sdk: fix ConsoleSpanExporter (#455)

19d573a ("Add io and formatter options to console exporter (#412)")
changed the way spans are printed by using write() instead of print().
In Python 3.x sys.stdout is line-buffered, so the spans were not being printed
to the console at the right timing.

This commit fixes that by adding an explicit flush() call at the end of the
export function , it also changes the default formatter to include a line break.

To be precise, only one of the changes was needed to solve the problem, but as
a matter of completness both are included, i.e, to handle the case where the
formatter chosen by the user doesn't append a line break.

opentelemetry-sdk/src/opentelemetry/sdk/trace/export/__init__.py

@@ -14,6 +14,7 @@
 
 import collections
 import logging
+import os
 import sys
 import threading
 import typing
@@ -270,12 +271,14 @@ class ConsoleSpanExporter(SpanExporter):
     def __init__(
         self,
         out: typing.IO = sys.stdout,
-        formatter: typing.Callable[[Span], str] = str,
+        formatter: typing.Callable[[Span], str] = lambda span: str(span)
+        + os.linesep,
     ):
         self.out = out
         self.formatter = formatter
 
     def export(self, spans: typing.Sequence[Span]) -> SpanExportResult:
         for span in spans:
             self.out.write(self.formatter(span))
+        self.out.flush()
         return SpanExportResult.SUCCESS

opentelemetry-sdk/tests/trace/export/test_export.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import os
 import time
 import unittest
 from logging import WARNING
@@ -288,8 +289,9 @@ def test_export(self):  # pylint: disable=no-self-use
         span = trace.Span("span name", mock.Mock())
         with mock.patch.object(exporter, "out") as mock_stdout:
             exporter.export([span])
-        mock_stdout.write.assert_called_once_with(str(span))
+        mock_stdout.write.assert_called_once_with(str(span) + os.linesep)
         self.assertEqual(mock_stdout.write.call_count, 1)
+        self.assertEqual(mock_stdout.flush.call_count, 1)
 
     def test_export_custom(self):  # pylint: disable=no-self-use
         """Check that console exporter uses custom io, formatter."""