How to verify my docker-compose.yml?

Question

I am setting up a container stack using docker-compose. Yet by running

docker-compose up

I get a rather cryptic error message:

Traceback (most recent call last):
  File "/usr/local/bin/docker-compose", line 9, in <module>
    load_entry_point('docker-compose==1.4.2', 'console_scripts', 'docker-compose')()
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/main.py", line 39, in main
    command.sys_dispatch()
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/docopt_command.py", line 21, in sys_dispatch
    self.dispatch(sys.argv[1:], None)
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/command.py", line 27, in dispatch
    super(Command, self).dispatch(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/docopt_command.py", line 24, in dispatch
    self.perform_command(*self.parse(argv, global_options))
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/command.py", line 57, in perform_command
    verbose=options.get('--verbose'))
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/command.py", line 78, in get_project
    config.load(config_details),
  File "/usr/local/lib/python2.7/dist-packages/compose/config.py", line 138, in load
    for service_name, service_dict in list(dictionary.items()):
AttributeError: 'NoneType' object has no attribute 'items'

How to debug? I have checked that the yml is valid, yet it doesn't work.

Answer 1

I forgot to save my docker-compose.yml, so it was empty.

I stumbled across the solution in this issue about making the error message better which was describing the behavior.

One gets a nice message if no docker-compose.yml exists at all:

Can't find a suitable configuration file in this directory or any parent. Are you in the right directory?
Supported filenames: docker-compose.yml, docker-compose.yaml, fig.yml, fig.yaml

It was anounced on the docker-compose's issue list that there will be a nicer error message in the upcomming 1.5 release:

Top level object needs to be a dictionary. Check your .yml file that
you have defined a service at the top level.

As of version 1.7.1 the error message indicating that your file is empty reads:

ERROR: Top level object in './docker-compose.yml' needs to be an
object not '<type 'NoneType'>'.

---

Given the latest versions, the error message is very straightforward:

$ docker -v
Docker version 19.03.13, build 4484c46d9d
$ docker build .
failed to solve with frontend dockerfile.v0: failed to create LLB definition: the Dockerfile cannot be empty

So for ease of use, consider upgrading your docker engine.