如果你想在运行docker-compose up时将打印输出添加到Flask输出中，请将以下内容添加到docker compose文件中。

web:
  environment:
    - PYTHONUNBUFFERED=1

https://docs.docker.com/compose/environment-variables/

因为我还没有看到这个答案:

你也可以在输出stdout后刷新它:

import time

if __name__ == '__main__':
    while True:
        print('cleaner is up', flush=True)
        time.sleep(5)

通常，我们将其重定向到一个特定的文件(通过从主机挂载一个卷并将其写入该文件)。

使用-t添加tty也可以。你得从码头工人的日志里查到。

使用大的日志输出，我没有任何问题，缓冲区存储所有没有把它放在dockers日志。

如果将打印更改为日志记录，则可以在分离映像上看到日志。

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"]

请看这篇文章，详细解释了这种行为的原因:

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”.

GNU libc (glibc)使用以下规则进行缓冲:

Stream               Type          Behavior
stdin                input         line-buffered
stdout (TTY)         output        line-buffered
stdout (not a TTY)   output        fully-buffered
stderr               output        unbuffered

因此，如果使用-t, from docker document，它将分配一个伪tty，然后stdout变成行缓冲，因此docker run——name=myapp -it myappimage可以看到一行输出。

而且，如果只使用-d，没有分配tty，那么，stdout是完全缓冲的，一行App启动肯定不能刷新缓冲区。

然后，使用-dt使stdout行缓冲或在python中添加-u来刷新缓冲区是修复它的方法。

在我的例子中，使用-u运行Python并没有改变任何东西。然而，关键在于将PYTHONUNBUFFERED=1设置为环境变量:

docker run --name=myapp -e PYTHONUNBUFFERED=1 -d myappimage

[编辑]:更新PYTHONUNBUFFERED=0到PYTHONUNBUFFERED=1后Lars的评论。这不会改变行为并增加清晰度。