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Editing Docker-compose files with Python

Docker-Compose files declare the state of a Docker stack and are used to configure Docker Containers. In this Blog I will show you how to edit them with Python, so you can change them programmatically.

Packages for editing YAML

Docker-Compose files are written in YAML, but unlike JSON Pythons Standard Library does not come with a parser for YAML. But there are some in the Python Package Index. My first instinct was PyYAML but I does come with a downside: It sorts the keys alphabetically in the output. This means that the Version of the Docker-Compose file would be at the bottom of the file. This behaviour is can be changed in Version 5.1 of PyYAML: yaml.dump(data, default_flow_style=False, sort_keys=False).

There is also ruamel.yaml which saves some roundtrips while emitting YAML files and thus leaves the structure of the file mostly intact. Thus I decided to give it a try.

Example: Changing the Version of an Image

This example shows how to change the Version of a Docker Image with a little commandline script. The script takes a path and a version as positional arguments

import argparse
import os
import ruamel.yaml
yaml = ruamel.yaml.YAML()
yaml.preserve_quotes = True
yaml.indent(sequence=3, offset=1)

def get_docker_compose_files(path):
    Walks the given path and collects all docker-compose.* files and yields
    for dirpath, dirnames, files in os.walk(path):
        for file in files:
            if 'docker-compose.' in file:
                yield os.path.join(dirpath, file) 

def change_version(version, path):
    Changes the Version of the Example Service
    Note: Example could be a good candidate for an additional commandline
    docker_compose_files = get_docker_compose_files(path)
    for docker_compose in docker_compose_files:
        with open(docker_compose, 'r') as ymlfile:
            docker_config = yaml.load(ymlfile)
            image = docker_config['services']['example']['image']
            string_split = image.split(':')
            new_image_string = ':'.join(string_split)
            docker_config['services']['example']['image'] = new_image_string
        with open(docker_compose, 'w') as newconf:
            yaml.dump(docker_config, newconf)

def main():
    parser = argparse.ArgumentParser()
    args = parser.parse_args()
    version = args.version
    path = args.path
    change_version(version, path)

if __name__ == '__main__':


The resulting docker-compose file will look almost the same as the input file and this keeps standard conventions like devlaring the version first and the networks or volumes last.

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