Viewing file:  calc.py (2.96 KB) -rw-r--r--
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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)
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