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SLY (Sly Lex-Yacc)

SLY is a 100% Python implementation of the lex and yacc tools commonly used to write parsers and compilers. Parsing is based on the same LALR(1) algorithm used by many yacc tools. Here are a few notable features:

  • SLY provides very extensive error reporting and diagnostic information to assist in parser construction. The original implementation was developed for instructional purposes. As a result, the system tries to identify the most common types of errors made by novice users.
  • SLY provides full support for empty productions, error recovery, precedence specifiers, and moderately ambiguous grammars.
  • SLY uses various Python metaprogramming features to specify lexers and parsers. There are no generated files or extra steps involved. You simply write Python code and run it.
  • SLY can be used to build parsers for "real" programming languages. Although it is not ultra-fast due to its Python implementation, SLY can be used to parse grammars consisting of several hundred rules (as might be found for a language like C).

SLY originates from the PLY project. However, it's been modernized a bit. In fact, don't expect any code previously written for PLY to work. That said, most of the things that were possible in PLY are also possible in SLY.

SLY is a modern library for performing lexing and parsing. It implements the LALR(1) parsing algorithm, commonly used for parsing and compiling various programming languages.


SLY requires the use of Python 3.6 or greater. Older versions of Python are not supported.

An Example

SLY is probably best illustrated by an example. Here's what it looks like to write a parser that can evaluate simple arithmetic expressions and store variables:

# -----------------------------------------------------------------------------
# -----------------------------------------------------------------------------

from sly import Lexer, Parser

class CalcLexer(Lexer):
    ignore = ' \t'

    # Tokens
    NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
    NUMBER = r'\d+'

    # Special symbols
    PLUS = r'\+'
    MINUS = r'-'
    TIMES = r'\*'
    DIVIDE = r'/'
    ASSIGN = r'='
    LPAREN = r'\('
    RPAREN = r'\)'

    # Ignored pattern
    ignore_newline = r'\n+'

    # Extra action for newlines
    def ignore_newline(self, t):
        self.lineno += t.value.count('\n')

    def error(self, t):
        print("Illegal character '%s'" % t.value[0])
        self.index += 1

class CalcParser(Parser):
    tokens = CalcLexer.tokens

    precedence = (
        ('left', PLUS, MINUS),
        ('left', TIMES, DIVIDE),
        ('right', UMINUS),

    def __init__(self):
        self.names = { }

    @_('NAME ASSIGN expr')
    def statement(self, p):
        self.names[p.NAME] = p.expr

    def statement(self, p):

    @_('expr PLUS expr')
    def expr(self, p):
        return p.expr0 + p.expr1

    @_('expr MINUS expr')
    def expr(self, p):
        return p.expr0 - p.expr1

    @_('expr TIMES expr')
    def expr(self, p):
        return p.expr0 * p.expr1

    @_('expr DIVIDE expr')
    def expr(self, p):
        return p.expr0 / p.expr1

    @_('MINUS expr %prec UMINUS')
    def expr(self, p):
        return -p.expr

    @_('LPAREN expr RPAREN')
    def expr(self, p):
        return p.expr

    def expr(self, p):
        return int(p.NUMBER)

    def expr(self, p):
            return self.names[p.NAME]
        except LookupError:
            print(f'Undefined name {p.NAME!r}')
            return 0

if __name__ == '__main__':
    lexer = CalcLexer()
    parser = CalcParser()
    while True:
            text = input('calc > ')
        except EOFError:
        if text:


Further documentation can be found at



For a detailed overview of parsing theory, consult the excellent book "Compilers : Principles, Techniques, and Tools" by Aho, Sethi, and Ullman. The topics found in "Lex & Yacc" by Levine, Mason, and Brown may also be useful.

The GitHub page for SLY can be found at:

Please direct bug reports and pull requests to the GitHub page. To contact me directly, send email to or contact me on Twitter (@dabeaz).

-- Dave


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