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 # ----------------------------------------------------------------------------- # calc.py # # A calculator parser that makes use of closures. The function make_calculator() # returns a function that accepts an input string and returns a result. All # lexing rules, parsing rules, and internal state are held inside the function. # ----------------------------------------------------------------------------- import sys sys.path.insert(0,"../..") if sys.version_info[0] >= 3: raw_input = input # Make a calculator function def make_calculator(): import ply.lex as lex import ply.yacc as yacc # ------- Internal calculator state variables = { } # Dictionary of stored variables # ------- Calculator tokenizing rules tokens = ( 'NAME','NUMBER', ) literals = ['=','+','-','*','/', '(',')'] t_ignore = " \t" t_NAME = r'[a-zA-Z_][a-zA-Z0-9_]*' def t_NUMBER(t): r'\d+' t.value = int(t.value) return t def t_newline(t): r'\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 lexer = lex.lex() # ------- Calculator parsing rules precedence = ( ('left','+','-'), ('left','*','/'), ('right','UMINUS'), ) def p_statement_assign(p): 'statement : NAME "=" expression' variables[p[1]] = p[3] p[0] = None def p_statement_expr(p): 'statement : expression' p[0] = p[1] def p_expression_binop(p): '''expression : expression '+' expression | expression '-' expression | expression '*' expression | expression '/' expression''' 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] def p_expression_uminus(p): "expression : '-' expression %prec UMINUS" p[0] = -p[2] def p_expression_group(p): "expression : '(' expression ')'" 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] = variables[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") # Build the parser parser = yacc.yacc() # ------- Input function def input(text): result = parser.parse(text,lexer=lexer) return result return input # Make a calculator object and use it calc = make_calculator() while True: try: s = raw_input("calc > ") except EOFError: break r = calc(s) if r: print(r)