lexer, parser , trying to make an expression calculator - python-3.x

Question

As the title says am trying to make an expression calculator using Ply , I still didnt finish the complete code just a part of it , but until now . I get this error :
Traceback (most recent call last): File "EX3.py", line 69, in
module
parser = yacc.yacc() File "/Users/mostafa.osama2/anaconda3/lib/python3.6/site-packages/ply/yacc.py",
line 3317, in yacc
raise YaccError('Unable to build parser') ply.yacc.YaccError: Unable to build parser
here is my code :
import ply.lex as lex
import ply.yacc as yacc
import sys
tokens = ['INT','FLOAT' , 'NAME' , 'PLUS' , 'MINUS' , 'DIVIDE' , 'MULTIPLY' ,
'EQUALS']
#list of tokens , for grammar checking
t_PLUS = r'\+'
t_MINUS = r'\-'
t_MULTIPLY = r'\*'
t_DIVIDE = r'\/'
t_EQUALS = r'\='
t_ignore = r' ' # used for ignoring spaces between numbers and operators
#has to match name of token,
def t_FLOAT(t):
r'\d+.\d+' # 1.2 is a float , 1.any number is a float
t.value = float(t.value)
return t
def t_INT(t):
r'\d+'
t.value = int(t.value)
return t # t is our token object
def t_NAME(t):
r'[a-zA-Z_][a-zA-Z_0-9]*' #star means 0 or more, first char is a-zA-z , second character is a-zA-z0-9
t.type = 'NAME'
return t
def t_error(t):
print("Illegal characters!")
t.lexer.skip(1) # skips 1 token onwards
lexer = lex.lex()
def p_calc(p): # p is a tuple
'''
calc : expression
| empty
'''
print(p[1])
def p_expression(p):
'''
expression : expression PLUS expression
| expression DIVIDE expression
| expression PLUS expression
| expression MINUS expression
'''
p[0] = (p[2] , p[1] , p[3])
def p_expression(p):
'''
expression : INT
| FLOAT
'''
p[0] = p[1]
def p_empty(p):
'''
empty:
'''
p[0] = None
parser = yacc.yacc()
while True:
try:
s = input('')
except EOFError: # when u press contorl D on keyboard
break
parser.parse(s)

Answer 1

Ply insists that parsing rules have whitespace between the name of the non-terminal and the colon. So empty: is not valid; you must write empty :
Also, as reported by Ply, you define two functions named p_expression. Ply requires that all parsing functions have different names (otherwise it has no way to call them), but it doesn't care what the names are, as long as they start with p_. So change one of the names.
Finally, you have two rules for addition, and no rule for multiplication. Ply will complain about the duplicate addition rule (after you fix the other problems). It will also complain that you are missing a p_error function.