Process escape sequences in a string in Python

Question

Sometimes when I get input from a file or the user, I get a string with escape sequences in it. I would like to process the escape sequences in the same way that Python processes escape sequences in string literals.

For example, let's say myString is defined as:

>>> myString = "spam\\neggs"
>>> print(myString)
spam\neggs

I want a function (I'll call it process) that does this:

>>> print(process(myString))
spam
eggs

It's important that the function can process all of the escape sequences in Python (listed in a table in the link above).

Does Python have a function to do this?

Answer 1

This should be the accepted answer for python 3.

def unescape_escape_sequences(s):
    return s.encode('utf-8').decode('unicode_escape').encode('latin1').decode('utf8')

Here is why this works:

1. First we unescape the escape sequences while leaving utf-8 characters unchanged as bytes (s.encode('utf-8').decode('unicode_escape')). Unfortunately, this produces a string (i.e. character symbols) under the assumption that the bytes should be interpreted as if they are 'latin1' encoded (rather than utf-8/unicode encoding we started with) (see 'unicode_escape' in the docs).
2. Next we convert the wrongly latin1-decoded string back to bytes using the 'latin1' codec (.encode('latin1')). The resulting bytes are the same as when we started, except for the removed escape characters.
3. Finally, we re-convert from bytes to character symbols (i.e. a string), this time with the assumption that the bytes are encoded as utf-8 (as they were intended all along) (.decode('utf8')).

This handles the edge cases where other answers fail, AND relies only on documented functions in the python 3 standard library.

Particularly, this handles the edge case where the input string includes (1) non-latin1 unicode characters and (2) escape sequences to unescape.

unescape_escape_sequences("naïve \\t test") 
# 'naïve \t test'
unescape_escape_sequences(r'Δ\nΔ')
# 'Δ\nΔ'
unescape_escape_sequences("juancarlo\\tañez")
# 'juancarlo\tañez'
unescape_escape_sequences('Ernő \\t Rubik')
# 'Ernő \t Rubik'
unescape_escape_sequences("naïve \\t test")
# 'naïve \t test'
unescape_escape_sequences("spam\\neggs")
# 'spam\neggs'

These other answers from this post fail in (all but the last of) the above cases:

s = 'Ernő \\t Rubik'
bytes(s, "utf-8").decode("unicode_escape") # incorrect output
s.encode('latin-1').decode('unicode_escape') # error

Finally, this answer avoids using codecs.escape_decode() which is not documented in python 3.

Be aware you may run into trouble if your input string is not unicode/utf-8.

Answer 2

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('Ernő \\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):
        try:
            return codecs.decode(match.group(0), 'unicode-escape')
        except UnicodeDecodeError:
            # In case we matched the wrong thing after a double-backslash
            return match.group(0)

    return ESCAPE_SEQUENCE_RE.sub(decode_match, s)

And with that:

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

Answer 3

Quote the string properly so that it looks like the equivalent Python string literal, and then use ast.literal_eval. This is safe, but much trickier to get right than you might expect.

It's easy enough to add a " to the beginning and end of the string, but we also need to make sure that any " inside the string are properly escaped. If we want fully Python-compliant translation, we need to account for the deprecated behaviour of invalid escape sequences.

It works out that we need to add one backslash to:

• any sequence of an even number of backslashes followed by a double-quote (so that we escape a quote if needed, but don't escape a backslash and un-escape the quote if it was already escaped); as well as

• a sequence of an odd number of backslashes at the end of the input (because otherwise a backslash would escape our enclosing double-quote).

Here is an acid-test input showing a bunch of difficult cases:

>>> text = r'''\\ \ \" \\" \\\" \'你好'\n\u062a\xff\N{LATIN SMALL LETTER A}"''' + '\\'
>>> text
'\\\\ \\ \\" \\\\" \\\\\\" \\\'你好\'\\n\\u062a\\xff\\N{LATIN SMALL LETTER A}"\\'
>>> print(text)
\\ \ \" \\" \\\" \'你好'\n\u062a\xff\N{LATIN SMALL LETTER A}"\

I was eventually able to work out a regex that handles all these cases properly, allowing literal_eval to be used:

>>> def parse_escapes(text):
...     fixed_escapes = re.sub(r'(?<!\\)(\\\\)*("|\\$)', r'\\\1\2', text)
...     return ast.literal_eval(f'"{fixed_escapes}"')
... 

Testing the results:

>>> parse_escapes(text)
'\\ \\ " \\" \\" \'你好\'\nتÿa"\\'
>>> print(parse_escapes(text))
\ \ " \" \" '你好'
تÿa"\

This should correctly handle everything - strings containing both single and double quotes, every weird situation with backslashes, and non-ASCII characters in the input. (I admit it's a bit difficult to verify the results by eye!)

Answer 4

The (currently) accepted answer by Jerub is correct for python2, but incorrect and may produce garbled results (as Apalala points out in a comment to that solution), for python3. That's because the unicode_escape codec requires its source to be coded in latin-1, not utf-8, as per the official python docs. Hence, in python3 use:

>>> myString="špåm\\nëðþ\\x73"
>>> print(myString)
špåm\nëðþ\x73
>>> decoded_string = myString.encode('latin-1','backslashreplace').decode('unicode_escape')
>>> print(decoded_string)
špåm
ëðþs

This method also avoids the extra unnecessary roundtrip between strings and bytes in metatoaster's comments to Jerub's solution (but hats off to metatoaster for recognizing the bug in that solution).

Answer 5

This is a bad way of doing it, but it worked for me when trying to interpret escaped octals passed in a string argument.

input_string = eval('b"' + sys.argv[1] + '"')

It's worth mentioning that there is a difference between eval and ast.literal_eval (eval being way more unsafe). See Using python's eval() vs. ast.literal_eval()?

Answer 6

Below code should work for \n is required to be displayed on the string.

import string

our_str = 'The String is \\n, \\n and \\n!'
new_str = string.replace(our_str, '/\\n', '/\n', 1)
print(new_str)

Answer 7

The actually correct and convenient answer for python 3:

>>> import codecs
>>> myString = "spam\\neggs"
>>> print(codecs.escape_decode(bytes(myString, "utf-8"))[0].decode("utf-8"))
spam
eggs
>>> myString = "naïve \\t test"
>>> print(codecs.escape_decode(bytes(myString, "utf-8"))[0].decode("utf-8"))
naïve    test

Details regarding codecs.escape_decode:

• codecs.escape_decode is a bytes-to-bytes decoder
• codecs.escape_decode decodes ascii escape sequences, such as: b"\\n" -> b"\n", b"\\xce" -> b"\xce".
• codecs.escape_decode does not care or need to know about the byte object's encoding, but the encoding of the escaped bytes should match the encoding of the rest of the object.

Background:

• @rspeer is correct: unicode_escape is the incorrect solution for python3. This is because unicode_escape decodes escaped bytes, then decodes bytes to unicode string, but receives no information regarding which codec to use for the second operation.
• @Jerub is correct: avoid the AST or eval.
• I first discovered codecs.escape_decode from this answer to "how do I .decode('string-escape') in Python3?". As that answer states, that function is currently not documented for python 3.

Answer 8

The correct thing to do is use the 'string-escape' code to decode the string.

>>> myString = "spam\\neggs"
>>> decoded_string = bytes(myString, "utf-8").decode("unicode_escape") # python3 
>>> decoded_string = myString.decode('string_escape') # python2
>>> print(decoded_string)
spam
eggs

Don't use the AST or eval. Using the string codecs is much safer.

Answer 9

The ast.literal_eval function comes close, but it will expect the string to be properly quoted first.

Of course Python's interpretation of backslash escapes depends on how the string is quoted ("" vs r"" vs u"", triple quotes, etc) so you may want to wrap the user input in suitable quotes and pass to literal_eval. Wrapping it in quotes will also prevent literal_eval from returning a number, tuple, dictionary, etc.

Things still might get tricky if the user types unquoted quotes of the type you intend to wrap around the string.