Question

I am trying to replace some content in a file with the current working directory using python 3.3. I have:

def ReplaceInFile(filename, replaceRegEx, replaceWithRegEx):
    ''' Open a file and use a re.sub to replace content within it in place '''
    with fileinput.input(filename, inplace=True) as f:
        for line in f:
            line = re.sub(replaceRegEx, replaceWithRegEx, line)
            #sys.stdout.write (line)
            print(line, end='')

and I am using it like so:

ReplaceInFile(r'Path\To\File.iss', r'(#define RootDir\s+)(.+)', r'\g<1>' + os.getcwd())

Unfortunately for me, my path is C:\Tkbt\Launch, so the substitution that I get is:

#define RootDir C:  kbt\Launch

i.e. it’s interpreting \t as tab.

So it looks to me like I need to tell python to double escape everything from os.getcwd(). I thought maybe .decode('unicode_escape') might be the answer but it is not. Can anybody help me out?

I’m hoping there’s a solution that isn’t “find replace each '\' with '\\'“.

Answer 1

You’ll have to resort to .replace('\\', '\\\\') I am afraid, that’s the only option you have to make this work.

• Using encoding to unicode_escape then decode again from ASCII would have been nice, if it worked:

  replacepattern = r'\g<1>' + os.getcwd().encode('unicode_escape').decode('ascii')
  

  This does the right thing with paths:

  >>> print(re.sub(r'(#define RootDir\s+)(.+)', r'\g<1>' + r'C:\Path\to\File.iss'.encode('unicode_escape').decode('ascii'), '#define Root
  #define RootDir C:\Path\to\File.iss
  

  but not with existing non-ASCII characters because re.sub() does not process \u or \x escapes.

• Don’t use re.escape() to escape special characters in a string, that escapes a little too much:

  >>> print(re.sub(r'(#define RootDir\s+)(.+)', r'\g<1>' + re.escape(r'C:\Path\To\File.iss'), '#define RootDir foo/bar/baz'))
  #define RootDir C\:\Path\To\File\.iss
  

  note the \: there.

Only .replace() results in a working replacement pattern, including non-ASCII characters:

>>> print(re.sub(r'(#define RootDir\s+)(.+)', r'\g<1>' + 'C:\\Path\\To\\File-with-non-
ASCII-\xef.iss'.replace('\\', '\\\\'), '#define Root
#define RootDir C:\Path\To\File-with-non-ASCII-ï.iss