Python app does not print anything when running detached in docker
Question:
I have a Python (2.7) app which is started in my dockerfile:
CMD ["python","main.py"]
main.py prints some strings when it is started and goes into a loop afterwards:
print "App started"
while True:
time.sleep(1)
As long as I start the container with the -it flag, everything works as expected:
$ docker run --name=myapp -it myappimage
> App started
And I can see the same output via logs later:
$ docker logs myapp
> App started
If I try to run the same container with the -d flag, the container seems to start normally, but I can’t see any output:
$ docker run --name=myapp -d myappimage
> b82db1120fee5f92c80000f30f6bdc84e068bafa32738ab7adb47e641b19b4d1
$ docker logs myapp
$ (empty)
But the container still seems to run;
$ docker ps
Container Status ...
myapp up 4 minutes ...
Attach does not display anything either:
$ docker attach --sig-proxy=false myapp
(working, no output)
Any ideas whats going wrong? Does “print” behave differently when ran in background?
Docker version:
Client version: 1.5.0
Client API version: 1.17
Go version (client): go1.4.2
Git commit (client): a8a31ef
OS/Arch (client): linux/arm
Server version: 1.5.0
Server API version: 1.17
Go version (server): go1.4.2
Git commit (server): a8a31ef
Answers:
As a quick fix, try this:
from __future__ import print_function
# some code
print("App started", file=sys.stderr)
This works for me when I encounter the same problems. But, to be honest, I don’t know why does this error happen.
Finally I found a solution to see Python output when running daemonized in Docker, thanks to @ahmetalpbalkan over at GitHub. Answering it here myself for further reference :
Using unbuffered output with
CMD ["python","-u","main.py"]
instead of
CMD ["python","main.py"]
solves the problem; you can see the output (both, stderr and stdout) via
docker logs myapp
now!
In my case, running Python with -u
didn’t change anything. What did the trick, however, was to set PYTHONUNBUFFERED=1
as environment variable:
docker run --name=myapp -e PYTHONUNBUFFERED=1 -d myappimage
[Edit]: Updated PYTHONUNBUFFERED=0
to PYTHONUNBUFFERED=1
after Lars’s comment. This doesn’t change the behavior and adds clarity.
See this article which explain detail reason for the behavior:
There are typically three modes for buffering:
- If a file descriptor is unbuffered then no buffering occurs whatsoever, and function calls that read or write data occur immediately (and will block).
- If a file descriptor is fully-buffered then a fixed-size buffer is used, and read or write calls simply read or write from the buffer. The buffer isn’t flushed until it fills up.
- If a file descriptor is line-buffered then the buffering waits until it sees a newline character. So data will buffer and buffer until a n is seen, and then all of the data that buffered is flushed at that point in time. In reality there’s typically a maximum size on the buffer (just as in the fully-buffered case), so the rule is actually more like “buffer until a newline character is seen or 4096 bytes of data are encountered, whichever occurs first”.
And GNU libc (glibc) uses the following rules for buffering:
Stream Type Behavior
stdin input line-buffered
stdout (TTY) output line-buffered
stdout (not a TTY) output fully-buffered
stderr output unbuffered
So, if use -t
, from docker document, it will allocate a pseudo-tty, then stdout
becomes line-buffered
, thus docker run --name=myapp -it myappimage
could see the one-line output.
And, if just use -d
, no tty was allocated, then, stdout
is fully-buffered
, one line App started
surely not able to flush the buffer.
Then, use -dt
to make stdout line buffered
or add -u
in python to flush the buffer
is the way to fix it.
You can see logs on detached image if you change print
to logging
.
main.py:
import time
import logging
print "App started"
logging.warning("Log app started")
while True:
time.sleep(1)
Dockerfile:
FROM python:2.7-stretch
ADD . /app
WORKDIR /app
CMD ["python","main.py"]
Usually, we redirect it to a specific file (by mounting a volume from host and writing it to that file).
Adding a tty using -t is also fine. You need to pick it up in docker logs.
Using large log outputs, I did not have any issue with buffer storing all without putting it in dockers log.
Since I haven’t seen this answer yet:
You can also flush stdout after you print to it:
import time
if __name__ == '__main__':
while True:
print('cleaner is up', flush=True)
time.sleep(5)
I had to use PYTHONUNBUFFERED=1
in my docker-compose.yml file to see the output from django runserver.
If you want to add your print output to your Flask output when running docker-compose up
, add the following to your docker compose file.
web:
environment:
- PYTHONUNBUFFERED=1
Try to add these two environment variables to your solution PYTHONUNBUFFERED=1
and PYTHONIOENCODING=UTF-8
If you aren’t using docker-compose
and just normal docker
instead, you can add this to your Dockerfile
that is hosting a flask app
ARG FLASK_ENV="production"
ENV FLASK_ENV="${FLASK_ENV}"
PYTHONUNBUFFERED="true"
CMD [ "flask", "run" ]
When using python manage.py runserver
for a Django application, adding environment variable PYTHONUNBUFFERED=1
solve my problem. print('helloworld', flush=True)
also works for me.
However, python -u
doesn’t work for me.
If anybody is running the python application with conda you should add --no-capture-output
to the command since conda buffers to stdout by default.
ENTRYPOINT ["conda", "run", "--no-capture-output", "-n", "my-app", "python", "main.py"]
I have a Python (2.7) app which is started in my dockerfile:
CMD ["python","main.py"]
main.py prints some strings when it is started and goes into a loop afterwards:
print "App started"
while True:
time.sleep(1)
As long as I start the container with the -it flag, everything works as expected:
$ docker run --name=myapp -it myappimage
> App started
And I can see the same output via logs later:
$ docker logs myapp
> App started
If I try to run the same container with the -d flag, the container seems to start normally, but I can’t see any output:
$ docker run --name=myapp -d myappimage
> b82db1120fee5f92c80000f30f6bdc84e068bafa32738ab7adb47e641b19b4d1
$ docker logs myapp
$ (empty)
But the container still seems to run;
$ docker ps
Container Status ...
myapp up 4 minutes ...
Attach does not display anything either:
$ docker attach --sig-proxy=false myapp
(working, no output)
Any ideas whats going wrong? Does “print” behave differently when ran in background?
Docker version:
Client version: 1.5.0
Client API version: 1.17
Go version (client): go1.4.2
Git commit (client): a8a31ef
OS/Arch (client): linux/arm
Server version: 1.5.0
Server API version: 1.17
Go version (server): go1.4.2
Git commit (server): a8a31ef
As a quick fix, try this:
from __future__ import print_function
# some code
print("App started", file=sys.stderr)
This works for me when I encounter the same problems. But, to be honest, I don’t know why does this error happen.
Finally I found a solution to see Python output when running daemonized in Docker, thanks to @ahmetalpbalkan over at GitHub. Answering it here myself for further reference :
Using unbuffered output with
CMD ["python","-u","main.py"]
instead of
CMD ["python","main.py"]
solves the problem; you can see the output (both, stderr and stdout) via
docker logs myapp
now!
In my case, running Python with -u
didn’t change anything. What did the trick, however, was to set PYTHONUNBUFFERED=1
as environment variable:
docker run --name=myapp -e PYTHONUNBUFFERED=1 -d myappimage
[Edit]: Updated PYTHONUNBUFFERED=0
to PYTHONUNBUFFERED=1
after Lars’s comment. This doesn’t change the behavior and adds clarity.
See this article which explain detail reason for the behavior:
There are typically three modes for buffering:
- If a file descriptor is unbuffered then no buffering occurs whatsoever, and function calls that read or write data occur immediately (and will block).
- If a file descriptor is fully-buffered then a fixed-size buffer is used, and read or write calls simply read or write from the buffer. The buffer isn’t flushed until it fills up.
- If a file descriptor is line-buffered then the buffering waits until it sees a newline character. So data will buffer and buffer until a n is seen, and then all of the data that buffered is flushed at that point in time. In reality there’s typically a maximum size on the buffer (just as in the fully-buffered case), so the rule is actually more like “buffer until a newline character is seen or 4096 bytes of data are encountered, whichever occurs first”.
And GNU libc (glibc) uses the following rules for buffering:
Stream Type Behavior
stdin input line-buffered
stdout (TTY) output line-buffered
stdout (not a TTY) output fully-buffered
stderr output unbuffered
So, if use -t
, from docker document, it will allocate a pseudo-tty, then stdout
becomes line-buffered
, thus docker run --name=myapp -it myappimage
could see the one-line output.
And, if just use -d
, no tty was allocated, then, stdout
is fully-buffered
, one line App started
surely not able to flush the buffer.
Then, use -dt
to make stdout line buffered
or add -u
in python to flush the buffer
is the way to fix it.
You can see logs on detached image if you change print
to logging
.
main.py:
import time
import logging
print "App started"
logging.warning("Log app started")
while True:
time.sleep(1)
Dockerfile:
FROM python:2.7-stretch
ADD . /app
WORKDIR /app
CMD ["python","main.py"]
Usually, we redirect it to a specific file (by mounting a volume from host and writing it to that file).
Adding a tty using -t is also fine. You need to pick it up in docker logs.
Using large log outputs, I did not have any issue with buffer storing all without putting it in dockers log.
Since I haven’t seen this answer yet:
You can also flush stdout after you print to it:
import time
if __name__ == '__main__':
while True:
print('cleaner is up', flush=True)
time.sleep(5)
I had to use PYTHONUNBUFFERED=1
in my docker-compose.yml file to see the output from django runserver.
If you want to add your print output to your Flask output when running docker-compose up
, add the following to your docker compose file.
web:
environment:
- PYTHONUNBUFFERED=1
Try to add these two environment variables to your solution PYTHONUNBUFFERED=1
and PYTHONIOENCODING=UTF-8
If you aren’t using docker-compose
and just normal docker
instead, you can add this to your Dockerfile
that is hosting a flask app
ARG FLASK_ENV="production"
ENV FLASK_ENV="${FLASK_ENV}"
PYTHONUNBUFFERED="true"
CMD [ "flask", "run" ]
When using python manage.py runserver
for a Django application, adding environment variable PYTHONUNBUFFERED=1
solve my problem. print('helloworld', flush=True)
also works for me.
However, python -u
doesn’t work for me.
If anybody is running the python application with conda you should add --no-capture-output
to the command since conda buffers to stdout by default.
ENTRYPOINT ["conda", "run", "--no-capture-output", "-n", "my-app", "python", "main.py"]