('duplicate file name: '+repr(fn))
    if fn!='-':
        if not exists(fn):
            print('file does not exist: '+repr(fn),file=stderr)
            dne += 1
if dne:
    exit('aborting due to '+str(dne)+' missing file'+'s'[dne==1:])
for fn in fns:
    if fn=='-':
        tn = None
        fi,fo = stdin,stdout
    else:
        tn = fn+'+'+str(int(secs()*3906250))
        fi = open(fn,'r')
        fo = open(tn,'w')
    changed = 0
    for ol in fi:
        nl = ol.replace(old,new)
        if nl!=ol:
            changed += 1
        fo.write(nl)
    fi.close()
    fo.close()
    if changed and fn!='-':
        run(['chmod','--quiet','--reference='+fn,tn])
        run(['chown','--quiet','--reference='+fn,tn])
        rename(fn,fn+'~')
        rename(tn,fn)
    else:
        if tn:
            remove(tn)

for fn in fns:
    with fileinput.input((fn,), inplace=True, backup='~') as input:
        for line in input:
            print(line.replace(old, new), end='')

documentation Wrote:Optional in-place filtering: if the keyword argument inplace=True is passed to fileinput.input() or to the FileInput constructor, the file is moved to a backup file and standard output is directed to the input file (if a file of the same name as the backup file already exists, it will be replaced silently). This makes it possible to write a filter that rewrites its input file in place. If the backup parameter is given (typically as backup='.<some extension>'), it specifies the extension for the backup file, and the backup file remains around; by default, the extension is '.bak' and it is deleted when the output file is closed. In-place filtering is disabled when standard input is read.

with fileinput.input(fns, inplace=True, backup='~') as ifh:
    for line in ifh:
        print(line.replace(old, new), end='')

• Write a series of unittests for this program.
  
• Handle arguments with specialized module such as argparse or click or plumbum.cli.