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PEP 617 -- New PEG parser for CPython

PEP:0617
Title:New PEG parser for CPython
Author:Guido van Rossum <guido at python.org>, Pablo Galindo <pablogsal at python.org>, Lysandros Nikolaou <lisandrosnik at gmail.com>
Discussions-To:Python-Dev <python-dev at python.org>
Status:Accepted
Type:Standards Track
Created:24-March-2020
Python-Version:3.9
Post-History:02-Apr-2020

Overview

This PEP proposes replacing the current LL(1)-based parser of CPython with a new PEG-based parser. This new parser would allow the elimination of multiple "hacks" that exist in the current grammar to circumvent the LL(1)-limitation. It would substantially reduce the maintenance costs in some areas related to the compiling pipeline such as the grammar, the parser and the AST generation. The new PEG parser will also lift the LL(1) restriction on the current Python grammar.

Background on LL(1) parsers

The current Python grammar is an LL(1)-based grammar. A grammar can be said to be LL(1) if it can be parsed by an LL(1) parser, which in turn is defined as a top-down parser that parses the input from left to right, performing leftmost derivation of the sentence, with just one token of lookahead. The traditional approach to constructing or generating an LL(1) parser is to produce a parse table which encodes the possible transitions between all possible states of the parser. These tables are normally constructed from the first sets and the follow sets of the grammar:

  • Given a rule, the first set is the collection of all terminals that can occur first in a full derivation of that rule. Intuitively, this helps the parser decide among the alternatives in a rule. For instance, given the rule

    rule: A | B
    

    if only A can start with the terminal a and only B can start with the terminal b and the parser sees the token b when parsing this rule, it knows that it needs to follow the non-terminal B.

  • An extension to this simple idea is needed when a rule may expand to the empty string. Given a rule, the follow set is the collection of terminals that can appear immediately to the right of that rule in a partial derivation. Intuitively, this solves the problem of the empty alternative. For instance, given this rule:

    rule: A 'b'
    

    if the parser has the token b and the non-terminal A can only start with the token a, then the parser can tell that this is an invalid program. But if A could expand to the empty string (called an ε-production), then the parser would recognise a valid empty A, since the next token b is in the follow set of A.

The current Python grammar does not contain ε-productions, so the follow sets are not needed when creating the parse tables. Currently, in CPython, a parser generator program reads the grammar and produces a parsing table representing a set of deterministic finite automata (DFA) that can be included in a C program, the parser. The parser is a pushdown automaton that uses this data to produce a Concrete Syntax Tree (CST) sometimes known directly as a "parse tree". In this process, the first sets are used indirectly when generating the DFAs.

LL(1) parsers and grammars are usually efficient and simple to implement and generate. However, it is not possible, under the LL(1) restriction, to express certain common constructs in a way natural to the language designer and the reader. This includes some in the Python language.

As LL(1) parsers can only look one token ahead to distinguish possibilities, some rules in the grammar may be ambiguous. For instance the rule:

rule: A | B

is ambiguous if the first sets of both A and B have some elements in common. When the parser sees a token in the input program that both A and B can start with, it is impossible for it to deduce which option to expand, as no further token of the program can be examined to disambiguate. The rule may be transformed to equivalent LL(1) rules, but then it may be harder for a human reader to grasp its meaning. Examples later in this document show that the current LL(1)-based grammar suffers a lot from this scenario.

Another broad class of rules precluded by LL(1) is left-recursive rules. A rule is left-recursive if it can derive to a sentential form with itself as the leftmost symbol. For instance this rule:

rule: rule 'a'

is left-recursive because the rule can be expanded to an expression that starts with itself. As will be described later, left-recursion is the natural way to express certain desired language properties directly in the grammar.

Background on PEG parsers

A PEG (Parsing Expression Grammar) grammar differs from a context-free grammar (like the current one) in the fact that the way it is written more closely reflects how the parser will operate when parsing it. The fundamental technical difference is that the choice operator is ordered. This means that when writing:

rule: A | B | C

a context-free-grammar parser (like an LL(1) parser) will generate constructions that given an input string will deduce which alternative (A, B or C) must be expanded, while a PEG parser will check if the first alternative succeeds and only if it fails, will it continue with the second or the third one in the order in which they are written. This makes the choice operator not commutative.

Unlike LL(1) parsers, PEG-based parsers cannot be ambiguous: if a string parses, it has exactly one valid parse tree. This means that a PEG-based parser cannot suffer from the ambiguity problems described in the previous section.

PEG parsers are usually constructed as a recursive descent parser in which every rule in the grammar corresponds to a function in the program implementing the parser and the parsing expression (the "expansion" or "definition" of the rule) represents the "code" in said function. Each parsing function conceptually takes an input string as its argument, and yields one of the following results:

  • A "success" result. This result indicates that the expression can be parsed by that rule and the function may optionally move forward or consume one or more characters of the input string supplied to it.
  • A "failure" result, in which case no input is consumed.

Notice that "failure" results do not imply that the program is incorrect or a parsing failure because as the choice operator is ordered, a "failure" result merely indicates "try the following option". A direct implementation of a PEG parser as a recursive descent parser will present exponential time performance in the worst case as compared with LL(1) parsers, because PEG parsers have infinite lookahead (this means that they can consider an arbitrary number of tokens before deciding for a rule). Usually, PEG parsers avoid this exponential time complexity with a technique called "packrat parsing" [1] which not only loads the entire program in memory before parsing it but also allows the parser to backtrack arbitrarily. This is made efficient by memoizing the rules already matched for each position. The cost of the memoization cache is that the parser will naturally use more memory than a simple LL(1) parser, which normally are table-based. We will explain later in this document why we consider this cost acceptable.

Rationale

In this section, we describe a list of problems that are present in the current parser machinery in CPython that motivates the need for a new parser.

Some rules are not actually LL(1)

Although the Python grammar is technically an LL(1) grammar (because is parsed by an LL(1) parser) several rules are not LL(1) and several workarounds are implemented in the grammar and in other parts of CPython to deal with this. For example, consider the rule for assignment expressions:

namedexpr_test: NAME [':=' test]

This simple rule is not compatible with the Python grammar as NAME is among the elements of the first set of the rule test. To work around this limitation the actual rule that appears in the current grammar is:

namedexpr_test: test [':=' test]

Which is a much broader rule than the previous one allowing constructs like [x for x in y] := [1,2,3]. The way the rule is limited to its desired form is by disallowing these unwanted constructions when transforming the parse tree to the abstract syntax tree. This is not only inelegant but a considerable maintenance burden as it forces the AST creation routines and the compiler into a situation in which they need to know how to separate valid programs from invalid programs, which should be a responsibility solely of the parser. This also leads to the actual grammar file not reflecting correctly what the actual grammar is (that is, the collection of all valid Python programs).

Similar workarounds appear in multiple other rules of the current grammar. Sometimes this problem is unsolvable. For instance, bpo-12782: Multiple context expressions do not support parentheses for continuation across lines shows how making an LL(1) rule that supports writing:

with (
    open("a_really_long_foo") as foo,
    open("a_really_long_baz") as baz,
    open("a_really_long_bar") as bar
):
  ...

is not possible since the first sets of the grammar items that can appear as context managers include the open parenthesis, making the rule ambiguous. This rule is not only consistent with other parts of the language (like the rule for multiple imports), but is also very useful to auto-formatting tools, as parenthesized groups are normally used to group elements to be formatted together (in the same way the tools operate on the contents of lists, sets...).

Complicated AST parsing

Another problem of the current parser is that there is a huge coupling between the AST generation routines and the particular shape of the produced parse trees. This makes the code for generating the AST especially complicated as many actions and choices are implicit. For instance, the AST generation code knows what alternatives of a certain rule are produced based on the number of child nodes present in a given parse node. This makes the code difficult to follow as this property is not directly related to the grammar file and is influenced by implementation details. As a result of this, a considerable amount of the AST generation code needs to deal with inspecting and reasoning about the particular shape of the parse trees that it receives.

Lack of left recursion

As described previously, a limitation of LL(1) grammars is that they cannot allow left-recursion. This makes writing some rules very unnatural and far from how programmers normally think about the program. For instance this construct (a simpler variation of several rules present in the current grammar):

expr: expr '+' term | term

cannot be parsed by an LL(1) parser. The traditional remedy is to rewrite the grammar to circumvent the problem:

expr: term ('+' term)*

The problem that appears with this form is that the parse tree is forced to have a very unnatural shape. This is because with this rule, for the input program a + b + c the parse tree will be flattened (['a', '+', 'b', '+', 'c']) and must be post-processed to construct a left-recursive parse tree ([['a', '+', 'b'], '+', 'c']). Being forced to write the second rule not only leads to the parse tree not correctly reflecting the desired associativity, but also imposes further pressure on later compilation stages to detect and post-process these cases.

Intermediate parse tree

The last problem present in the current parser is the intermediate creation of a parse tree or Concrete Syntax Tree that is later transformed to an Abstract Syntax Tree. Although the construction of a CST is very common in parser and compiler pipelines, in CPython this intermediate CST is not used by anything else (it is only indirectly exposed by the parser module and a surprisingly small part of the code in the CST production is reused in the module). Which is worse: the whole tree is kept in memory, keeping many branches that consist of chains of nodes with a single child. This has been shown to consume a considerable amount of memory (for instance in bpo-26415: Excessive peak memory consumption by the Python parser).

Having to produce an intermediate result between the grammar and the AST is not only undesirable but also makes the AST generation step much more complicated, raising considerably the maintenance burden.

The new proposed PEG parser

The new proposed PEG parser contains the following pieces:

  • A parser generator that can read a grammar file and produce a PEG parser written in Python or C that can parse said grammar.
  • A PEG meta-grammar that automatically generates a Python parser that is used for the parser generator itself (this means that there are no manually-written parsers).
  • A generated parser (using the parser generator) that can directly produce C and Python AST objects.

Left recursion

PEG parsers normally do not support left recursion but we have implemented a technique similar to the one described in Medeiros et al. [2] but using the memoization cache instead of static variables. This approach is closer to the one described in Warth et al. [3]. This allows us to write not only simple left-recursive rules but also more complicated rules that involve indirect left-recursion like:

rule1: rule2 | 'a'
rule2: rule3 | 'b'
rule3: rule1 | 'c'

and "hidden left-recursion" like:

rule: 'optional'? rule '@' some_other_rule

Syntax

The grammar consists of a sequence of rules of the form:

rule_name: expression

Optionally, a type can be included right after the rule name, which specifies the return type of the C or Python function corresponding to the rule:

rule_name[return_type]: expression

If the return type is omitted, then a void * is returned in C and an Any in Python.

Grammar Expressions

# comment

Python-style comments.

e1 e2

Match e1, then match e2.

rule_name: first_rule second_rule

e1 | e2

Match e1 or e2.

The first alternative can also appear on the line after the rule name for formatting purposes. In that case, a | must be used before the first alternative, like so:

rule_name[return_type]:
    | first_alt
    | second_alt

( e )

Match e.

rule_name: (e)

A slightly more complex and useful example includes using the grouping operator together with the repeat operators:

rule_name: (e1 e2)*

[ e ] or e?

Optionally match e.

rule_name: [e]

A more useful example includes defining that a trailing comma is optional:

rule_name: e (',' e)* [',']

e*

Match zero or more occurrences of e.

rule_name: (e1 e2)*

e+

Match one or more occurrences of e.

rule_name: (e1 e2)+

s.e+

Match one or more occurrences of e, separated by s. The generated parse tree does not include the separator. This is otherwise identical to (e (s e)*).

rule_name: ','.e+

&e

Succeed if e can be parsed, without consuming any input.

!e

Fail if e can be parsed, without consuming any input.

An example taken from the proposed Python grammar specifies that a primary consists of an atom, which is not followed by a . or a ( or a [:

primary: atom !'.' !'(' !'['

~

Commit to the current alternative, even if it fails to parse.

rule_name: '(' ~ some_rule ')' | some_alt

In this example, if a left parenthesis is parsed, then the other alternative won’t be considered, even if some_rule or ‘)’ fail to be parsed.

Variables in the Grammar

A subexpression can be named by preceding it with an identifier and an = sign. The name can then be used in the action (see below), like this:

rule_name[return_type]: '(' a=some_other_rule ')' { a }

Grammar actions

To avoid the intermediate steps that obscure the relationship between the grammar and the AST generation the proposed PEG parser allows directly generating AST nodes for a rule via grammar actions. Grammar actions are language-specific expressions that are evaluated when a grammar rule is successfully parsed. These expressions can be written in Python or C depending on the desired output of the parser generator. This means that if one would want to generate a parser in Python and another in C, two grammar files should be written, each one with a different set of actions, keeping everything else apart from said actions identical in both files. As an example of a grammar with Python actions, the piece of the parser generator that parses grammar files is bootstrapped from a meta-grammar file with Python actions that generate the grammar tree as a result of the parsing.

In the specific case of the new proposed PEG grammar for Python, having actions allows to directly describe how the AST is composed in the grammar itself, making it more clear and maintainable. This AST generation process is supported by the use of some helper functions that factor out common AST object manipulations and some other required operations that are not directly related to the grammar.

To indicate these actions each alternative can be followed by the action code inside curly-braces, which specifies the return value of the alternative:

rule_name[return_type]:
    | first_alt1 first_alt2 { first_alt1 }
    | second_alt1 second_alt2 { second_alt1 }

If the action is omitted and C code is being generated, then there are two different possibilities:

  1. If there’s a single name in the alternative, this gets returned.
  2. If not, a dummy name object gets returned (this case should be avoided).

If the action is omitted and Python code is being generated, then a list with all the parsed expressions gets returned (this is meant for debugging).

The full meta-grammar for the grammars supported by the PEG generator is:

start[Grammar]: grammar ENDMARKER { grammar }

grammar[Grammar]:
    | metas rules { Grammar(rules, metas) }
    | rules { Grammar(rules, []) }

metas[MetaList]:
    | meta metas { [meta] + metas }
    | meta { [meta] }

meta[MetaTuple]:
    | "@" NAME NEWLINE { (name.string, None) }
    | "@" a=NAME b=NAME NEWLINE { (a.string, b.string) }
    | "@" NAME STRING NEWLINE { (name.string, literal_eval(string.string)) }

rules[RuleList]:
    | rule rules { [rule] + rules }
    | rule { [rule] }

rule[Rule]:
    | rulename ":" alts NEWLINE INDENT more_alts DEDENT {
          Rule(rulename[0], rulename[1], Rhs(alts.alts + more_alts.alts)) }
    | rulename ":" NEWLINE INDENT more_alts DEDENT { Rule(rulename[0], rulename[1], more_alts) }
    | rulename ":" alts NEWLINE { Rule(rulename[0], rulename[1], alts) }

rulename[RuleName]:
    | NAME '[' type=NAME '*' ']' {(name.string, type.string+"*")}
    | NAME '[' type=NAME ']' {(name.string, type.string)}
    | NAME {(name.string, None)}

alts[Rhs]:
    | alt "|" alts { Rhs([alt] + alts.alts)}
    | alt { Rhs([alt]) }

more_alts[Rhs]:
    | "|" alts NEWLINE more_alts { Rhs(alts.alts + more_alts.alts) }
    | "|" alts NEWLINE { Rhs(alts.alts) }

alt[Alt]:
    | items '$' action { Alt(items + [NamedItem(None, NameLeaf('ENDMARKER'))], action=action) }
    | items '$' { Alt(items + [NamedItem(None, NameLeaf('ENDMARKER'))], action=None) }
    | items action { Alt(items, action=action) }
    | items { Alt(items, action=None) }

items[NamedItemList]:
    | named_item items { [named_item] + items }
    | named_item { [named_item] }

named_item[NamedItem]:
    | NAME '=' ~ item {NamedItem(name.string, item)}
    | item {NamedItem(None, item)}
    | it=lookahead {NamedItem(None, it)}

lookahead[LookaheadOrCut]:
    | '&' ~ atom {PositiveLookahead(atom)}
    | '!' ~ atom {NegativeLookahead(atom)}
    | '~' {Cut()}

item[Item]:
    | '[' ~ alts ']' {Opt(alts)}
    |  atom '?' {Opt(atom)}
    |  atom '*' {Repeat0(atom)}
    |  atom '+' {Repeat1(atom)}
    |  sep=atom '.' node=atom '+' {Gather(sep, node)}
    |  atom {atom}

atom[Plain]:
    | '(' ~ alts ')' {Group(alts)}
    | NAME {NameLeaf(name.string) }
    | STRING {StringLeaf(string.string)}

# Mini-grammar for the actions

action[str]: "{" ~ target_atoms "}" { target_atoms }

target_atoms[str]:
    | target_atom target_atoms { target_atom + " " + target_atoms }
    | target_atom { target_atom }

target_atom[str]:
    | "{" ~ target_atoms "}" { "{" + target_atoms + "}" }
    | NAME { name.string }
    | NUMBER { number.string }
    | STRING { string.string }
    | "?" { "?" }
    | ":" { ":" }

As an illustrative example this simple grammar file allows to directly generate a full parser that can parse simple arithmetic expressions and that returns a valid C-based Python AST:

start[mod_ty]: a=expr_stmt* $ { Module(a, NULL, p->arena) }
expr_stmt[stmt_ty]: a=expr NEWLINE { _Py_Expr(a, EXTRA) }
expr[expr_ty]:
    | l=expr '+' r=term { _Py_BinOp(l, Add, r, EXTRA) }
    | l=expr '-' r=term { _Py_BinOp(l, Sub, r, EXTRA) }
    | t=term { t }

term[expr_ty]:
    | l=term '*' r=factor { _Py_BinOp(l, Mult, r, EXTRA) }
    | l=term '/' r=factor { _Py_BinOp(l, Div, r, EXTRA) }
    | f=factor { f }

factor[expr_ty]:
    | '(' e=expr ')' { e }
    | a=atom { a }

atom[expr_ty]:
    | n=NAME { n }
    | n=NUMBER { n }
    | s=STRING { s }

Here EXTRA is a macro that expands to start_lineno, start_col_offset, end_lineno, end_col_offset, p->arena, those being variables automatically injected by the parser; p points to an object that holds on to all state for the parser.

A similar grammar written to target Python AST objects:

start: expr NEWLINE? ENDMARKER { ast.Expression(expr) }
expr:
    | expr '+' term { ast.BinOp(expr, ast.Add(), term) }
    | expr '-' term { ast.BinOp(expr, ast.Sub(), term) }
    | term { term }

term:
    | l=term '*' r=factor { ast.BinOp(l, ast.Mult(), r) }
    | term '/' factor { ast.BinOp(term, ast.Div(), factor) }
    | factor { factor }

factor:
    | '(' expr ')' { expr }
    | atom { atom }

atom:
    | NAME { ast.Name(id=name.string, ctx=ast.Load()) }
    | NUMBER { ast.Constant(value=ast.literal_eval(number.string)) }

Migration plan

This section describes the migration plan when porting to the new PEG-based parser if this PEP is accepted. The migration will be executed in a series of steps that allow initially to fallback to the previous parser if needed:

  1. Starting with Python 3.9 alpha 6, include the new PEG-based parser machinery in CPython with a command-line flag and environment variable that allows switching between the new and the old parsers together with explicit APIs that allow invoking the new and the old parsers independently. At this step, all Python APIs like ast.parse and compile will use the parser set by the flags or the environment variable and the default parser will be the new PEG-based parser.
  2. Between Python 3.9 and Python 3.10, the old parser and related code (like the "parser" module) will be kept until a new Python release happens (Python 3.10). In the meanwhile and until the old parser is removed, no new Python Grammar addition will be added that requires the PEG parser. This means that the grammar will be kept LL(1) until the old parser is removed.
  3. In Python 3.10, remove the old parser, the command-line flag, the environment variable and the "parser" module and related code.

Performance and validation

We have done extensive timing and validation of the new parser, and this gives us confidence that the new parser is of high enough quality to replace the current parser.

Validation

To start with validation, we regularly compile the entire Python 3.8 stdlib and compare every aspect of the resulting AST with that produced by the standard compiler. (In the process we found a few bugs in the standard parser's treatment of line and column numbers, which we have all fixed upstream via a series of issues and PRs.)

We have also occasionally compiled a much larger codebase (the approx. 3800 most popular packages on PyPI) and this has helped us find a (very) few additional bugs in the new parser.

(One area we have not explored extensively is rejection of all wrong programs. We have unit tests that check for a certain number of explicit rejections, but more work could be done, e.g. by using a fuzzer that inserts random subtle bugs into existing code. We're open to help in this area.)

Performance

We have tuned the performance of the new parser to come within 10% of the current parser both in speed and memory consumption. While the PEG/packrat parsing algorithm inherently consumes more memory than the current LL(1) parser, we have an advantage because we don't construct an intermediate CST.

Below are some benchmarks. These are focused on compiling source code to bytecode, because this is the most realistic situation. Returning an AST to Python code is not as representative, because the process to convert the internal AST (only accessible to C code) to an external AST (an instance of ast.AST) takes more time than the parser itself.

All measurements reported here are done on a recent MacBook Pro, taking the median of three runs. No particular care was taken to stop other applications running on the same machine.

The first timings are for our canonical test file, which has 100,000 lines endlessly repeating the following three lines:

1 + 2 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + ((((((11 * 12 * 13 * 14 * 15 + 16 * 17 + 18 * 19 * 20))))))
2*3 + 4*5*6
12 + (2 * 3 * 4 * 5 + 6 + 7 * 8)
  • Just parsing and throwing away the internal AST takes 1.16 seconds with a max RSS of 681 MiB.
  • Parsing and converting to ast.AST takes 6.34 seconds, max RSS 1029 MiB.
  • Parsing and compiling to bytecode takes 1.28 seconds, max RSS 681 MiB.
  • With the current parser, parsing and compiling takes 1.44 seconds, max RSS 836 MiB.

For this particular test file, the new parser is faster and uses less memory than the current parser (compare the last two bullets).

We also did timings with a more realistic payload, the entire Python 3.8 stdlib. This payload consists of 1,641 files, 749,570 lines, 27,622,497 bytes. (Though 11 files can't be compiled by any Python 3 parser due to encoding issues, sometimes intentional.)

  • Compiling and throwing away the internal AST took 2.141 seconds. That's 350,040 lines/sec, or 12,899,367 bytes/sec. The max RSS was 74 MiB (the largest file in the stdlib is much smaller than out canonical test file).
  • Compiling to bytecode took 3.290 seconds. That's 227,861 lines/sec, or 8,396,942 bytes/sec. Max RSS 77 MiB.
  • Compiling to bytecode using the current parser took 3.367 seconds. That's 222,620 lines/sec, or 8,203,780 bytes/sec. Max RSS 70 MiB.

Comparing the last two bullets we find that the new parser is slightly faster but uses slightly (about 10%) more memory. We believe this is acceptable. (Also, there are probably some more tweaks we can make to reduce memory usage.)

Rejected Alternatives

We did not seriously consider alternative ways to implement the new parser, but here's a brief discussion of LALR(1).

Thirty years ago the first author decided to go his own way with Python's parser rather than using LALR(1), which was the industry standard at the time (e.g. Bison and Yacc). The reasons were primarily emotional (gut feelings, intuition), based on past experience using Yacc in other projects, where grammar development took more effort than anticipated (in part due to shift-reduce conflicts). A specific criticism of Bison and Yacc that still holds is that their meta-grammar (the notation used to feed the grammar into the parser generator) does not support EBNF conveniences like [optional_clause] or (repeated_clause)*. Using a custom parser generator, a syntax tree matching the structure of the grammar could be generated automatically, and with EBNF that tree could match the "human-friendly" structure of the grammar.

Other variants of LR were not considered, nor was LL (e.g. ANTLR). PEG was selected because it was easy to understand given a basic understanding of recursive-descent parsing.

Source: https://github.com/python/peps/blob/master/pep-0617.rst