# GardenSnake - a parser generator demonstration program
#
# This implements a modified version of a subset of Python:
# - only 'def', 'return' and 'if' statements
# - 'if' only has 'then' clause (no elif nor else)
# - single-quoted strings only, content in raw format
# - numbers are decimal.Decimal instances (not integers or floats)
# - no print statment; use the built-in 'print' function
# - only < > == + - / * implemented (and unary + -)
# - assignment and tuple assignment work
# - no generators of any sort
# - no ... well, no quite a lot
# Why? I'm thinking about a new indentation-based configuration
# language for a project and wanted to figure out how to do it. Once
# I got that working I needed a way to test it out. My original AST
# was dumb so I decided to target Python's AST and compile it into
# Python code. Plus, it's pretty cool that it only took a day or so
# from sitting down with Ply to having working code.
# This uses David Beazley's Ply from http://www.dabeaz.com/ply/
# This work is hereby released into the Public Domain. To view a copy of
# the public domain dedication, visit
# http://creativecommons.org/licenses/publicdomain/ or send a letter to
# Creative Commons, 543 Howard Street, 5th Floor, San Francisco,
# California, 94105, USA.
#
# Portions of this work are derived from Python's Grammar definition
# and may be covered under the Python copyright and license
#
# Andrew Dalke / Dalke Scientific Software, LLC
# 30 August 2006 / Cape Town, South Africa
# Changelog:
# 30 August - added link to CC license; removed the "swapcase" encoding
# Modifications for inclusion in PLY distribution
import sys
sys.path.insert(0,"../..")
from ply import *
##### Lexer ######
#import lex
import decimal
tokens = (
'DEF',
'IF',
'NAME',
'NUMBER', # Python decimals
'STRING', # single quoted strings only; syntax of raw strings
'LPAR',
'RPAR',
'COLON',
'EQ',
'ASSIGN',
'LT',
'GT',
'PLUS',
'MINUS',
'MULT',
'DIV',
'RETURN',
'WS',
'NEWLINE',
'COMMA',
'SEMICOLON',
'INDENT',
'DEDENT',
'ENDMARKER',
)
#t_NUMBER = r'\d+'
# taken from decmial.py but without the leading sign
def t_NUMBER(t):
r"""(\d+(\.\d*)?|\.\d+)([eE][-+]? \d+)?"""
t.value = decimal.Decimal(t.value)
return t
def t_STRING(t):
r"'([^\\']+|\\'|\\\\)*'" # I think this is right ...
t.value=t.value[1:-1].decode("string-escape") # .swapcase() # for fun
return t
t_COLON = r':'
t_EQ = r'=='
t_ASSIGN = r'='
t_LT = r'<'
t_GT = r'>'
t_PLUS = r'\+'
t_MINUS = r'-'
t_MULT = r'\*'
t_DIV = r'/'
t_COMMA = r','
t_SEMICOLON = r';'
# Ply nicely documented how to do this.
RESERVED = {
"def": "DEF",
"if": "IF",
"return": "RETURN",
}
def t_NAME(t):
r'[a-zA-Z_][a-zA-Z0-9_]*'
t.type = RESERVED.get(t.value, "NAME")
return t
# Putting this before t_WS let it consume lines with only comments in
# them so the latter code never sees the WS part. Not consuming the
# newline. Needed for "if 1: #comment"
def t_comment(t):
r"[ ]*\043[^\n]*" # \043 is '#'
pass
# Whitespace
def t_WS(t):
r' [ ]+ '
if t.lexer.at_line_start and t.lexer.paren_count == 0:
return t
# Don't generate newline tokens when inside of parenthesis, eg
# a = (1,
# 2, 3)
def t_newline(t):
r'\n+'
t.lexer.lineno += len(t.value)
t.type = "NEWLINE"
if t.lexer.paren_count == 0:
return t
def t_LPAR(t):
r'\('
t.lexer.paren_count += 1
return t
def t_RPAR(t):
r'\)'
# check for underflow? should be the job of the parser
t.lexer.paren_count -= 1
return t
def t_error(t):
raise SyntaxError("Unknown symbol %r" % (t.value[0],))
print "Skipping", repr(t.value[0])
t.lexer.skip(1)
## I implemented INDENT / DEDENT generation as a post-processing filter
# The original lex token stream contains WS and NEWLINE characters.
# WS will only occur before any other tokens on a line.
# I have three filters. One tags tokens by adding two attributes.
# "must_indent" is True if the token must be indented from the
# previous code. The other is "at_line_start" which is True for WS
# and the first non-WS/non-NEWLINE on a line. It flags the check so
# see if the new line has changed indication level.
# Python's syntax has three INDENT states
# 0) no colon hence no need to indent
# 1) "if 1: go()" - simple statements have a COLON but no need for an indent
# 2) "if 1:\n go()" - complex statements have a COLON NEWLINE and must indent
NO_INDENT = 0
MAY_INDENT = 1
MUST_INDENT = 2
# only care about whitespace at the start of a line
def track_tokens_filter(lexer, tokens):
lexer.at_line_start = at_line_start = True
indent = NO_INDENT
saw_colon = False
for token in tokens:
token.at_line_start = at_line_start
if token.type == "COLON":
at_line_start = False
indent = MAY_INDENT
token.must_indent = False
elif token.type == "NEWLINE":
at_line_start = True
if indent == MAY_INDENT:
indent = MUST_INDENT
token.must_indent = False
elif token.type == "WS":
assert token.at_line_start == True
at_line_start = True
token.must_indent = False
else:
# A real token; only indent after COLON NEWLINE
if indent == MUST_INDENT:
token.must_indent = True
else:
token.must_indent = False
at_line_start = False
indent = NO_INDENT
yield token
lexer.at_line_start = at_line_start
def _new_token(type, lineno):
tok = lex.LexToken()
tok.type = type
tok.value = None
tok.lineno = lineno
return tok
# Synthesize a DEDENT tag
def DEDENT(lineno):
return _new_token("DEDENT", lineno)
# Synthesize an INDENT tag
def INDENT(lineno):
return _new_token("INDENT", lineno)
# Track the indentation level and emit the right INDENT / DEDENT events.
def indentation_filter(tokens):
# A stack of indentation levels; will never pop item 0
levels = [0]
token = None
depth = 0
prev_was_ws = False
for token in tokens:
## if 1:
## print "Process", token,
## if token.at_line_start:
## print "at_line_start",
## if token.must_indent:
## print "must_indent",
## print
# WS only occurs at the start of the line
# There may be WS followed by NEWLINE so
# only track the depth here. Don't indent/dedent
# until there's something real.
if token.type == "WS":
assert depth == 0
depth = len(token.value)
prev_was_ws = True
# WS tokens are never passed to the parser
continue
if token.type == "NEWLINE":
depth = 0
if prev_was_ws or token.at_line_start:
# ignore blank lines
continue
# pass the other cases on through
yield token
continue
# then it must be a real token (not WS, not NEWLINE)
# which can affect the indentation level
prev_was_ws = False
if token.must_indent:
# The current depth must be larger than the previous level
if not (depth > levels[-1]):
raise IndentationError("expected an indented block")
levels.append(depth)
yield INDENT(token.lineno)
elif token.at_line_start:
# Must be on the same level or one of the previous levels
if depth == levels[-1]:
# At the same level
pass
elif depth > levels[-1]:
raise IndentationError("indentation increase but not in new block")
else:
# Back up; but only if it matches a previous level
try:
i = levels.index(depth)
except ValueError:
raise IndentationError("inconsistent indentation")
for _ in range(i+1, len(levels)):
yield DEDENT(token.lineno)
levels.pop()
yield token
### Finished processing ###
# Must dedent any remaining levels
if len(levels) > 1:
assert token is not None
for _ in range(1, len(levels)):
yield DEDENT(token.lineno)
# The top-level filter adds an ENDMARKER, if requested.
# Python's grammar uses it.
def filter(lexer, add_endmarker = True):
token = None
tokens = iter(lexer.token, None)
tokens = track_tokens_filter(lexer, tokens)
for token in indentation_filter(tokens):
yield token
if add_endmarker:
lineno = 1
if token is not None:
lineno = token.lineno
yield _new_token("ENDMARKER", lineno)
# Combine Ply and my filters into a new lexer
class IndentLexer(object):
def __init__(self, debug=0, optimize=0, lextab='lextab', reflags=0):
self.lexer = lex.lex(debug=debug, optimize=optimize, lextab=lextab, reflags=reflags)
self.token_stream = None
def input(self, s, add_endmarker=True):
self.lexer.paren_count = 0
self.lexer.input(s)
self.token_stream = filter(self.lexer, add_endmarker)
def token(self):
try:
return self.token_stream.next()
except StopIteration:
return None
########## Parser (tokens -> AST) ######
# also part of Ply
#import yacc
# I use the Python AST
from compiler import ast
# Helper function
def Assign(left, right):
names = []
if isinstance(left, ast.Name):
# Single assignment on left
return ast.Assign([ast.AssName(left.name, 'OP_ASSIGN')], right)
elif isinstance(left, ast.Tuple):
# List of things - make sure they are Name nodes
names = []
for child in left.getChildren():
if not isinstance(child, ast.Name):
raise SyntaxError("that assignment not supported")
names.append(child.name)
ass_list = [ast.AssName(name, 'OP_ASSIGN') for name in names]
return ast.Assign([ast.AssTuple(ass_list)], right)
else:
raise SyntaxError("Can't do that yet")
# The grammar comments come from Python's Grammar/Grammar file
## NB: compound_stmt in single_input is followed by extra NEWLINE!
# file_input: (NEWLINE | stmt)* ENDMARKER
def p_file_input_end(p):
"""file_input_end : file_input ENDMARKER"""
p[0] = ast.Stmt(p[1])
def p_file_input(p):
"""file_input : file_input NEWLINE
| file_input stmt
| NEWLINE
| stmt"""
if isinstance(p[len(p)-1], basestring):
if len(p) == 3:
p[0] = p[1]
else:
p[0] = [] # p == 2 --> only a blank line
else:
if len(p) == 3:
p[0] = p[1] + p[2]
else:
p[0] = p[1]
# funcdef: [decorators] 'def' NAME parameters ':' suite
# ignoring decorators
def p_funcdef(p):
"funcdef : DEF NAME parameters COLON suite"
p[0] = ast.Function(None, p[2], tuple(p[3]), (), 0, None, p[5])
# parameters: '(' [varargslist] ')'
def p_parameters(p):
"""parameters : LPAR RPAR
| LPAR varargslist RPAR"""
if len(p) == 3:
p[0] = []
else:
p[0] = p[2]
# varargslist: (fpdef ['=' test] ',')* ('*' NAME [',' '**' NAME] | '**' NAME) |
# highly simplified
def p_varargslist(p):
"""varargslist : varargslist COMMA NAME
| NAME"""
if len(p) == 4:
p[0] = p[1] + p[3]
else:
p[0] = [p[1]]
# stmt: simple_stmt | compound_stmt
def p_stmt_simple(p):
"""stmt : simple_stmt"""
# simple_stmt is a list
p[0] = p[1]
def p_stmt_compound(p):
"""stmt : compound_stmt"""
p[0] = [p[1]]
# simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE
def p_simple_stmt(p):
"""simple_stmt : small_stmts NEWLINE
| small_stmts SEMICOLON NEWLINE"""
p[0] = p[1]
def p_small_stmts(p):
"""small_stmts : small_stmts SEMICOLON small_stmt
| small_stmt"""
if len(p) == 4:
p[0] = p[1] + [p[3]]
else:
p[0] = [p[1]]
# small_stmt: expr_stmt | print_stmt | del_stmt | pass_stmt | flow_stmt |
# import_stmt | global_stmt | exec_stmt | assert_stmt
def p_small_stmt(p):
"""small_stmt : flow_stmt
| expr_stmt"""
p[0] = p[1]
# expr_stmt: testlist (augassign (yield_expr|testlist) |
# ('=' (yield_expr|testlist))*)
# augassign: ('+=' | '-=' | '*=' | '/=' | '%=' | '&=' | '|=' | '^=' |
# '<<=' | '>>=' | '**=' | '//=')
def p_expr_stmt(p):
"""expr_stmt : testlist ASSIGN testlist
| testlist """
if len(p) == 2:
# a list of expressions
p[0] = ast.Discard(p[1])
else:
p[0] = Assign(p[1], p[3])
def p_flow_stmt(p):
"flow_stmt : return_stmt"
p[0] = p[1]
# return_stmt: 'return' [testlist]
def p_return_stmt(p):
"return_stmt : RETURN testlist"
p[0] = ast.Return(p[2])
def p_compound_stmt(p):
"""compound_stmt : if_stmt
| funcdef"""
p[0] = p[1]
def p_if_stmt(p):
'if_stmt : IF test COLON suite'
p[0] = ast.If([(p[2], p[4])], None)
def p_suite(p):
"""suite : simple_stmt
| NEWLINE INDENT stmts DEDENT"""
if len(p) == 2:
p[0] = ast.Stmt(p[1])
else:
p[0] = ast.Stmt(p[3])
def p_stmts(p):
"""stmts : stmts stmt
| stmt"""
if len(p) == 3:
p[0] = p[1] + p[2]
else:
p[0] = p[1]
## No using Python's approach because Ply supports precedence
# comparison: expr (comp_op expr)*
# arith_expr: term (('+'|'-') term)*
# term: factor (('*'|'/'|'%'|'//') factor)*
# factor: ('+'|'-'|'~') factor | power
# comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'
def make_lt_compare((left, right)):
return ast.Compare(left, [('<', right),])
def make_gt_compare((left, right)):
return ast.Compare(left, [('>', right),])
def make_eq_compare((left, right)):
return ast.Compare(left, [('==', right),])
binary_ops = {
"+": ast.Add,
"-": ast.Sub,
"*": ast.Mul,
"/": ast.Div,
"<": make_lt_compare,
">": make_gt_compare,
"==": make_eq_compare,
}
unary_ops = {
"+": ast.UnaryAdd,
"-": ast.UnarySub,
}
precedence = (
("left", "EQ", "GT", "LT"),
("left", "PLUS", "MINUS"),
("left", "MULT", "DIV"),
)
def p_comparison(p):
"""comparison : comparison PLUS comparison
| comparison MINUS comparison
| comparison MULT comparison
| comparison DIV comparison
| comparison LT comparison
| comparison EQ comparison
| comparison GT comparison
| PLUS comparison
| MINUS comparison
| power"""
if len(p) == 4:
p[0] = binary_ops[p[2]]((p[1], p[3]))
elif len(p) == 3:
p[0] = unary_ops[p[1]](p[2])
else:
p[0] = p[1]
# power: atom trailer* ['**' factor]
# trailers enables function calls. I only allow one level of calls
# so this is 'trailer'
def p_power(p):
"""power : atom
| atom trailer"""
if len(p) == 2:
p[0] = p[1]
else:
if p[2][0] == "CALL":
p[0] = ast.CallFunc(p[1], p[2][1], None, None)
else:
raise AssertionError("not implemented")
def p_atom_name(p):
"""atom : NAME"""
p[0] = ast.Name(p[1])
def p_atom_number(p):
"""atom : NUMBER
| STRING"""
p[0] = ast.Const(p[1])
def p_atom_tuple(p):
"""atom : LPAR testlist RPAR"""
p[0] = p[2]
# trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
def p_trailer(p):
"trailer : LPAR arglist RPAR"
p[0] = ("CALL", p[2])
# testlist: test (',' test)* [',']
# Contains shift/reduce error
def p_testlist(p):
"""testlist : testlist_multi COMMA
| testlist_multi """
if len(p) == 2:
p[0] = p[1]
else:
# May need to promote singleton to tuple
if isinstance(p[1], list):
p[0] = p[1]
else:
p[0] = [p[1]]
# Convert into a tuple?
if isinstance(p[0], list):
p[0] = ast.Tuple(p[0])
def p_testlist_multi(p):
"""testlist_multi : testlist_multi COMMA test
| test"""
if len(p) == 2:
# singleton
p[0] = p[1]
else:
if isinstance(p[1], list):
p[0] = p[1] + [p[3]]
else:
# singleton -> tuple
p[0] = [p[1], p[3]]
# test: or_test ['if' or_test 'else' test] | lambdef
# as I don't support 'and', 'or', and 'not' this works down to 'comparison'
def p_test(p):
"test : comparison"
p[0] = p[1]
# arglist: (argument ',')* (argument [',']| '*' test [',' '**' test] | '**' test)
# XXX INCOMPLETE: this doesn't allow the trailing comma
def p_arglist(p):
"""arglist : arglist COMMA argument
| argument"""
if len(p) == 4:
p[0] = p[1] + [p[3]]
else:
p[0] = [p[1]]
# argument: test [gen_for] | test '=' test # Really [keyword '='] test
def p_argument(p):
"argument : test"
p[0] = p[1]
def p_error(p):
#print "Error!", repr(p)
raise SyntaxError(p)
class GardenSnakeParser(object):
def __init__(self, lexer = None):
if lexer is None:
lexer = IndentLexer()
self.lexer = lexer
self.parser = yacc.yacc(start="file_input_end")
def parse(self, code):
self.lexer.input(code)
result = self.parser.parse(lexer = self.lexer)
return ast.Module(None, result)
###### Code generation ######
from compiler import misc,syntax,pycodegen
class GardenSnakeCompiler(object):
def __init__(self):
self.parser = GardenSnakeParser()
def compile(self, code, filename="<string>"):
tree = self.parser.parse(code)
#print tree
misc.set_filename(filename, tree)
syntax.check(tree)
gen = pycodegen.ModuleCodeGenerator(tree)
code = gen.getCode()
return code
####### Test code #######
compile = GardenSnakeCompiler().compile
code = r"""
print('LET\'S TRY THIS \\OUT')
#Comment here
def x(a):
print('called with',a)
if a == 1:
return 2
if a*2 > 10: return 999 / 4
# Another comment here
return a+2*3
ints = (1, 2,
3, 4,
5)
print('mutiline-expression', ints)
t = 4+1/3*2+6*(9-5+1)
print('predence test; should be 34+2/3:', t, t==(34+2/3))
print('numbers', 1,2,3,4,5)
if 1:
8
a=9
print(x(a))
print(x(1))
print(x(2))
print(x(8),'3')
print('this is decimal', 1/5)
print('BIG DECIMAL', 1.234567891234567e12345)
"""
# Set up the GardenSnake run-time environment
def print_(*args):
print "-->", " ".join(map(str,args))
globals()["print"] = print_
compiled_code = compile(code)
exec compiled_code in globals()
print "Done"
|