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Fast and accurate Python dependency management

Project description

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Fast and safe dependency management for Python applications.

Motivation

The de-facto standard way to keep track of Python dependencies is with a requirements.txt file, listing the required packages and specifying what versions of them can be used. There are two strategies for specifying versions in a requirements.txt file: adding only the top-level dependencies and constraints you know to be necessary, or adding every recursive dependency and pinning them to specific versions you know work. The first strategy makes installing dependencies non-repeatable. The second makes upgrading difficult, and is hard to manage with standard python tools.

Dotlock enables you to do both: keep track of top-level requirements and known constraints in package.json, and generate repeatable requirement sets in package.lock.json by running a single command: dotlock lock.

Dotlock is partly inspired by pipenv, which also provides dependency-locking functionality. However, dotlock attempts to improve over pipenv in the following ways:

  • Accuracy: pipenv only locks to the level of versions, not specific distributions. This is why a Pipfile.lock will often contain multiple hashes for the same dependency, and means you do not know exactly what distribution will be installed when you run pipenv lock.
  • Speed: pipenv lock is very slow in my experience.
  • Reliability: pipenv does a lot of stuff, but it also has a lot of bugs.
  • Extras Support: pipenv only supports “default” dependencies and “dev” dependencies; dotlock supports arbitrary extra dependency groups, e.g. dotlock install --extras tests.

Under the hood, pipenv is essentially a complicated wrapper for pip, relying on it for metadata discovery and extraction, dependency resolution, dependency downloading and installation. To improve on pipenv, dotlock handles most of these itself, relying on pip only to install already-downloaded dependencies.

Installation

Dotlock can be installed with pip, i.e. pip install dotlock. It can be installed in an application’s virtual environment, at the user level, or globally.

Development Setup

On your development machine, run dotlock init to create a virtualenv and a skeleton package.lock file. Add your sources and dependencies to package.lock:

{
    "sources": [
        // PyPI-like package index hosting the dependencies.
        // If multiple indexes are included, each is tried in order during dependency resolution.
        "https://pypi.org/pypi"
    ],
    "default": {
        // Requirements in the form "package-name": "specifier".
        // Version specifiers may be "*", or a version number preceded by any of <, <=, >, >=, or ==.
        // Multiple specifiers can be separated by commas, e.g. ">=2.1,<3.0".
        "setuptools": ">=39.0",
        "virtualenv": "*",
        // Git, Mercurial and Subversion dependencies are also supported.
        "requests": "git+git://github.com/requests/requests@v2.19.1",
        // If you need extras or markers, supply a dictionary instead of a string.
        "idna_ssl": {
            "specifier": "*",
            "marker": "python_version < 3.7",  // See PEP 496
            "extras": ["tests"],
        },
        // Local file paths can be used too, but this loses integrity guarantees.
        "mypackage": "~/projects/mypackage"
    },
    "extras": {
        // You can specify groups of additional dependencies that will be installed by
        // dotlock install --extras [names]
        "dev": {
            "ipython": "*"
        },
        "tests": {
            "pytest": "*"
        }
    }
}

Then you can lock and install your dependencies:

dotlock lock  # Creates package.lock.json.
dotlock install  # Installs exactly the distributions in package.lock.json.
# Either source venv/bin/activate to enter the virtualenv, or use dotlock run.
dotlock run [program] [args]  # Runs [program] in the virtualenv.

For more information, run dotlock -h or dotlock [command] -h.

Developing in Different Environments

If your development environment differs significantly from your target deployed environment, e.g. you use a different operating system or a different version of Python, you will have to do some extra work and lose some of the benefits of dotlock.

In order to resolve dependencies and select distributions correctly, dotlock needs to know certain features of the deployed environment. Run dotlock dump-env on the deployed environment to create an env.json file. This file should live alongside your package.json file, and will be used by dotlock lock.

Since package.lock.json contains only the distributions appropriate for your deployed environment, running dotlock install on an incompatible environment will error. Instead, you can run dotlock install --skip-lock, which will bypass package.lock.json, looking just at package.json.

Deployment

There are two ways to install your locked dependencies during deployment:

  • Install dotlock and run dotlock install in the application root directory.
  • Use dotlock bundle to create bundle.tar.gz and install.sh prior to deployment, include these files in the deployment, and run ./install.sh during deployment.

Using dotlock bundle is preferred because it does not require installing dotlock in the deployed environment and does not depend on external services during deploy.

Once the dependencies are installed, run your application with one of:

  • source venv/bin/activate; [program] [args]
  • Assuming dotlock is installed: dotlock run [program] [args]

Roadmap and Limitations

Planned features:

  • Interpolate environment variables in sources
  • Allow specifying indices for individual packages

Features under consideration:

  • Support virtualenvs other than ./venv
  • Support versions of Python before 3.6
  • Support locking for other platforms. This is not possible to do with perfect reliability, since the dependencies discovered by running setup.py may differ depending on what platform the script is run on.

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