Molt: Dependency and environment manager for Python projects

Work in progress.

Specify a Python

All subcommands take a --py flag before it to specify the Python to use. The Python command can be:

Prefixed by a dash (e.g. -3.6). This is passed directly to the Python launcher (py command) if available.
Treated as a path if containing path separators.
A command to be looked up in PATH.

Examples:

# Runs "init" by invoking `py -2` as Python.
molt --py -2 init

# Runs "run" by invoking the specified executable.
molt --py /usr/bin/python run

# Runs "init" by looking up `python3.6` in PATH.
molt --py python3.6 init

Subcommands

`molt init`

Creates a Python environment for the project. An environment is created in ./__pypackages__/<compatibility-tag>.

The compatibility tag is used to identify Python package compatibility, based on PEP 425. The strictest compatibility tag is used to identify a platform. The value of which for a given Python interpreter can be unambiguously determined by invoking Brett Cannon’s pep425 tool:

import pep425
print(next(pep425.sys_tags()))

`molt install`

Install packages into the environment from molt.lock.json.

`molt run`

molt run <command> executes a command available in the environment. This is NOT a regular executable call; Molt manages commands during package installation, and resolve them at runtime to emulate Setuptools and pip’s entry point scripts.

Note that only commands installed via entry points work with molt run.

`molt py`

Access the base interpreter. For example, molt --py=python3.6 py myscript.py would use interpreter python3.6 to execute file myscript.py.

`molt lock`

Generate molt.lock.json from the manidest.

`molt latest`

List latest version (globally and compatible) of a package on the package index. Useful if you want to add a pinned package to the manifest, but don’t immediately know what version to use.

Manifest

TODO.

Lock file

See design/lock-file.md.

Interoperability

Other project management tools

molt install from Pipfile.lock, poetry.lock, or requirements.txt is supported. The former two have precedence over requirements.txt; behaviour is undetermined if both are found. This is done by converting them to molt.lock.json, and install from that.

Tip: The auto-generated molt.lock.json can be ignored locally by adding it to the project’s .git/info/exclude, so you can use Molt to develop Pipenv or Poetry projects without converting wholesale.

Locking into those files is not supported. You’ll need to use the respective tool to generate a new lock file.

Try it out

Requires Cargo, and a Python interpreter with pip available. The Python interpreter should be either available in PATH, discoverable by py, or set in the environment variable MOLT_BUILD_PYTHON.

cargo build --release

This gives you a binary in target/release. Copy it somewhere in your PATH.

Run lints/tests

Rust requirements:

Clippy

cargo clippy
cargo test

Python requirements:

flake8 python vendor
black python vendor
tox

uranusjr/molt