FFmpeg Writer: handle redirected logs when checking for errors (#890)

* FFmpeg Writer: handle redirected logs when checking for errors

 * Previously we were looking for `'foo' in None` resulting in a TypeError (#877)
 * Now, if an `IOError` occurs when running FFmpeg to write video clips, **but** there is no output returned, assume we are redirected, and read the logs in from wherever they were actually placed.

moviepy/video/io/ffmpeg_writer.py

@@ -81,7 +81,7 @@ def __init__(
 
         if logfile is None:
             logfile = sp.PIPE
-
+        self.logfile = logfile
         self.filename = filename
         self.codec = codec
         self.ext = self.filename.split(".")[-1]
@@ -137,7 +137,12 @@ def write_frame(self, img_array):
         try:
             self.proc.stdin.write(img_array.tobytes())
         except IOError as err:
+            logs = self.logfile.name
             _, ffmpeg_error = self.proc.communicate()
+            if not ffmpeg_error:
+                with open(logs, "rb") as f:
+                    ffmpeg_error = f.read()
+
             error = str(err) + (
                 "\n\nMoviePy error: FFMPEG encountered "
                 "the following error while writing file %s:"

tests/test_VideoClip.py

@@ -27,6 +27,17 @@ def test_check_codec():
     close_all_clips(locals())
 
 
+def test_errors_with_redirected_logs():
+    """Checks error cases return helpful messages even when logs redirected
+    See https://github.com/Zulko/moviepy/issues/877"""
+    clip = VideoFileClip("media/big_buck_bunny_432_433.webm")
+    location = os.path.join(TMP_DIR, "logged-write.mp4")
+    with pytest.raises(IOError) as e:
+        clip.write_videofile(location, codec="nonexistent-codec", write_logfile=True)
+    assert "Unknown encoder 'nonexistent-codec'" in str(e.value)
+    close_all_clips(locals())
+
+
 def test_save_frame():
     clip = VideoFileClip("media/big_buck_bunny_432_433.webm")
     location = os.path.join(TMP_DIR, "save_frame.png")