docker stucks when executing time.sleep(1) in a python loop

Question

With the use of docker-compose and python:2.7, it runs correctly when executing only while 1 loop and time.sleep(1) separately.

But it stucks when executing them together.

Here is the docker version and the file content on my mac

tmp docker -v
Docker version 1.12.5, build 7392c3b
tmp cat docker-compose.yml
version: '2'
services:
    test:
        image: python:2.7
        command: [python, -c, "print 0\nwhile 1:\n\tprint 1\n\tbreak"]
tmp docker-compose up
Creating network "tmp_default" with the default driver
Creating tmp_test_1
Attaching to tmp_test_1
test_1  | 0
test_1  | 1
tmp_test_1 exited with code 0
tmp cat docker-compose.yml
version: '2'
services:
    test:
        image: python:2.7
        command: [python, -c, "print 0\nimport time\nprint time.sleep(1)"]
tmp docker-compose up
Recreating tmp_test_1
Attaching to tmp_test_1
test_1  | 0
test_1  | None
tmp_test_1 exited with code 0
tmp cat docker-compose.yml
version: '2'
services:
    test:
        image: python:2.7
        command: [python, -c, "print 0\nimport time\nwhile 1:\n\tprint time.sleep(1)"]
tmp docker-compose up
Recreating tmp_test_1
Attaching to tmp_test_1

and here it stucks.

Hope to know the reason and the method to fix it, thanks.

Answer 1

Add -u flag to python in order to have unbuffered stdout:

command: [python, -uc, "print 0\nimport time\nwhile 1:\n\tprint time.sleep(1)"]