Lark Parser Usage Demo

This repository demonstrates how the Lark parser toolkit can be used in a project.

• Implements stand-alone and dependent versions of the same grammar.
• A single lark.Transformer is shared by both implementations.
• The tox.ini file includes a pre-testing step that rebuilds the stand-alone parser module before each test.
• The environment settings define an environment variable (LARK_DEMO_MODE) to determine which parser is checked (either standalone or dependency).
• The test suite runs the same tests for each mode.

You can learn more about Lark on PyPI or GitHub.

The tox.ini file

We want to build the stand-alone module before we begin testing. We do this by adding the build_standalone environment to the top of the list (tox builds environments and runs tests in the order given). We also use the skipsdist = True setting because this demo is not a proper Python package:

[tox]
envlist =
    build_standalone  # <- Pre-testing build step.
    py3-standalone
    py3-dependency
recreate = False
skipsdist = True

...

The testenv:build_standalone section requires Lark as a dependency. The commands setting runs Lark's stand-alone tool taking a grammar file as input (json.lark) and generating a stand-alone parser module as output (standalone_module.py):

...

[testenv:build_standalone]
description =
    Generate stand-alone parser module (pre-test build step).
deps =
    lark
commands =
    python -m lark.tools.standalone --out="standalone_module.py" json.lark

...

The other testing environments set the environment variable that determines which version is loaded (see the Define make_parser() function section of json_parser_main.py to see how this is implemented):

...

[testenv:py3-standalone]
description =
    Run tests for stand-alone parser (no dependencies).
setenv =
    LARK_DEMO_MODE = standalone
deps =
    # None (Once built, no dependencies are required.)


[testenv:py3-dependency]
description =
    Run tests for parser with dependency (requires `lark`).
setenv =
    LARK_DEMO_MODE = dependency
deps =
    lark

Sources

This demo project was adapted from pieces of code given in the Lark documentation and Erez Shinan's blog post linked below:

• Create a stand-alone LALR(1) parser in Python:
  • http://blog.erezsh.com/create-a-stand-alone-lalr1-parser-in-python/
• Standalone Parser (from Docs » Examples for Lark):
  • https://lark-parser.readthedocs.io/en/latest/examples/standalone/json_parser_main.html

MIT License

The code in this project builds on work form the Lark project which uses the MIT License. All code presented here is presented using this same license and is copyright by all contributing authors.