Exact equivalent of `b'...'.decode("utf-8", "backslashreplace")` in Python 2

I attempted a more complete backport of the cpython implementation

This handles both UnicodeDecodeError (from .decode()) as well as UnicodeEncodeError from .encode() and UnicodeTranslateError from .translate():

from __future__ import unicode_literals

import codecs


def _bytes_repr(c):
    """py2: bytes, py3: int"""
    if not isinstance(c, int):
        c = ord(c)
    return '\\x{:x}'.format(c)


def _text_repr(c):
    d = ord(c)
    if d >= 0x10000:
        return '\\U{:08x}'.format(d)
    else:
        return '\\u{:04x}'.format(d)


def backslashescape_backport(ex):
    s, start, end = ex.object, ex.start, ex.end
    c_repr = _bytes_repr if isinstance(ex, UnicodeDecodeError) else _text_repr
    return ''.join(c_repr(c) for c in s[start:end]), end


codecs.register_error('backslashescape_backport', backslashescape_backport)

print(b'\xc2\xa1\xa1after'.decode('utf-8', 'backslashescape_backport'))
print(u'\u2603'.encode('latin1', 'backslashescape_backport'))

You can write your own error handler. Here's a solution that I tested on Python 2.7, 3.3 and 3.6:

from __future__ import print_function
import codecs
import sys

print(sys.version)

def myreplace(ex):
    # The error handler receives the UnicodeDecodeError, which contains arguments of the
    # string and start/end indexes of the bad portion.
    bstr,start,end = ex.object,ex.start,ex.end

    # The return value is a tuple of Unicode string and the index to continue conversion.
    # Note: iterating byte strings returns int on 3.x but str on 2.x
    return u''.join('\\x{:02x}'.format(c if isinstance(c,int) else ord(c))
                    for c in bstr[start:end]),end

codecs.register_error('myreplace',myreplace)
print(b'\xc2\xa1\xa1ABC'.decode("utf-8", "myreplace"))

Output:

C:\>py -2.7 test.py
2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:42:59) [MSC v.1500 32 bit (Intel)]
¡\xa1ABC

C:\>py -3.3 test.py
3.3.5 (v3.3.5:62cf4e77f785, Mar  9 2014, 10:35:05) [MSC v.1600 64 bit (AMD64)]
¡\xa1ABC

C:\>py -3.6 test.py
3.6.1 (v3.6.1:69c0db5, Mar 21 2017, 18:41:36) [MSC v.1900 64 bit (AMD64)]
¡\xa1ABC