core: support writing ulog to bytes handle

Previously only writing to files was possible. It's useful to be able to
write to BytesIO for e.g. returning generated ULog files without having
to write an intermediary file.

pyulog/core.py

@@ -3,6 +3,7 @@
 import struct
 import copy
 import sys
+import contextlib
 import numpy as np
 #pylint: disable=too-many-instance-attributes, unused-argument, missing-docstring
 #pylint: disable=protected-access, too-many-branches
@@ -236,9 +237,14 @@ def get_dataset(self, name, multi_instance=0):
         return [elem for elem in self._data_list
                 if elem.name == name and elem.multi_id == multi_instance][0]
 
-    def write_ulog(self, path):
+    def write_ulog(self, log_file):
         """ write current data back into a ulog file """
-        with open(path, "wb") as ulog_file:
+        if isinstance(log_file, str):
+            handle = open(log_file, "wb")
+        else:
+            handle = contextlib.nullcontext(log_file)
+
+        with handle as ulog_file:
             # Definition section
             self._write_file_header(ulog_file)
             self._write_flags(ulog_file)

test/test_ulog.py

@@ -6,6 +6,7 @@
 import inspect
 import unittest
 import tempfile
+from io import BytesIO
 
 from ddt import ddt, data
 
@@ -59,4 +60,33 @@ def test_write_ulog(self, base_name):
 
         assert copied == original
 
+    @data('sample')
+    def test_write_ulog_memory(self, base_name):
+        '''
+        Test that the write_ulog method can write bytes to memory.
+        '''
+        ulog_file_name = os.path.join(TEST_PATH, base_name + '.ulg')
+        original = pyulog.ULog(ulog_file_name)
+        with BytesIO() as bytes_handle:
+            original.write_ulog(bytes_handle)
+            bytes_handle.seek(0)
+            copied = pyulog.ULog(bytes_handle)
+
+        for original_key, original_value in original.__dict__.items():
+            copied_value = getattr(copied, original_key)
+            if original_key == '_sync_seq_cnt':
+                # Sync messages are counted on parse, but otherwise dropped, so
+                # we don't rewrite them
+                assert copied_value == 0
+            elif original_key == '_appended_offsets':
+                # Abruptly ended messages just before offsets are dropped, so
+                # we don't rewrite appended offsets
+                assert copied_value == []
+            elif original_key == '_incompat_flags':
+                # Same reasoning on incompat_flags[0] as for '_appended_offsets'
+                assert copied_value[0] == original_value[0] & 0xFE # pylint: disable=unsubscriptable-object
+                assert copied_value[1:] == original_value[1:] # pylint: disable=unsubscriptable-object
+            else:
+                assert copied_value == original_value
+
 # vim: set et fenc=utf-8 ft=python ff=unix sts=4 sw=4 ts=4