[LNT] Python 3 support: fix convert to JSON

[thopre]

Summary

Plistlib and json modules have conflicting requirements for their dump
methods: Plistlib expects its file handle to be a writable binary file
object while json expects it to be a writable text file object.

This commit solves this issue by dumping the JSON into a temporary
string which is encoded to UTF-8 (JSON format RFC 7159 requires UTF-8,
UTF-16 or UTF-32 encoding and UTF-8 is the recommended default) before
being output. This allows to keep delegating file opening to Click and
avoid a refactoring to abstract the opening mode between supported
formats.

[thopre]

Ping?

[hubert.reinterpretcast]

LGTM.

lnt/formats/JSONFormat.py

27

According to https://docs.python.org/3/library/json.html, the default argument for json.dumps's ensure_ascii parameter is true. I guess this means both Python 2 and Python 3 will behave the same here with encode(), so we're okay to just use that.

lnt/formats/JSONFormat.py

 
 def _load_format(path_or_file):
     if isinstance(path_or_file, str):
         path_or_file = open(path_or_file)
 
     return json.load(path_or_file)
 
 
+def _dump_format(obj, fp):
+    # The json module produces str objects but fp is opened in binary mode
+    # (since Plistlib only dump to binary mode files) so we first dump into
+    # a string a convert to UTF-8 before outputing.
+    json_str = json.dumps(obj)
+    fp.write(json_str.encode())
+
+
 format = {
     'name': 'json',
     'predicate': _matches_format,
     'read': _load_format,
-    'write': json.dump,
+    'write': _dump_format,
 }