Python exceptions in Docker logs marked as stream: stdout

Question

I want parse and handle all errors from docker container, but python exceptions marked as stdout, when I expect stderr.

As example simple app.py

Answer 1

I had a misconception. I thought that the command of docker CLI does not affect the main logs (/var/lib/docker/containers/.../...-json.log)

But in case with:

docker run -it my_python python /var/app.py

json.log content:

{"log":"Traceback (most recent call last):\n","stream":"stdout","time":"2015-06-18T10:02:55.842010241Z"}
{"log":"  File \"/var/app.py\", line 1, in \u003cmodule\u003e\n","stream":"stdout","time":"2015-06-18T10:02:55.842252975Z"}
{"log":"    raise Exception(\"error\")\n","stream":"stdout","time":"2015-06-18T10:02:55.842423153Z"}
{"log":"Exception: error\n","stream":"stdout","time":"2015-06-18T10:02:55.842754372Z"}

But if I run container in background, stream become stderr:

docker run -d my_python python /var/app.py

{"log":"Traceback (most recent call last):\n","stream":"stderr","time":"2015-06-18T10:02:18.905673576Z"}
{"log":"  File \"/var/app.py\", line 1, in \u003cmodule\u003e\n","stream":"stderr","time":"2015-06-18T10:02:18.90575399Z"}
{"log":"    raise Exception(\"error\")\n","stream":"stderr","time":"2015-06-18T10:02:18.905802834Z"}
{"log":"Exception: error\n","stream":"stderr","time":"2015-06-18T10:02:18.90616668Z"}

I think this behavior implicitly.