#!/usr/bin/env python3

# Change the #! line occurring in Python scripts. The new interpreter
# pathname must be given with a -i option.
#
# Command line arguments are files or directories to be processed.
# Directories are searched recursively for files whose name looks
# like a python module.
# Symbolic links are always ignored (except as explicit directory
# arguments).
# The original file is kept as a back-up (with a "~" attached to its name),
# -n flag can be used to disable this.
#
# Undoubtedly you can do this using find and sed or perl, but this is
# a nice example of Python code that recurses down a directory tree
# and uses regular expressions. Also note several subtleties like
# preserving the file's mode and avoiding to even write a temp file
# when no changes are needed for a file.
#
# NB: by changing only the function fixfile() you can turn this
# into a program for a different change to Python programs...

import sys
import re
import os
from stat import *
import getopt

err = sys.stderr.write
dbg = err
rep = sys.stdout.write

new_interpreter = None
preserve_timestamps = False
create_backup = True


def main():
 global new_interpreter
 global preserve_timestamps
 global create_backup
 usage = ('usage: %s -i /interpreter -p -n file-or-directory ...\n' %
 sys.argv[0])
 try:
 opts, args = getopt.getopt(sys.argv[1:], 'i:pn')
 except getopt.error as msg:
 err(str(msg) + '\n')
 err(usage)
 sys.exit(2)
 for o, a in opts:
 if o == '-i':
 new_interpreter = a.encode()
 if o == '-p':
 preserve_timestamps = True
 if o == '-n':
 create_backup = False
 if not new_interpreter or not new_interpreter.startswith(b'/') or \
 not args:
 err('-i option or file-or-directory missing\n')
 err(usage)
 sys.exit(2)
 bad = 0
 for arg in args:
 if os.path.isdir(arg):
 if recursedown(arg): bad = 1
 elif os.path.islink(arg):
 err(arg + ': will not process symbolic links\n')
 bad = 1
 else:
 if fix(arg): bad = 1
 sys.exit(bad)

ispythonprog = re.compile(r'^[a-zA-Z0-9_]+\.py$')
def ispython(name):
 return bool(ispythonprog.match(name))

def recursedown(dirname):
 dbg('recursedown(%r)\n' % (dirname,))
 bad = 0
 try:
 names = os.listdir(dirname)
 except OSError as msg:
 err('%s: cannot list directory: %r\n' % (dirname, msg))
 return 1
 names.sort()
 subdirs = []
 for name in names:
 if name in (os.curdir, os.pardir): continue
 fullname = os.path.join(dirname, name)
 if os.path.islink(fullname): pass
 elif os.path.isdir(fullname):
 subdirs.append(fullname)
 elif ispython(name):
 if fix(fullname): bad = 1
 for fullname in subdirs:
 if recursedown(fullname): bad = 1
 return bad

def fix(filename):
## dbg('fix(%r)\n' % (filename,))
 try:
 f = open(filename, 'rb')
 except IOError as msg:
 err('%s: cannot open: %r\n' % (filename, msg))
 return 1
 line = f.readline()
 fixed = fixline(line)
 if line == fixed:
 rep(filename+': no change\n')
 f.close()
 return
 head, tail = os.path.split(filename)
 tempname = os.path.join(head, '@' + tail)
 try:
 g = open(tempname, 'wb')
 except IOError as msg:
 f.close()
 err('%s: cannot create: %r\n' % (tempname, msg))
 return 1
 rep(filename + ': updating\n')
 g.write(fixed)
 BUFSIZE = 8*1024
 while 1:
 buf = f.read(BUFSIZE)
 if not buf: break
 g.write(buf)
 g.close()
 f.close()

 # Finishing touch -- move files

 mtime = None
 atime = None
 # First copy the file's mode to the temp file
 try:
 statbuf = os.stat(filename)
 mtime = statbuf.st_mtime
 atime = statbuf.st_atime
 os.chmod(tempname, statbuf[ST_MODE] & 0o7777)
 except OSError as msg:
 err('%s: warning: chmod failed (%r)\n' % (tempname, msg))
 # Then make a backup of the original file as filename~
 if create_backup:
 try:
 os.rename(filename, filename + '~')
 except OSError as msg:
 err('%s: warning: backup failed (%r)\n' % (filename, msg))
 else:
 try:
 os.remove(filename)
 except OSError as msg:
 err('%s: warning: removing failed (%r)\n' % (filename, msg))
 # Now move the temp file to the original file
 try:
 os.rename(tempname, filename)
 except OSError as msg:
 err('%s: rename failed (%r)\n' % (filename, msg))
 return 1
 if preserve_timestamps:
 if atime and mtime:
 try:
 os.utime(filename, (atime, mtime))
 except OSError as msg:
 err('%s: reset of timestamp failed (%r)\n' % (filename, msg))
 return 1
 # Return success
 return 0

def fixline(line):
 if not line.startswith(b'#!'):
 return line
 if b"python" not in line:
 return line
 return b'#! ' + new_interpreter + b'\n'

if __name__ == '__main__':
 main()