how this parsing engine works.
"""
+from contextlib import contextmanager
# Local imports
-from . import token
+from . import grammar, token, tokenize
from typing import (
+ cast,
+ Any,
Optional,
- Text,
Union,
Tuple,
Dict,
List,
+ Iterator,
Callable,
Set,
+ TYPE_CHECKING,
)
from blib2to3.pgen2.grammar import Grammar
-from blib2to3.pytree import NL, Context, RawNode, Leaf, Node
+from blib2to3.pytree import convert, NL, Context, RawNode, Leaf, Node
+if TYPE_CHECKING:
+ from blib2to3.pgen2.driver import TokenProxy
-Results = Dict[Text, NL]
+
+Results = Dict[str, NL]
Convert = Callable[[Grammar, RawNode], Union[Node, Leaf]]
DFA = List[List[Tuple[int, int]]]
DFAS = Tuple[DFA, Dict[int, int]]
return Node(type=node[0], children=node[3], context=node[2])
+# A placeholder node, used when parser is backtracking.
+DUMMY_NODE = (-1, None, None, None)
+
+
+def stack_copy(
+ stack: List[Tuple[DFAS, int, RawNode]]
+) -> List[Tuple[DFAS, int, RawNode]]:
+ """Nodeless stack copy."""
+ return [(dfa, label, DUMMY_NODE) for dfa, label, _ in stack]
+
+
+class Recorder:
+ def __init__(self, parser: "Parser", ilabels: List[int], context: Context) -> None:
+ self.parser = parser
+ self._ilabels = ilabels
+ self.context = context # not really matter
+
+ self._dead_ilabels: Set[int] = set()
+ self._start_point = self.parser.stack
+ self._points = {ilabel: stack_copy(self._start_point) for ilabel in ilabels}
+
+ @property
+ def ilabels(self) -> Set[int]:
+ return self._dead_ilabels.symmetric_difference(self._ilabels)
+
+ @contextmanager
+ def switch_to(self, ilabel: int) -> Iterator[None]:
+ with self.backtrack():
+ self.parser.stack = self._points[ilabel]
+ try:
+ yield
+ except ParseError:
+ self._dead_ilabels.add(ilabel)
+ finally:
+ self.parser.stack = self._start_point
+
+ @contextmanager
+ def backtrack(self) -> Iterator[None]:
+ """
+ Use the node-level invariant ones for basic parsing operations (push/pop/shift).
+ These still will operate on the stack; but they won't create any new nodes, or
+ modify the contents of any other existing nodes.
+
+ This saves us a ton of time when we are backtracking, since we
+ want to restore to the initial state as quick as possible, which
+ can only be done by having as little mutatations as possible.
+ """
+ is_backtracking = self.parser.is_backtracking
+ try:
+ self.parser.is_backtracking = True
+ yield
+ finally:
+ self.parser.is_backtracking = is_backtracking
+
+ def add_token(self, tok_type: int, tok_val: str, raw: bool = False) -> None:
+ func: Callable[..., Any]
+ if raw:
+ func = self.parser._addtoken
+ else:
+ func = self.parser.addtoken
+
+ for ilabel in self.ilabels:
+ with self.switch_to(ilabel):
+ args = [tok_type, tok_val, self.context]
+ if raw:
+ args.insert(0, ilabel)
+ func(*args)
+
+ def determine_route(self, value: Optional[str] = None, force: bool = False) -> Optional[int]:
+ alive_ilabels = self.ilabels
+ if len(alive_ilabels) == 0:
+ *_, most_successful_ilabel = self._dead_ilabels
+ raise ParseError("bad input", most_successful_ilabel, value, self.context)
+
+ ilabel, *rest = alive_ilabels
+ if force or not rest:
+ return ilabel
+ else:
+ return None
+
+
class ParseError(Exception):
"""Exception to signal the parser is stuck."""
def __init__(
- self, msg: Text, type: Optional[int], value: Optional[Text], context: Context
+ self, msg: str, type: Optional[int], value: Optional[str], context: Context
) -> None:
Exception.__init__(
- self, "%s: type=%r, value=%r, context=%r" % (msg, type, value, context)
+ self, f"{msg}: type={type!r}, value={value!r}, context={context!r}"
)
self.msg = msg
self.type = type
self.context = context
-class Parser(object):
+class Parser:
"""Parser engine.
The proper usage sequence is:
to be converted. The syntax tree is converted from the bottom
up.
+ **post-note: the convert argument is ignored since for Black's
+ usage, convert will always be blib2to3.pytree.convert. Allowing
+ this to be dynamic hurts mypyc's ability to use early binding.
+ These docs are left for historical and informational value.
+
A concrete syntax tree node is a (type, value, context, nodes)
tuple, where type is the node type (a token or symbol number),
value is None for symbols and a string for tokens, context is
"""
self.grammar = grammar
+ # See note in docstring above. TL;DR this is ignored.
self.convert = convert or lam_sub
+ self.is_backtracking = False
- def setup(self, start: Optional[int] = None) -> None:
+ def setup(self, proxy: "TokenProxy", start: Optional[int] = None) -> None:
"""Prepare for parsing.
This *must* be called before starting to parse.
self.stack: List[Tuple[DFAS, int, RawNode]] = [stackentry]
self.rootnode: Optional[NL] = None
self.used_names: Set[str] = set()
+ self.proxy = proxy
- def addtoken(self, type: int, value: Optional[Text], context: Context) -> bool:
+ def addtoken(self, type: int, value: str, context: Context) -> bool:
"""Add a token; return True iff this is the end of the program."""
# Map from token to label
- ilabel = self.classify(type, value, context)
+ ilabels = self.classify(type, value, context)
+ assert len(ilabels) >= 1
+
+ # If we have only one state to advance, we'll directly
+ # take it as is.
+ if len(ilabels) == 1:
+ [ilabel] = ilabels
+ return self._addtoken(ilabel, type, value, context)
+
+ # If there are multiple states which we can advance (only
+ # happen under soft-keywords), then we will try all of them
+ # in parallel and as soon as one state can reach further than
+ # the rest, we'll choose that one. This is a pretty hacky
+ # and hopefully temporary algorithm.
+ #
+ # For a more detailed explanation, check out this post:
+ # https://tree.science/what-the-backtracking.html
+
+ with self.proxy.release() as proxy:
+ counter, force = 0, False
+ recorder = Recorder(self, ilabels, context)
+ recorder.add_token(type, value, raw=True)
+
+ next_token_value = value
+ while recorder.determine_route(next_token_value) is None:
+ if not proxy.can_advance(counter):
+ force = True
+ break
+
+ next_token_type, next_token_value, *_ = proxy.eat(counter)
+ if next_token_type in (tokenize.COMMENT, tokenize.NL):
+ counter += 1
+ continue
+
+ if next_token_type == tokenize.OP:
+ next_token_type = grammar.opmap[next_token_value]
+
+ recorder.add_token(next_token_type, next_token_value)
+ counter += 1
+
+ ilabel = cast(int, recorder.determine_route(next_token_value, force=force))
+ assert ilabel is not None
+
+ return self._addtoken(ilabel, type, value, context)
+
+ def _addtoken(self, ilabel: int, type: int, value: str, context: Context) -> bool:
# Loop until the token is shifted; may raise exceptions
while True:
dfa, state, node = self.stack[-1]
arcs = states[state]
# Look for a state with this label
for i, newstate in arcs:
- t, v = self.grammar.labels[i]
- if ilabel == i:
+ t = self.grammar.labels[i][0]
+ if t >= 256:
+ # See if it's a symbol and if we're in its first set
+ itsdfa = self.grammar.dfas[t]
+ itsstates, itsfirst = itsdfa
+ if ilabel in itsfirst:
+ # Push a symbol
+ self.push(t, itsdfa, newstate, context)
+ break # To continue the outer while loop
+
+ elif ilabel == i:
# Look it up in the list of labels
- assert t < 256
# Shift a token; we're done with it
self.shift(type, value, newstate, context)
# Pop while we are in an accept-only state
states, first = dfa
# Done with this token
return False
- elif t >= 256:
- # See if it's a symbol and if we're in its first set
- itsdfa = self.grammar.dfas[t]
- itsstates, itsfirst = itsdfa
- if ilabel in itsfirst:
- # Push a symbol
- self.push(t, self.grammar.dfas[t], newstate, context)
- break # To continue the outer while loop
+
else:
if (0, state) in arcs:
# An accepting state, pop it and try something else
# No success finding a transition
raise ParseError("bad input", type, value, context)
- def classify(self, type: int, value: Optional[Text], context: Context) -> int:
- """Turn a token into a label. (Internal)"""
+ def classify(self, type: int, value: str, context: Context) -> List[int]:
+ """Turn a token into a label. (Internal)
+
+ Depending on whether the value is a soft-keyword or not,
+ this function may return multiple labels to choose from."""
if type == token.NAME:
# Keep a listing of all used names
- assert value is not None
self.used_names.add(value)
# Check for reserved words
- ilabel = self.grammar.keywords.get(value)
- if ilabel is not None:
- return ilabel
+ if value in self.grammar.keywords:
+ return [self.grammar.keywords[value]]
+ elif value in self.grammar.soft_keywords:
+ assert type in self.grammar.tokens
+ return [
+ self.grammar.soft_keywords[value],
+ self.grammar.tokens[type],
+ ]
+
ilabel = self.grammar.tokens.get(type)
if ilabel is None:
raise ParseError("bad token", type, value, context)
- return ilabel
+ return [ilabel]
- def shift(
- self, type: int, value: Optional[Text], newstate: int, context: Context
- ) -> None:
+ def shift(self, type: int, value: str, newstate: int, context: Context) -> None:
"""Shift a token. (Internal)"""
- dfa, state, node = self.stack[-1]
- assert value is not None
- assert context is not None
- rawnode: RawNode = (type, value, context, None)
- newnode = self.convert(self.grammar, rawnode)
- if newnode is not None:
+ if self.is_backtracking:
+ dfa, state, _ = self.stack[-1]
+ self.stack[-1] = (dfa, newstate, DUMMY_NODE)
+ else:
+ dfa, state, node = self.stack[-1]
+ rawnode: RawNode = (type, value, context, None)
+ newnode = convert(self.grammar, rawnode)
assert node[-1] is not None
node[-1].append(newnode)
- self.stack[-1] = (dfa, newstate, node)
+ self.stack[-1] = (dfa, newstate, node)
def push(self, type: int, newdfa: DFAS, newstate: int, context: Context) -> None:
"""Push a nonterminal. (Internal)"""
- dfa, state, node = self.stack[-1]
- newnode: RawNode = (type, None, context, [])
- self.stack[-1] = (dfa, newstate, node)
- self.stack.append((newdfa, 0, newnode))
+ if self.is_backtracking:
+ dfa, state, _ = self.stack[-1]
+ self.stack[-1] = (dfa, newstate, DUMMY_NODE)
+ self.stack.append((newdfa, 0, DUMMY_NODE))
+ else:
+ dfa, state, node = self.stack[-1]
+ newnode: RawNode = (type, None, context, [])
+ self.stack[-1] = (dfa, newstate, node)
+ self.stack.append((newdfa, 0, newnode))
def pop(self) -> None:
"""Pop a nonterminal. (Internal)"""
- popdfa, popstate, popnode = self.stack.pop()
- newnode = self.convert(self.grammar, popnode)
- if newnode is not None:
+ if self.is_backtracking:
+ self.stack.pop()
+ else:
+ popdfa, popstate, popnode = self.stack.pop()
+ newnode = convert(self.grammar, popnode)
if self.stack:
dfa, state, node = self.stack[-1]
assert node[-1] is not None
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