Ericson Willians Ericson Willians - 1 year ago 161
Python Question

How can I create an abstract syntax tree considering '|'? (Ply / Yacc)

Considering the following grammar:

expr : expr '+' term | expr '-' term | term
term : term '*' factor | term '/' factor | factor
factor : '(' expr ')' | identifier | number

This is my code using ply:

from ply import lex, yacc

tokens = [

t_ignore = r" \t"
t_identifier = r"^[a-zA-Z]+$"
t_number = r"[+-]?(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?"
t_plus = r"\+"
t_minus = r"-"
t_mult = r"\*"
t_div = r"/"

def p_stmt(p):
"""stmt : expr"""
p[0] = ("stmt", p[1])

def p_expr(p):
"""expr : expr plus term
| expr minus term
| term"""
p[0] = ("expr", p[1], p[2]) # Problem here <<<

def p_term(p):
"""term : term mult factor
| term div factor
| factor"""

def p_factor(p):
"""factor : '(' expr ')'
| identifier
| number"""

if __name__ == "__main__":
data = "32 + 10"
result = yacc.parse(data)

How am I supposed to build an AST with the expression if I can't access the operators? I could separate the functions like p_expr_plus, but in this case, I would eliminate operator precedence. The docs are not so helpful, since I'm a beginner and can't solve this problem. The best material I've found on the subject is this, but it does not consider the complexity of operator precedence.

EDIT: I can't access p2 or p[3], since I get an IndexError (It's matching the term only). In the PDF I've linked, they explicitly put the operator inside the tuple, like: ('+', p1, p2), and thus, evincing my problem considering precedence (I can't separate the functions, the expression is the expression, there should be a way to consider the pipes and access any operator).

Answer Source

As far as I can see, in p[0] = ("expr", p[1], p[2]), p1 would be the left hand expression, p[2] would be the operator, and p[3] (that you aren't using) would be the right hand term.

Just use p[2] to determine the operator, add p[3], since you will need it, and you should be good to go.

Also, you must verify how many items p has, since if the last rule, | term""" is matched, p will only have two items instead of four.

Take a look at a snippet from the GardenSnake example:

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])
        p[0] = p[1]
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