ReplaceFile = windll.kernel32.ReplaceFileW
ReplaceFile.restype = BOOL
ReplaceFile.argtypes = [
	LPWSTR,
	LPWSTR,
	LPWSTR,
	DWORD,
	LPVOID,
	LPVOID,
	]

REPLACEFILE_WRITE_THROUGH = 0x1
REPLACEFILE_IGNORE_MERGE_ERRORS = 0x2
REPLACEFILE_IGNORE_ACL_ERRORS = 0x4

from jaraco.windows.api.filesystem import ReplaceFile
import os

open('orig-file', 'w').write('some content')
open('replacing-file', 'w').write('new content')
ReplaceFile('orig-file', 'replacing-file', 'orig-backup', 0, 0, 0)
assert open('orig-file').read() == 'new content'
assert open('orig-backup').read() == 'some content'
assert not os.path.exists('replacing-file')

import fileinput
for line in fileinput.input(filename,inplace=True, backup='.bak'):
    # inplace=True causes the original file to be moved to a backup
    # standard output is redirected to the original file.
    # backup='.bak' specifies the extension for the backup file.

    # manipulate line
    newline=process(line)
    print(newline)

newcontents=process(contents)
for line in fileinput.input(filename,inplace=True, backup='.bak'):
    print(newcontents)
    break

from boltons.fileutils import atomic_save

with atomic_save('/path/to/file.txt') as f:
    f.write('this will only overwrite if it succeeds!n')