Pegen is the parser generator used in CPython to produce the parser used by the interpreter. It allows to produce PEG parsers from a description of a formal Grammar.
Install with pip or your favorite PyPi package manager.
pip install pegen
The documentation is available here.
Given a grammar file compatible with pegen (you can write your own or start with one in the data directory), you
can easily generate a parser by running:
python -m pegen <path-to-grammar-file> -o parser.py
This will generate a file called parser.py in the current directory. This can be used to parse code using the grammar that
we just used:
python parser.py <file-with-code-to-parse>
As a demo: generate a Python parser from data/python.gram, and use the generated parser to parse and run tests/demo.py
make demo
See the instructions in the CONTRIBUTING.md file.
This repository exists to distribute a version of the Python PEG parser generator used by CPython that can be installed via PyPI, with some improvements. Although the official PEG generator included in CPython can generate both Python and C code, this distribution of the generator only allows to generate Python code. This is due to the fact that the C code generated by the generator included in CPython includes a lot of implementation details and private headers that are not available for general usage.
The official PEG generator for Python 3.9 and later is now included in the CPython repo under
Tools/peg_generator/. We aim to keep this repo in sync with the
Python generator from that version of pegen.
See also PEP 617.
- The
srcdirectory contains thepegensource (the package itself). - The
testsdirectory contains the test suite for thepegenpackage. - The
datadirectory contains some example grammars compatible withpegen. This includes a pure-Python version of the Python grammar. - The
docsdirectory contains the documentation for the package. - The
scriptsdirectory contains some useful scripts that can be used for visualizing grammars, benchmarking and other usages relevant to the development of the generator itself. - The
storiesdirectory contains the backing files and examples for Guido's series on PEG parser.
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 Python function corresponding to the rule:
rule_name[return_type]: expression
If the return type is omitted, then Any is returned.
Python-style comments.
Match e1, then match e2.
rule_name: first_rule second_rule
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
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)*
Optionally match e.
rule_name: [e]
A more useful example includes defining that a trailing comma is optional:
rule_name: e (',' e)* [',']
Match zero or more occurrences of e.
rule_name: (e1 e2)*
Match one or more occurrences of e.
rule_name: (e1 e2)+
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+
Succeed if e can be parsed, without consuming any input.
Fail if e can be parsed, without consuming any input.
An example taken from the 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.
PEG parsers normally do not support left recursion but Pegen implements a technique that allows left recursion using the memoization cache. 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
A sub-expression 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 }
To avoid the intermediate steps that obscure the relationship between the grammar and the AST generation the 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. 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 PEG grammar for Python, having actions allows directly describing 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 ommited, a default action is generated:
-
If there's a single name in the rule in the rule, it gets returned.
-
If there is more than one name in the rule, a collection with all parsed expressions gets returned.
This default behaviour is primarily made for very simple situations and for debugging purposes.
As an illustrative example this simple grammar file allows directly generating a full parser that can parse simple arithmetic expressions and that returns a valid Python AST:
start[ast.Module]: a=expr_stmt* ENDMARKER { ast.Module(body=a or []) }
expr_stmt: a=expr NEWLINE { ast.Expr(value=a, EXTRA) }
expr:
| l=expr '+' r=term { ast.BinOp(left=l, op=ast.Add(), right=r, EXTRA) }
| l=expr '-' r=term { ast.BinOp(left=l, op=ast.Sub(), right=r, EXTRA) }
| term
term:
| l=term '*' r=factor { ast.BinOp(left=l, op=ast.Mult(), right=r, EXTRA) }
| l=term '/' r=factor { ast.BinOp(left=l, op=ast.Div(), right=r, EXTRA) }
| factor
factor:
| '(' e=expr ')' { e }
| atom
atom:
| NAME
| NUMBER
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