pyqgis - Non-ASCII characters not displaying correctly after Python manipulation

I'm using QGIS 2.18 on a Mac (Sierra).

I'm doing some pretty extensive data manipulation using a virtual column, and using the function editor (so Python) to do so: one of my functions, for example, is to change street-name abbreviations to long-format (e.g.: All => Allée), and in French.

The result is encoded in UTF-8, and I can't seem to find a means to translate it into whatever encoding is needed to display it correctly on a map... The result of any string containing an accented character is 'null' in the attributes table.

Yet in another 'street name' layer, pulled directly from a PostGIS database, even accented labels (street names) display correctly.

Is this a common problem, and is there any way around this?

#!/usr/bin/env python

# -*- coding: utf-8 -*-


from qgis.core import *
from qgis.gui import *
from qgis.utils import qgsfunction
import re

@qgsfunction(args='auto', group='Custom')
def streettype_format(input, feature, parent):

    rep = {
        "CITE":"Cité",
        "ALL":"Allée",
        "RUE":"Rue",
        "RPT":"Rond-point",
        "SENT":"Sentier",
        "VLA":"Villa",
        "TERR":"Terrace",
        "CHEM":"Chemin",
        "CAR":"Carrefour",

        "HAM":"Hameau",
        "BD":"Boulevard",
        "AV":"Avenue",
        "CRS":"Cours",
        "VOIE":"Voie",
        "CHAU":"Chaussée",
        "ARC":"Arcade",
        "GAL":"Galérie"
    }
    rep = dict((re.escape(k), v) for k, v in rep.iteritems())

    pattern = re.compile("|".join(rep.keys()))
    input = pattern.sub(lambda m: rep[re.escape(m.group(0))], input)
    return input

Also: from some d*cking around in the Python console:

>>>> import sys
>>>>sys.getdefaultencoding()
ascii
>>>>u"Héy"
u'H\xe9y'

>>>>"Héy"
'H\xc3\xa9y'