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Editing: calc.py

# -----------------------------------------------------------------------------
# calc.py
#
# A simple calculator with variables.   This is from O'Reilly's
# "Lex and Yacc", p. 63.
#
# This example uses unicode strings for tokens, docstrings, and input.
# -----------------------------------------------------------------------------

import sys
sys.path.insert(0,"../..")

tokens = (
    'NAME','NUMBER',
    'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
    'LPAREN','RPAREN',
    )

# Tokens

t_PLUS    = ur'\+'
t_MINUS   = ur'-'
t_TIMES   = ur'\*'
t_DIVIDE  = ur'/'
t_EQUALS  = ur'='
t_LPAREN  = ur'\('
t_RPAREN  = ur'\)'
t_NAME    = ur'[a-zA-Z_][a-zA-Z0-9_]*'

def t_NUMBER(t):
    ur'\d+'
    try:
        t.value = int(t.value)
    except ValueError:
        print "Integer value too large", t.value
        t.value = 0
    return t

t_ignore = u" \t"

def t_newline(t):
    ur'\n+'
    t.lexer.lineno += t.value.count("\n")
    
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

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

# dictionary of names
names = { }

def p_statement_assign(p):
    'statement : NAME EQUALS expression'
    names[p[1]] = p[3]

def p_statement_expr(p):
    'statement : expression'
    print p[1]

def p_expression_binop(p):
    '''expression : expression PLUS expression
                  | expression MINUS expression
                  | expression TIMES expression
                  | expression DIVIDE expression'''
    if p[2] == u'+'  : p[0] = p[1] + p[3]
    elif p[2] == u'-': p[0] = p[1] - p[3]
    elif p[2] == u'*': p[0] = p[1] * p[3]
    elif p[2] == u'/': p[0] = p[1] / p[3]

def p_expression_uminus(p):
    'expression : MINUS expression %prec UMINUS'
    p[0] = -p[2]

def p_expression_group(p):
    'expression : LPAREN expression RPAREN'
    p[0] = p[2]

def p_expression_number(p):
    'expression : NUMBER'
    p[0] = p[1]

def p_expression_name(p):
    'expression : NAME'
    try:
        p[0] = names[p[1]]
    except LookupError:
        print "Undefined name '%s'" % p[1]
        p[0] = 0

def p_error(p):
    if p:
        print "Syntax error at '%s'" % p.value
    else:
        print "Syntax error at EOF"

import ply.yacc as yacc
yacc.yacc()

while 1:
    try:
        s = raw_input('calc > ')
    except EOFError:
        break
    if not s: continue
    yacc.parse(unicode(s))

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