1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
|
# -----------------------------------------------------------------------------
# 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.
# -----------------------------------------------------------------------------
# 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 = input("calc > ")
except EOFError:
break
r = calc(s)
if r:
print(r)
|
