Hello dear plyers,
Please help me solve a little ambiguity!
I've been trying to write a parser for python for my personal
enjoyment. I'm aware that it's hard.
As you probably know, Python's assignment is rather complex. You can
use multiple assignments on one line, in two forms:
1) a=b=c=d=4
2) a,b=3,4
I wrote a syntax for it (which I added, in minimal form, to the bottom
of this message), and it seems okay.
It can even parse statements like: "c,d=x,y=a+b"
However, it fails to parse "c,d=a,b"
My guess is that PLY thinks it's getting another variable declaration,
and cannot back down from this assumption.
I tried for several days and cannot get all forms to work.
Perhaps I'm doing something wrong (or not doing it sufficiently
right :) )
I would really appreciate pointers.
Here is the script:
#---------------------------------------
tokens = (
'NAME','NUMBER',
'PLUS','ASSIGN', 'COMMA', 'NL'
)
# Tokens
t_PLUS = r'\+'
t_COMMA = r','
t_ASSIGN = r'='
t_NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
t_NUMBER = r'\d+'
# Ignored characters
t_ignore = " \t"
#t_NL = r'\n+'
def t_NL(t):
r'\n+'
t.lexer.lineno += t.value.count("\n")
return t
def t_error(t):
print "Illegal character '%s'" % t.value[0]
t.lexer.skip(1)
# Build the lexer
import ply.lex as lex
lex.lex()
# Parsing rules
def p_statements(t):
'''statements : statement
| statement statements
'''
t[0] = t[1:]
def p_statement(t):
'''statement : assign_list NL
| NL
'''
t[0] = t[1:]
def p_assign_list(t):
"""assign_list : var_list ASSIGN expr_list
| var_list ASSIGN assign_list
"""
t[0] = t[1:]
def p_var_list(t):
"""var_list : NAME
| NAME COMMA var_list
"""
t[0] = ['var_list'] + t[1:]
def p_expr_list(t):
"""expr_list : expr
| expr COMMA expr_list
"""
t[0] = ['expr_list'] + t[1:]
def p_expr(t):
"""expr : NAME
| expr PLUS expr
| NUMBER
"""
t[0] = t[1:]
def p_error(t):
if t:
print "Syntax error at '%s'" % t.value, t.lineno, t.lexpos
import ply.yacc as yacc
yacc.yacc()
print yacc.parse("""
c,d=x,y=a+b
c,d=1,2
c,d=a,b
""")
# first two work, but last throws an exception!
#--------------------------------------- END OF SCRIPT
Thanks,
Erez
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