#!/usr/bin/env python3

# Change the #! line (shebang) 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.

# Sometimes you may find shebangs with flags such as `#! /usr/bin/env python -si`.
# Normally, pathfix overwrites the entire line, including the flags.
# To change interpreter and keep flags from the original shebang line, use -k.
# If you want to keep flags and add to them one single literal flag, use option -a.


# 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
keep_flags = False
add_flags = b''


def main():
 global new_interpreter
 global preserve_timestamps
 global create_backup
 global keep_flags
 global add_flags

 usage = ('usage: %s -i /interpreter -p -n -k -a file-or-directory ...\n' %
 sys.argv[0])
 try:
 opts, args = getopt.getopt(sys.argv[1:], 'i:a:kpn')
 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 o == '-k':
 keep_flags = True
 if o == '-a':
 add_flags = a.encode()
 if b' ' in add_flags:
 err("-a option doesn't support whitespaces")
 sys.exit(2)
 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)


def ispython(name):
 return name.endswith('.py')


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

 # 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 parse_shebang(shebangline):
 shebangline = shebangline.rstrip(b'\n')
 start = shebangline.find(b' -')
 if start == -1:
 return b''
 return shebangline[start:]


def populate_flags(shebangline):
 old_flags = b''
 if keep_flags:
 old_flags = parse_shebang(shebangline)
 if old_flags:
 old_flags = old_flags[2:]
 if not (old_flags or add_flags):
 return b''
 # On Linux, the entire string following the interpreter name
 # is passed as a single argument to the interpreter.
 # e.g. "#! /usr/bin/python3 -W Error -s" runs "/usr/bin/python3 "-W Error -s"
 # so shebang should have single '-' where flags are given and
 # flag might need argument for that reasons adding new flags is
 # between '-' and original flags
 # e.g. #! /usr/bin/python3 -sW Error
 return b' -' + add_flags + old_flags


def fixline(line):
 if not line.startswith(b'#!'):
 return line

 if b"python" not in line:
 return line

 flags = populate_flags(line)
 return b'#! ' + new_interpreter + flags + b'\n'


if __name__ == '__main__':
 main()