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# Parser for ASDL [1] definition files. Reads in an ASDL description and parses
# it into an AST that describes it.
# The EBNF we're parsing here: Figure 1 of the paper [1]. Extended to support
# modules and attributes after a product. Words starting with Capital letters
# are terminals. Literal tokens are in "double quotes". Others are
# non-terminals. Id is either TokenId or ConstructorId.
# module ::= "module" Id "{" [definitions] "}"
# definitions ::= { TypeId "=" type }
# type ::= product | sum
# product ::= fields ["attributes" fields]
# fields ::= "(" { field, "," } field ")"
# field ::= TypeId ["?" | "*"] [Id]
# sum ::= constructor { "|" constructor } ["attributes" fields]
# constructor ::= ConstructorId [fields]
# [1] "The Zephyr Abstract Syntax Description Language" by Wang, et. al. See
from collections import namedtuple
import re
__all__ = [
'builtin_types', 'parse', 'AST', 'Module', 'Type', 'Constructor',
'Field', 'Sum', 'Product', 'VisitorBase', 'Check', 'check']
# The following classes define nodes into which the ASDL description is parsed.
# Note: this is a "meta-AST". ASDL files (such as Python.asdl) describe the AST
# structure used by a programming language. But ASDL files themselves need to be
# parsed. This module parses ASDL files and uses a simple AST to represent them.
# See the EBNF at the top of the file to understand the logical connection
# between the various node types.
builtin_types = {'identifier', 'string', 'bytes', 'int', 'object', 'singleton',
class AST:
def __repr__(self):
raise NotImplementedError
class Module(AST):
def __init__(self, name, dfns): = name
self.dfns = dfns
self.types = { type.value for type in dfns}
def __repr__(self):
return 'Module({}, {0.dfns})'.format(self)
class Type(AST):
def __init__(self, name, value): = name
self.value = value
def __repr__(self):
return 'Type({}, {0.value})'.format(self)
class Constructor(AST):
def __init__(self, name, fields=None): = name
self.fields = fields or []
def __repr__(self):
return 'Constructor({}, {0.fields})'.format(self)
class Field(AST):
def __init__(self, type, name=None, seq=False, opt=False):
self.type = type = name
self.seq = seq
self.opt = opt
def __repr__(self):
if self.seq:
extra = ", seq=True"
elif self.opt:
extra = ", opt=True"
extra = ""
if is None:
return 'Field({0.type}{1})'.format(self, extra)
return 'Field({0.type}, {}{1})'.format(self, extra)
class Sum(AST):
def __init__(self, types, attributes=None):
self.types = types
self.attributes = attributes or []
def __repr__(self):
if self.attributes:
return 'Sum({0.types}, {0.attributes})'.format(self)
return 'Sum({0.types})'.format(self)
class Product(AST):
def __init__(self, fields, attributes=None):
self.fields = fields
self.attributes = attributes or []
def __repr__(self):
if self.attributes:
return 'Product({0.fields}, {0.attributes})'.format(self)
return 'Product({0.fields})'.format(self)
# A generic visitor for the meta-AST that describes ASDL. This can be used by
# emitters. Note that this visitor does not provide a generic visit method, so a
# subclass needs to define visit methods from visitModule to as deep as the
# interesting node.
# We also define a Check visitor that makes sure the parsed ASDL is well-formed.
class VisitorBase(object):
"""Generic tree visitor for ASTs."""
def __init__(self):
self.cache = {}
def visit(self, obj, *args):
klass = obj.__class__
meth = self.cache.get(klass)
if meth is None:
methname = "visit" + klass.__name__
meth = getattr(self, methname, None)
self.cache[klass] = meth
if meth:
meth(obj, *args)
except Exception as e:
print("Error visiting %r: %s" % (obj, e))
class Check(VisitorBase):
"""A visitor that checks a parsed ASDL tree for correctness.
Errors are printed and accumulated.
def __init__(self):
super(Check, self).__init__()
self.cons = {}
self.errors = 0
self.types = {}
def visitModule(self, mod):
for dfn in mod.dfns:
def visitType(self, type):
self.visit(type.value, str(
def visitSum(self, sum, name):
for t in sum.types:
self.visit(t, name)
def visitConstructor(self, cons, name):
key = str(
conflict = self.cons.get(key)
if conflict is None:
self.cons[key] = name
print('Redefinition of constructor {}'.format(key))
print('Defined in {} and {}'.format(conflict, name))
self.errors += 1
for f in cons.fields:
self.visit(f, key)
def visitField(self, field, name):
key = str(field.type)
l = self.types.setdefault(key, [])
def visitProduct(self, prod, name):
for f in prod.fields:
self.visit(f, name)
def check(mod):
"""Check the parsed ASDL tree for correctness.
Return True if success. For failure, the errors are printed out and False
is returned.
v = Check()
for t in v.types:
if t not in mod.types and not t in builtin_types:
v.errors += 1
uses = ", ".join(v.types[t])
print('Undefined type {}, used in {}'.format(t, uses))
return not v.errors
# The ASDL parser itself comes next. The only interesting external interface
# here is the top-level parse function.
def parse(filename):
"""Parse ASDL from the given file and return a Module node describing it."""
with open(filename) as f:
parser = ASDLParser()
return parser.parse(
# Types for describing tokens in an ASDL specification.
class TokenKind:
"""TokenKind is provides a scope for enumerated token kinds."""
(ConstructorId, TypeId, Equals, Comma, Question, Pipe, Asterisk,
LParen, RParen, LBrace, RBrace) = range(11)
operator_table = {
'=': Equals, ',': Comma, '?': Question, '|': Pipe, '(': LParen,
')': RParen, '*': Asterisk, '{': LBrace, '}': RBrace}
Token = namedtuple('Token', 'kind value lineno')
class ASDLSyntaxError(Exception):
def __init__(self, msg, lineno=None):
self.msg = msg
self.lineno = lineno or '<unknown>'
def __str__(self):
return 'Syntax error on line {0.lineno}: {0.msg}'.format(self)
def tokenize_asdl(buf):
"""Tokenize the given buffer. Yield Token objects."""
for lineno, line in enumerate(buf.splitlines(), 1):
for m in re.finditer(r'\s*(\w+|--.*|.)', line.strip()):
c =
if c[0].isalpha():
# Some kind of identifier
if c[0].isupper():
yield Token(TokenKind.ConstructorId, c, lineno)
yield Token(TokenKind.TypeId, c, lineno)
elif c[:2] == '--':
# Comment
# Operators
op_kind = TokenKind.operator_table[c]
except KeyError:
raise ASDLSyntaxError('Invalid operator %s' % c, lineno)
yield Token(op_kind, c, lineno)
class ASDLParser:
"""Parser for ASDL files.
Create, then call the parse method on a buffer containing ASDL.
This is a simple recursive descent parser that uses tokenize_asdl for the
def __init__(self):
self._tokenizer = None
self.cur_token = None
def parse(self, buf):
"""Parse the ASDL in the buffer and return an AST with a Module root.
self._tokenizer = tokenize_asdl(buf)
return self._parse_module()
def _parse_module(self):
if self._at_keyword('module'):
raise ASDLSyntaxError(
'Expected "module" (found {})'.format(self.cur_token.value),
name = self._match(self._id_kinds)
defs = self._parse_definitions()
return Module(name, defs)
def _parse_definitions(self):
defs = []
while self.cur_token.kind == TokenKind.TypeId:
typename = self._advance()
type = self._parse_type()
defs.append(Type(typename, type))
return defs
def _parse_type(self):
if self.cur_token.kind == TokenKind.LParen:
# If we see a (, it's a product
return self._parse_product()
# Otherwise it's a sum. Look for ConstructorId
sumlist = [Constructor(self._match(TokenKind.ConstructorId),
while self.cur_token.kind == TokenKind.Pipe:
# More constructors
return Sum(sumlist, self._parse_optional_attributes())
def _parse_product(self):
return Product(self._parse_fields(), self._parse_optional_attributes())
def _parse_fields(self):
fields = []
while self.cur_token.kind == TokenKind.TypeId:
typename = self._advance()
is_seq, is_opt = self._parse_optional_field_quantifier()
id = (self._advance() if self.cur_token.kind in self._id_kinds
else None)
fields.append(Field(typename, id, seq=is_seq, opt=is_opt))
if self.cur_token.kind == TokenKind.RParen:
elif self.cur_token.kind == TokenKind.Comma:
return fields
def _parse_optional_fields(self):
if self.cur_token.kind == TokenKind.LParen:
return self._parse_fields()
return None
def _parse_optional_attributes(self):
if self._at_keyword('attributes'):
return self._parse_fields()
return None
def _parse_optional_field_quantifier(self):
is_seq, is_opt = False, False
if self.cur_token.kind == TokenKind.Asterisk:
is_seq = True
elif self.cur_token.kind == TokenKind.Question:
is_opt = True
return is_seq, is_opt
def _advance(self):
""" Return the value of the current token and read the next one into
cur_val = None if self.cur_token is None else self.cur_token.value
self.cur_token = next(self._tokenizer)
except StopIteration:
self.cur_token = None
return cur_val
_id_kinds = (TokenKind.ConstructorId, TokenKind.TypeId)
def _match(self, kind):
"""The 'match' primitive of RD parsers.
* Verifies that the current token is of the given kind (kind can
be a tuple, in which the kind must match one of its members).
* Returns the value of the current token
* Reads in the next token
if (isinstance(kind, tuple) and self.cur_token.kind in kind or
self.cur_token.kind == kind
value = self.cur_token.value
return value
raise ASDLSyntaxError(
'Unmatched {} (found {})'.format(kind, self.cur_token.kind),
def _at_keyword(self, keyword):
return (self.cur_token.kind == TokenKind.TypeId and
self.cur_token.value == keyword)