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A python implementation of the web assembly interpreter

Read more in the documentation on ReadTheDocs. View the change log.


pip install py-wasm

Developer Setup

If you would like to hack on py-wasm, please check out the Ethereum Development Tactical Manual for information on how we do:

  • Testing
  • Pull Requests
  • Code Style
  • Documentation

Development Environment Setup

You can set up your dev environment with:

git clone
cd py-wasm
virtualenv -p python3 venv
. venv/bin/activate
pip install -e .[dev]

Testing Setup

During development, you might like to have tests run on every file save.

Show flake8 errors on file change:

# Test flake8
when-changed -v -s -r -1 wasm/ tests/ -c "clear; flake8 wasm tests && echo 'flake8 success' || echo 'error'"

Run multi-process tests in one command, but without color:

# in the project root:
pytest --numprocesses=4 --looponfail --maxfail=1
# the same thing, succinctly:
pytest -n 4 -f --maxfail=1

Run in one thread, with color and desktop notifications:

cd venv
ptw --onfail "notify-send -t 5000 'Test failure ⚠⚠⚠⚠⚠' 'python 3 test on py-wasm failed'" ../tests ../wasm

Release setup

For Debian-like systems:

apt install pandoc

To release a new version:

make release bump=$$VERSION_PART_TO_BUMP$$

How to bumpversion

The version format for this repo is {major}.{minor}.{patch} for stable, and {major}.{minor}.{patch}-{stage}.{devnum} for unstable (stage can be alpha or beta).

To issue the next version in line, specify which part to bump, like make release bump=minor or make release bump=devnum. This is typically done from the master branch, except when releasing a beta (in which case the beta is released from master, and the previous stable branch is released from said branch). To include changes made with each release, update "docs/releases.rst" with the changes, and apply commit directly to master before release.

If you are in a beta version, make release bump=stage will switch to a stable.

To issue an unstable version when the current version is stable, specify the new version explicitly, like make release bump="--new-version 4.0.0-alpha.1 devnum"

Development and Testing

The test suite in this library is run using pytest.

pytest tests/

Part of the test suite includes the spec tests from the official Web Assembly spec. These are found under ./tests/spec.

It is often useful to view logging output when running tests. This can be done with:

pytest tests/spec/ --log-cli-level=debug

When trying to diagnose a specific failure in a spec test it can be useful to run the tests in a branch that is currently passing, capture the logging output, and then compare it to the logging output of the test in the failing branch. In order to make it easier to get the relevant logging output, you can use the flag --stop-after-command-line=123 where 123 is the line for the failing command. The full command would look something like:

pytest tests/spec/ --log-cli-level=debug --stop-after-command-line=123 -k f32.wast

This sets the logging output to DEBUG level, stops the test suite after it passes command line 123 and only runs the spec tests from the f32.wast spec test file.

There are a few spec tests that take noticeably longer than the others. You can omit these from the test run by adding the flag --skip-slow-spec.


A python implementation of the web assembly interpreter





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