Fix make_lines excludes fields with empty strings (#655) (#766)

* Fix make_lines excludes fields with empty strings (#655)

Converting to unicode required something to be done with None values. They were
converted to empty strings which were subsequently ignored. This makes it
impossible to write an explicitly empty string, which should be possible. This
change distinguishes between None and empty strings.

* Fix linting failure due to long comment line

Co-authored-by: Greg Schrock <gschrock@128technology.com>

Author: gregschrock

influxdb/line_protocol.py

@@ -104,9 +104,11 @@ def _is_float(value):
 
 
 def _escape_value(value):
-    value = _get_unicode(value)
+    if value is None:
+        return ''
 
-    if isinstance(value, text_type) and value != '':
+    value = _get_unicode(value)
+    if isinstance(value, text_type):
         return quote_ident(value)
 
     if isinstance(value, integer_types) and not isinstance(value, bool):

influxdb/tests/test_line_protocol.py

@@ -117,6 +117,24 @@ def test_make_lines_unicode(self):
             'test,unicode_tag=\'Привет!\' unicode_val="Привет!"\n'
         )
 
+    def test_make_lines_empty_field_string(self):
+        """Test make lines with an empty string field."""
+        data = {
+            "points": [
+                {
+                    "measurement": "test",
+                    "fields": {
+                        "string": "",
+                    }
+                }
+            ]
+        }
+
+        self.assertEqual(
+            line_protocol.make_lines(data),
+            'test string=""\n'
+        )
+
     def test_tag_value_newline(self):
         """Test make lines with tag value contains newline."""
         data = {