Comprehensive Guide to Lark-Parser for Python Developers

The Lark parser is a powerful, yet simple-to-use, parsing library for Python. It’s designed to handle all parsing needs, including context-free grammars, fast lexing, and more. With Lark, you can build parsers for a wide range of languages and data formats, thanks to its rich set of APIs and features. In this guide, we’ll explore some of the most useful APIs provided by Lark with practical code snippets, and we’ll conclude with a complete application example.

Getting Started

To begin using Lark, first install it via pip:

  $ pip install lark-parser

Basic Usage

Here is a basic example of how to define and use a Lark parser:

from lark import Lark

parser = Lark('''
start: WORD "," WORD "!"
WORD: /[a-zA-Z]+/
%ignore " "
''', start='start')

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

Intermediate Usage

With Lark, you can create more complex grammars:

from lark import Lark

grammar = '''
start: sentence
sentence: noun verb noun
noun: "cat" | "dog"
verb: "chases" | "eats"
%ignore " "
'''

parser = Lark(grammar, start='start')
print(parser.parse("cat chases dog"))
print(parser.parse("dog eats cat"))

Advanced Usage

Lark also allows for the definition of more advanced parsing constructs:

from lark import Lark, Transformer

grammar = '''
?start: sum
        | NAME "=" sum    -> assign_var
sum: product
        | sum "+" product -> add
product: atom
        | product "*" atom -> mul
?atom: NUMBER           -> number
        | "-" atom          -> neg
        | NAME            -> var
        | "(" sum ")"
%import common.CNAME -> NAME
%import common.NUMBER
%import common.WS
%ignore WS
'''

parser = Lark(grammar)

class CalculateTree(Transformer):
    from operator import add, mul
    number = float

    def __init__(self):
        self.vars = {}

    def assign_var(self, name, value):
        self.vars[name] = value
        return value

    def var(self, name):
        try:
            return self.vars[name]
        except KeyError:
            raise Exception(f"Variable {name} not found")

calc = CalculateTree()
tree = parser.parse("a = 5 * (3 + 2)")
result = calc.transform(tree)
print(result)  # Outputs: 25.0

Complete Application Example

Below is an example of a full application using some of the Lark parser’s features:

from lark import Lark, Transformer

grammar = '''
start: "begin" statement "end"
statement: "print" "(" STRING ")"
STRING: /"[^"]*"/
%import common.WS
%ignore WS
'''

class ExecuteCommands(Transformer):
    def statement(self, args):
        command, raw_text = args
        if command == 'print':
            print(eval(raw_text))

parser = Lark(grammar, parser='lalr')
transformer = ExecuteCommands()

code = '''
begin
print("Hello, World!")
end
'''

tree = parser.parse(code)
transformer.transform(tree)

This code defines a simple scripting language that can print messages when executed.

By better understanding the capabilities and flexibility of the Lark parser, you can create efficient and robust parsing solutions for a variety of applications.

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