Functional testing of console apps with Docker

For one of my apps I’d been manually testing some basic functions in a bunch of environments, and it was a huge pain. So I decided to automatize it. As a simplest solution I chose to run an environment in Docker and interact with them through pexpect.

First of all I tried to use docker-py, but it’s almost impossible to interact with app run in Docker container, started from docker-py with pexpect. So I just used Docker binary:

from contextlib import contextmanager
import subprocess
import shutil
from tempfile import mkdtemp
from pathlib import Path
import sys
import pexpect

# Absolute path to your source root:
root = str(Path(__file__).parent.parent.parent.resolve())


def _build_container(tag, dockerfile):
    """Creates a temporary folder with Dockerfile, builds an image and
    removes the folder.
    
    """
    tmpdir = mkdtemp()
    with Path(tmpdir).joinpath('Dockerfile').open('w') as file:
        file.write(dockerfile)
    if subprocess.call(['docker', 'build', '--tag={}'.format(tag), tmpdir],
                       cwd=root) != 0:
        raise Exception("Can't build a container")
    shutil.rmtree(tmpdir)


@contextmanager
def spawn(tag, dockerfile, cmd):
    """Yields spawn object for `cmd` ran inside a Docker container with an
    image build with `tag` and `dockerfile`. Source root is available in `/src`.
    
    """
    _build_container(tag, dockerfile)
    proc = pexpect.spawnu('docker run --volume {}:/src --tty=true '
                          '--interactive=true {} {}'.format(root, tag, cmd))
    proc.logfile = sys.stdout

    try:
        yield proc
    finally:
        proc.terminate()

_build_container is a bit tricky, but it’s because Docker binary can build an image only for file named Dockerfile.

This code can be used for running something inside a Docker container very simple, code for printing content of your source root inside the container will be:

with spawn(u'ubuntu-test', u'FROM ubuntu:latest', u'bash') as proc:
    proc.sendline(u'ls /src')

Back to testing, if we want to test that some application can print version, you can easily write py.test test like this:

container = (u'ubuntu-python', u'''
FROM ubuntu:latest
RUN apt-get update
RUN apt-get install -yy python
''')


def test_version():
    """Ensure that app can print current version."""
    tag, dockerfile = container
    with spawn(tag, dockerfile, u'bash') as proc:
        proc.sendline(u'cd /src')
        proc.sendline(u'pip install .')
        proc.sendline(u'app --version')
        # Checks that `version:` is in the output:
        assert proc.expect([pexpect.TIMEOUT, u'version:'])

You can notice the strange assert proc.expect([pexpect.TIMEOUT, u'version:']) construction, it works very simple, if there’s version: in output, expect returns 1, if timeout came first - 0.

Also you can notice that all strings are in unicode (u''), it’s for compatibility with Python 2. If you use only Python 3, you can remove all u''.

Examples.