Source code for hypothesis.extra.lark

# This file is part of Hypothesis, which may be found at
# Most of this work is copyright (C) 2013-2021 David R. MacIver
# (, but it contains contributions by others. See
# CONTRIBUTING.rst for a full list of people who may hold copyright, and
# consult the git log if you need to determine who owns an individual
# contribution.
# This Source Code Form is subject to the terms of the Mozilla Public License,
# v. 2.0. If a copy of the MPL was not distributed with this file, You can
# obtain one at


This extra can be used to generate strings matching any context-free grammar,
using the `Lark parser library <>`_.

It currently only supports Lark's native EBNF syntax, but we plan to extend
this to support other common syntaxes such as ANTLR and :rfc:`5234` ABNF.
Lark already `supports loading grammars
from `nearley.js <>`_, so you may not have to write
your own at all.

Note that as Lark is at version 0.x, this module *may* break API compatibility
in minor releases if supporting the latest version of Lark would otherwise be
infeasible.  We may also be quite aggressive in bumping the minimum version of
Lark, unless someone volunteers to either fund or do the maintainence.

from inspect import getfullargspec
from typing import Dict, Optional

import attr
import lark
from lark.grammar import NonTerminal, Terminal

from hypothesis.errors import InvalidArgument
from hypothesis.internal.conjecture.utils import calc_label_from_name
from hypothesis.internal.validation import check_type
from hypothesis.strategies._internal import SearchStrategy, core as st

__all__ = ["from_lark"]

class DrawState:
    """Tracks state of a single draw from a lark grammar.

    Currently just wraps a list of tokens that will be emitted at the
    end, but as we support more sophisticated parsers this will need
    to track more state for e.g. indentation level.

    # The text output so far as a list of string tokens resulting from
    # each draw to a non-terminal.
    result = attr.ib(default=attr.Factory(list))

def get_terminal_names(terminals, rules, ignore_names):
    """Get names of all terminals in the grammar.

    The arguments are the results of calling ``Lark.grammar.compile()``,
    so you would think that the ``terminals`` and ``ignore_names`` would
    have it all... but they omit terminals created with ``@declare``,
    which appear only in the expansion(s) of nonterminals.
    names = { for t in terminals} | set(ignore_names)
    for rule in rules:
        names |= { for t in rule.expansion if isinstance(t, Terminal)}
    return names

class LarkStrategy(SearchStrategy):
    """Low-level strategy implementation wrapping a Lark grammar.

    See ``from_lark`` for details.

    def __init__(self, grammar, start, explicit):
        assert isinstance(grammar, lark.lark.Lark)
        if start is None:
            start = grammar.options.start
        if not isinstance(start, list):
            start = [start]
        self.grammar = grammar

        # This is a total hack, but working around the changes is a nicer user
        # experience than breaking for anyone who doesn't instantly update their
        # installation of Lark alongside Hypothesis.
        compile_args = getfullargspec(grammar.grammar.compile).args
        if "terminals_to_keep" in compile_args:
            terminals, rules, ignore_names = grammar.grammar.compile(start, ())
        elif "start" in compile_args:  # pragma: no cover
            # Support lark <= 0.10.0, without the terminals_to_keep argument.
            terminals, rules, ignore_names = grammar.grammar.compile(start)
        else:  # pragma: no cover
            # This branch is to support lark <= 0.7.1, without the start argument.
            terminals, rules, ignore_names = grammar.grammar.compile()

        self.names_to_symbols = {}

        for r in rules:
            t = r.origin
            self.names_to_symbols[] = t

        for t in terminals:
            self.names_to_symbols[] = Terminal(

        self.start = st.sampled_from([self.names_to_symbols[s] for s in start])

        self.ignored_symbols = tuple(self.names_to_symbols[n] for n in ignore_names)

        self.terminal_strategies = {
   st.from_regex(t.pattern.to_regexp(), fullmatch=True)
            for t in terminals
        unknown_explicit = set(explicit) - get_terminal_names(
            terminals, rules, ignore_names
        if unknown_explicit:
            raise InvalidArgument(
                "The following arguments were passed as explicit_strategies, "
                "but there is no such terminal production in this grammar: %r"
                % (sorted(unknown_explicit),)

        nonterminals = {}

        for rule in rules:
            nonterminals.setdefault(, []).append(tuple(rule.expansion))

        for v in nonterminals.values():

        self.nonterminal_strategies = {
            k: st.sampled_from(v) for k, v in nonterminals.items()

        self.__rule_labels = {}

    def do_draw(self, data):
        state = DrawState()
        start = data.draw(self.start)
        self.draw_symbol(data, start, state)
        return "".join(state.result)

    def rule_label(self, name):
            return self.__rule_labels[name]
        except KeyError:
            return self.__rule_labels.setdefault(
                name, calc_label_from_name("LARK:%s" % (name,))

    def draw_symbol(self, data, symbol, draw_state):
        if isinstance(symbol, Terminal):
                strategy = self.terminal_strategies[]
            except KeyError:
                raise InvalidArgument(
                    "Undefined terminal %r. Generation does not currently support "
                    "use of %%declare unless you pass `explicit`, a dict of "
                    'names-to-strategies, such as `{%r: st.just("")}`'
                    % (,
            assert isinstance(symbol, NonTerminal)
            expansion = data.draw(self.nonterminal_strategies[])
            for e in expansion:
                self.draw_symbol(data, e, draw_state)
                self.gen_ignore(data, draw_state)

    def gen_ignore(self, data, draw_state):
        if self.ignored_symbols and data.draw_bits(2) == 3:
            emit = data.draw(st.sampled_from(self.ignored_symbols))
            self.draw_symbol(data, emit, draw_state)

    def calc_has_reusable_values(self, recur):
        return True

def check_explicit(name):
    def inner(value):
        check_type(str, value, "value drawn from " + name)
        return value

    return inner

[docs]@st.cacheable @st.defines_strategy(force_reusable_values=True) def from_lark( grammar: lark.lark.Lark, *, start: Optional[str] = None, explicit: Optional[Dict[str, st.SearchStrategy[str]]] = None, ) -> st.SearchStrategy[str]: """A strategy for strings accepted by the given context-free grammar. ``grammar`` must be a ``Lark`` object, which wraps an EBNF specification. The Lark EBNF grammar reference can be found `here <>`_. ``from_lark`` will automatically generate strings matching the nonterminal ``start`` symbol in the grammar, which was supplied as an argument to the Lark class. To generate strings matching a different symbol, including terminals, you can override this by passing the ``start`` argument to ``from_lark``. Note that Lark may remove unreachable productions when the grammar is compiled, so you should probably pass the same value for ``start`` to both. Currently ``from_lark`` does not support grammars that need custom lexing. Any lexers will be ignored, and any undefined terminals from the use of ``%declare`` will result in generation errors. To define strategies for such terminals, pass a dictionary mapping their name to a corresponding strategy as the ``explicit`` argument. The :pypi:`hypothesmith` project includes a strategy for Python source, based on a grammar and careful post-processing. """ check_type(lark.lark.Lark, grammar, "grammar") if explicit is None: explicit = {} else: check_type(dict, explicit, "explicit") explicit = { k:"explicit[%r]=%r" % (k, v))) for k, v in explicit.items() } return LarkStrategy(grammar, start, explicit)