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Lark - a parsing toolkit for Python

Lark is a parsing toolkit for Python, built with a focus on ergonomics, performance and modularity.

Lark can parse all context-free languages. To put it simply, it means that it is capable of parsing almost any programming language out there, and to some degree most natural languages too.

Who is it for?

What can it do?

And many more features. Read ahead and find out!

Most importantly, Lark will save you time and prevent you from getting parsing headaches.

Quick links

Install Lark

$ pip install lark --upgrade

Lark has no dependencies.

Tests

Syntax Highlighting

Lark provides syntax highlighting for its grammar files (*.lark):

Clones

These are implementations of Lark in other languages. They accept Lark grammars, and provide similar utilities.

Hello World

Here is a little program to parse "Hello, World!" (Or any other similar phrase):

from lark import Lark

l = Lark('''start: WORD "," WORD "!"

            %import common.WORD   // imports from terminal library
            %ignore " "           // Disregard spaces in text
         ''')

print( l.parse("Hello, World!") )

And the output is:

Tree(start, [Token(WORD, 'Hello'), Token(WORD, 'World')])

Notice punctuation doesn't appear in the resulting tree. It's automatically filtered away by Lark.

Fruit flies like bananas

Lark is great at handling ambiguity. Here is the result of parsing the phrase "fruit flies like bananas":

fruitflies.png

Read the code here, and see more examples here.

List of main features

See the full list of features here

Comparison to other libraries

Performance comparison

Lark is fast and light (lower is better)

Run-time Comparison

Memory Usage Comparison

Check out the JSON tutorial for more details on how the comparison was made.

For thorough 3rd-party benchmarks, checkout the Python Parsing Benchmarks repo.

Feature comparison

LibraryAlgorithmGrammarBuilds tree?Supports ambiguity?Can handle every CFG?Line/Column trackingGenerates Stand-alone
LarkEarley/LALR(1)EBNFYes!Yes!Yes!Yes!Yes! (LALR only)
PLYLALR(1)BNFNoNoNoNoNo
PyParsingPEGCombinatorsNoNoNo*NoNo
ParsleyPEGEBNFNoNoNo*NoNo
ParsimoniousPEGEBNFYesNoNo*NoNo
ANTLRLL(*)EBNFYesNoYes?YesNo

(* PEGs cannot handle non-deterministic grammars. Also, according to Wikipedia, it remains unanswered whether PEGs can really parse all deterministic CFGs)

Projects using Lark

Full list

License

Lark uses the MIT license.

(The standalone tool is under MPL2)

Contributors

Lark accepts pull-requests. See How to develop Lark

Big thanks to everyone who contributed so far:

<img src="https://contributors-img.web.app/image?repo=lark-parser/lark" /> <a href="https://github.com/lark-parser/lark/graphs/contributors"> </a>

Sponsor

If you like Lark, and want to see us grow, please consider sponsoring us!

Contact the author

Questions about code are best asked on gitter or in the issues.

For anything else, I can be reached by email at erezshin at gmail com.

-- Erez