Switch branches/tags
Nothing to show
Clone or download
Pull request Compare This branch is 2 commits ahead, 166 commits behind python-parsy:master.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Parsy is an easy way to combine simple, small parsers into complex, larger parsers. If it means anything to you, it's a monadic parser combinator library for LL(infinity) grammars in the spirit of Parsec, Parsnip, and Parsimmon.

Parsy requires Python 3.3 or greater.

This repo is no longer maintained. Instead, see


Simple parser generators

  • string(expected_string)

    Returns a parser that expects the expected_string and produces that string value.

  • regex(exp, [flags=0])

    Returns a parser that expects the given exp, and produces the matched string. exp can be a compiled regular expression, or a string which will be compiled with the given flags.

  • success(val)

    Returns a parser that does not consume any of the stream, but produces val.

Parser methods

  • parser.parse(string)

    Attempts to parse the given string. If the parse is successful and consumes the entire string, the result is returned - otherwise, a ParseError is raised.

  • parser.parse_partial(string)

    Similar to parse, except that it does not require the entire string to be consumed. Returns a tuple of (result, rest_of_string), where rest_of_string is the part of the string that was left over.

  • parser | other_parser

    Returns a parser that tries parser and, if it fails, backtracks and tries other_parser. These can be chained together.

    The resulting parser will produce the value produced by the first successful parser.

>>> parser = string('x') | string('y') | string('z')
>>> parser.parse('x')
>>> parser.parse('y')
>>> parser.parse('z')
  • parser.then(other_parser) (also parser >> other_parser)

    Returns a parser which, if parser succeeds, will continue parsing with other_parser. This will produce the value produced by other_parser

>>> (string('x') >> string('y')).parse('xy')
  • parser.skip(other_parser) (also parser << other_parser)

    Similar to then (or >>), except the resulting parser will use the value produced by the first parser.

>>> (string('x') << string('y')).parse('xy')
  • parser.many()

    Returns a parser that expects parser 0 or more times, and produces a list of the results. Note that this parser can never fail - only produce an empty list.

>>> parser = regex(r'[a-z]').many()
>>> parser.parse('')
>>> parser.parse('abc')
['a', 'b', 'c']
  • parser.times(min [, max=min])

    Returns a parser that expects parser at least min times, and at most max times, and produces a list of the results. If only one argument is given, the parser is expected exactly that number of times.

  • parser.at_most(n)

    Returns a parser that expects parser at most n times, and produces a list of the results.

  • parser.at_least(n)

    Returns a parser that expects parser at least n times, and produces a list of the results.


    Returns a parser that transforms the produced value of parser with fn.

>>> regex(r'[0-9]+').map(int).parse('1234')
  • parser.result(val)

    Returns a parser that, if parser succeeds, always produces val.

>>> string('foo').result(42).parse('foo')
  • parser.bind(fn)

    Returns a parser which, if parser is successful, passes the result to fn, and continues with the parser returned from fn. This is the monadic binding operation.

Generating a parser

The most powerful way to construct a parser is to use the generate decorator. parsy.generate creates a parser from a generator that should yield parsers. These parsers are applied successively and their results are sent back to the generator using .send() protocol. The generator should return the result or another parser, which is equivalent to applying it and returning its result.

from parsy import generate

def form():
    Parse an s-expression form, like (a b c).
    An equivalent to lparen >> expr.many() << rparen
    yield lparen
    exprs = yield expr.many()
    yield rparen
    return exprs

def exact_number():
    Parse specified number of expressions, like
    4: a b c d
    num = int(yield regex(r'[0-9]+')) # or .map(int)
    yield string(':')
    return expr.times(num)

Note that there is no guarantee that the entire function is executed: if any of the yielded parsers fails, parsy will try to backtrack to an alternative parser.