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I am trying to read json containing a UTF-8 string from a file. It works when running it from a shell with LANG=en_GB.utf8 set, but fails from the empty environment.
> python3 --version
Python 3.4.0
> cat test.json 
{ "test": "Ümläute" }
> env -u LANG python3 -c 'import json; json.load(open("test.json"))'
Traceback (most recent call last):
 File "<string>", line 1, in <module>
 File "/usr/lib/python3.4/json/__init__.py", line 265, in load
 return loads(fp.read(),
 File "/usr/lib/python3.4/encodings/ascii.py", line 26, in decode
 return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 11: ordinal not in range(128)
> > env LANG=en_GB.UTF-8 python3 -c 'import json; print(json.load(open("test.json")))'
{'test': 'Ümläute'}

By default, open("test.json") will open the file in text mode, decoding from the encoding determined by your environment. See <https://docs.python.org/3.4/library/functions.html#open>, in particular, "The default encoding is platform dependent (whatever locale.getpreferredencoding() returns)".
If your file’s encoding is determined some other way, you should specify that other encoding. For JSON, I understand UTF-8 is generally always used, so you should call open("test.json", encoding="utf-8").