How to un-escape a backslash-escaped string?

Question

Suppose I have a string which is a backslash-escaped version of another string. Is there an easy way, in Python, to unescape the string? I could, for example, do:

>>> escaped_str = '"Hello,\nworld!"'
>>> raw_str = eval(escaped_str)
>>> print raw_str
Hello,
world!
>>>

However that involves passing a (possibly untrusted) string to eval() which is a security risk. Is there a function in the standard lib which takes a string and produces a string with no security implications?

Asked By: Nick

||

Source

Answer 1

>>> print '"Hello,\nworld!"'.decode('string_escape')
"Hello,
world!"

Answered By: ChristopheD

Answer 2

You can use ast.literal_eval which is safe:

Safely evaluate an expression node or a string containing a Python
expression. The string or node provided may only consist of the
following Python literal structures: strings, numbers, tuples, lists,
dicts, booleans, and None. (END)

Like this:

>>> import ast
>>> escaped_str = '"Hello,\nworld!"'
>>> print ast.literal_eval(escaped_str)
Hello,
world!

Answered By: jathanism

Answer 3

In python 3, str objects don’t have a decode method and you have to use a bytes object. ChristopheD’s answer covers python 2.

# create a `bytes` object from a `str`
my_str = "Hello,\nworld"
# (pick an encoding suitable for your str, e.g. 'latin1')
my_bytes = my_str.encode("utf-8")

# or directly
my_bytes = b"Hello,\nworld"

print(my_bytes.decode("unicode_escape"))
# "Hello,
# world"

Answered By: asachet

Answer 4

All given answers will break on general Unicode strings. The following works for Python3 in all cases, as far as I can tell:

from codecs import encode, decode
sample = u'mon€y\nröcks'
result = decode(encode(sample, 'latin-1', 'backslashreplace'), 'unicode-escape')
print(result)

In recent Python versions, this also works without the import:

sample = u'mon€y\nröcks'
result = sample.encode('latin-1', 'backslashreplace').decode('unicode-escape')

As suggested by obataku, you can also use the literal_eval method from the ast module like so:

import ast
sample = u'mon€y\nröcks'
print(ast.literal_eval(F'"{sample}"'))

Or like this when your string really contains a string literal (including the quotes):

import ast
sample = u'"mon€y\nröcks"'
print(ast.literal_eval(sample))

However, if you are uncertain whether the input string uses double or single quotes as delimiters, or when you cannot assume it to be properly escaped at all, then literal_eval may raise a SyntaxError while the encode/decode method will still work.

Answered By: Jesko Hüttenhain

Answer 5

For Python3, consider:

my_string.encode('raw_unicode_escape').decode('unicode_escape')

The ‘raw_unicode_escape’ codec encodes to latin1, but first replaces all other Unicode code points with an escaped 'uXXXX' or 'UXXXXXXXX' form. Importantly, it differs from the normal ‘unicode_escape’ codec in that it does not touch existing backslashes.

So when the normal ‘unicode_escape’ decoder is applied, both the newly-escaped code points and the originally-escaped elements are treated equally, and the result is an unescaped native Unicode string.

(The ‘raw_unicode_escape’ decoder appears to pay attention only to the 'uXXXX' and 'UXXXXXXXX' forms, ignoring all other escapes.)

Documentation:
https://docs.python.org/3/library/codecs.html?highlight=codecs#text-encodings

Answered By: Jander

Answer 6

custom string parser to decode only some backslash-escapes, in this case " and '

def backslash_decode(src):
    "decode backslash-escapes"
    slashes = 0 # count backslashes
    dst = ""
    for loc in range(0, len(src)):
        char = src[loc]
        if char == "\":
            slashes += 1
            if slashes == 2:
                dst += char # decode backslash
                slashes = 0
        elif slashes == 0:
            dst += char # normal char
        else: # slashes == 1
            if char == '"':
                dst += char # decode double-quote
            elif char == "'":
                dst += char # decode single-quote
            else:
                dst += "\" + char # keep backslash-escapes like n or t
            slashes = 0
    return dst

src = "a" + "\\" + r"'" + r'"' + r"n" + r"t" + r"x" + "z" # input
exp = "a" + "\"   +  "'"  +  '"'  + r"n" + r"t" + r"x" + "z" # expected output

res = backslash_decode(src)

print(res)
assert res == exp

Answered By: milahu

How to un-escape a backslash-escaped string?

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