Compose: [1.27.0+] docker-compose config truncates version ('3' instead of '3.x')

[Toilal]

Description of the issue

Not sure if it's a bug or a new feature, but ... Since 1.27.0+, docker-compose config returns only the major digit from version field.

Context information (for bug reports)

Output of docker-compose version

docker-compose version 1.27.0, build unknown
docker-py version: 4.3.1
CPython version: 3.7.9
OpenSSL version: OpenSSL 1.1.1  11 Sep 2018

Output of docker version

Docker version 19.03.12, build 48a66213fe

Steps to reproduce the issue

1. Create a docker-compose.yml file with the following content.

version: '3.7'

1. Run docker-compose config.

2. Check the output.

Observed result

services: {}
version: '3'

Expected result

services: {}
version: '3.7'

Stacktrace / full error message

No error

Additional information

Ubuntu 18.04, docker-compose 1.27.0 installed from pip on Python 3.7.9.

[ulyssessouza]

Hello @Toilal!
Thank you for your report!

Actually that's because since the latest compose-spec the version is not important anymore, since it's a merge of all the previous ones. We are also thinking about removing this from docker-compose config since it's not important anymore.

[t3easy]

This is actually a problem for docker swarm / services like Play with docker at the moment because only using '3' throws errors for ports and volume config

ERROR: The Compose file '/var/run/pwd/uploads/stack.yml' is invalid because:
services.web.ports is invalid: Invalid port "{'published': 80, 'target': 80}", should be [[remote_ip:]remote_port[-remote_port]:]port[/protocol]
services.web.ports contains an invalid type, it should be a string, or a number
services.web.volumes contains an invalid type, it should be a string
services.web.volumes contains an invalid type, it should be a string

See
https://labs.play-with-docker.com/?stack=https://raw.githubusercontent.com/t3easy/docker-typo3/7835138e9ae4e36f21d4cb644e2c93b2e52335df/.docker/pwd/stack.yml

vs:
https://labs.play-with-docker.com/?stack=https://raw.githubusercontent.com/t3easy/docker-typo3/28e4e22e43354fb2e77cdeddb760c12c4ba28866/.docker/pwd/stack.yml

[ulyssessouza]

Ok. We will make it a 3.9 then. Does it work for you?

[sgammill]

docker 19.03 can't stack-deploy version 3.9. You could output the version that was used in the input.

IIRC, version "3" is interpreted as "3.0", which would explain the errors mentioned above.

[strophy]

docker-compose 1.27.0, docker 19.03.8

Confirming @sgammill theory above. I'm using docker-compose config as a pre-processor as described here to work around stack-deploy being unable to interpolate values from an .env file. The resulting output uses long ports syntax and does not specify a minor version for the compose file, so docker assumes version 3.0, throwing the error services.core.ports.0 must be a string or number

Relevant section of the output:

services:
  core:
    command:
     ...
    image: ...
    ports:
    - published: 19999
      target: 19999
version: '3'

+1 docker-compose config should output the same version that was used in the input.

[ulyssessouza]

Well, we will then just propagate the version on the file, but you could just use something simpler for this environment variable interpolation. Like https://github.com/a8m/envsubst for example