[python] Process escape sequences in a string in Python

unicode_escape doesn't work in general

It turns out that the string_escape or unicode_escape solution does not work in general -- particularly, it doesn't work in the presence of actual Unicode.

If you can be sure that every non-ASCII character will be escaped (and remember, anything beyond the first 128 characters is non-ASCII), unicode_escape will do the right thing for you. But if there are any literal non-ASCII characters already in your string, things will go wrong.

unicode_escape is fundamentally designed to convert bytes into Unicode text. But in many places -- for example, Python source code -- the source data is already Unicode text.

The only way this can work correctly is if you encode the text into bytes first. UTF-8 is the sensible encoding for all text, so that should work, right?

The following examples are in Python 3, so that the string literals are cleaner, but the same problem exists with slightly different manifestations on both Python 2 and 3.

>>> s = 'naïve \\t test'
>>> print(s.encode('utf-8').decode('unicode_escape'))
naïve   test

Well, that's wrong.

The new recommended way to use codecs that decode text into text is to call codecs.decode directly. Does that help?

>>> import codecs
>>> print(codecs.decode(s, 'unicode_escape'))
naïve   test

Not at all. (Also, the above is a UnicodeError on Python 2.)

The unicode_escape codec, despite its name, turns out to assume that all non-ASCII bytes are in the Latin-1 (ISO-8859-1) encoding. So you would have to do it like this:

>>> print(s.encode('latin-1').decode('unicode_escape'))
naïve    test

But that's terrible. This limits you to the 256 Latin-1 characters, as if Unicode had never been invented at all!

>>> print('Erno \\t Rubik'.encode('latin-1').decode('unicode_escape'))
UnicodeEncodeError: 'latin-1' codec can't encode character '\u0151'
in position 3: ordinal not in range(256)

Adding a regular expression to solve the problem

(Surprisingly, we do not now have two problems.)

What we need to do is only apply the unicode_escape decoder to things that we are certain to be ASCII text. In particular, we can make sure only to apply it to valid Python escape sequences, which are guaranteed to be ASCII text.

The plan is, we'll find escape sequences using a regular expression, and use a function as the argument to re.sub to replace them with their unescaped value.

import re
import codecs

ESCAPE_SEQUENCE_RE = re.compile(r'''
    ( \\U........      # 8-digit hex escapes
    | \\u....          # 4-digit hex escapes
    | \\x..            # 2-digit hex escapes
    | \\[0-7]{1,3}     # Octal escapes
    | \\N\{[^}]+\}     # Unicode characters by name
    | \\[\\'"abfnrtv]  # Single-character escapes
    )''', re.UNICODE | re.VERBOSE)

def decode_escapes(s):
    def decode_match(match):
        return codecs.decode(match.group(0), 'unicode-escape')

    return ESCAPE_SEQUENCE_RE.sub(decode_match, s)

And with that:

>>> print(decode_escapes('Erno \\t Rubik'))
Erno     Rubik