4.2 Python PLY With Yacc and its Values

 Character Literals

If desired, a grammar may contain tokens defined as single character literals. For example:

def p_binary_operators(p):
    '''expression : expression '+' term
                  | expression '-' term
       term       : term '*' factor
                  | term '/' factor'''
    if p[2] == '+':
        p[0] = p[1] + p[3]
    elif p[2] == '-':
        p[0] = p[1] - p[3]
    elif p[2] == '*':
        p[0] = p[1] * p[3]
    elif p[2] == '/':
        p[0] = p[1] / p[3]

A character literal must be enclosed in quotes such as '+'. In addition, if literals are used, they must be declared in the corresponding lex file through the use of a special literals declaration.

# Literals.  Should be placed in module given to lex()
literals = ['+','-','*','/' ]

Character literals are limited to a single character. Thus, it is not legal to specify literals such as '<=' or '=='. For this, use the normal lexing rules (e.g., define a rule such as t_EQ = r'==').

Example:

Precedence

To resolve ambiguity, especially in expression grammars, yacc.py allows individual tokens to be assigned a precedence level and associativity. This is done by adding a variable precedence to the grammar file like this:

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

This declaration specifies that PLUS/MINUS have the same precedence level and are left-associative and that TIMES/DIVIDE have the same precedence and are left-associative. Within the precedence declaration, tokens are ordered from lowest to highest precedence. Thus, this declaration specifies that TIMES/DIVIDE have higher precedence than PLUS/MINUS (since they appear later in the precedence specification).

The precedence specification works by associating a numerical precedence level value and associativity direction to the listed tokens. For example, in the above example you get:

PLUS      : level = 1,  assoc = 'left'
MINUS     : level = 1,  assoc = 'left'
TIMES     : level = 2,  assoc = 'left'
DIVIDE    : level = 2,  assoc = 'left'

These values are then used to attach a numerical precedence value and associativity direction to each grammar rule. This is always determined by looking at the precedence of the right-most terminal symbol. For example:

expression : expression PLUS expression                 # level = 1, left
           | expression MINUS expression                # level = 1, left
           | expression TIMES expression                # level = 2, left
           | expression DIVIDE expression               # level = 2, left
           | LPAREN expression RPAREN                   # level = None (not specified)
           | NUMBER                                     # level = None (not specified)

Example: