Pratt

Pratt is a Python library for creating Pratt parsers. Pratt parsers are recursive descent operator precedence parsers. Such parsers are well suited to parsing expressions and integrate well into regular recursive descent parsers.

A minimal parser using Pratt looks like this:

from pratt import Grammar, Parser

def get_token_type(token):
    return token[0]

def handle_unexpected_token(token):
    # Custom exception type omitted for brevity
    raise SyntaxErrror('oops: {!r}'.format(token))

grammar = Grammar(get_token_type=lambda token: token[0]
grammar.symbol('end')

@grammar.literal('int')
def handle_int(token):
    return int(token[1])

@grammar.infix('+', 10)
def add(token, left, right):
    return left + right


tokens = [('int', 1), ('+', '+'), ('int', 1)]
parser = Parser(grammar, tokens)
result = parser.parse()
print(result)
# 2

A more realistic and well commented example for parsing mathematical expressions, is examples/math_expr.py, in the Pratt source code repository.

As you can see Pratt itself provides no utilities for tokenization. Doing so in a way that satifies everyones needs is fairly complex and therefore out of scope for this project. On the other hand Pratt makes no assumptions about your tokens, allowing easy integration with whatever solution you want to use for that purpose.

Pratt is licensed under the 3-clause BSD license.