单行计算器，无法确定最后一个功能

Question

I'm programming a single line calculator in python. It needs to be able to take in an expression of any length and with multiple operators. It then returns the answer while following the order of operations.

ie "2 / 4 - 2 * 3 + 4" and it returns -1.5

This is what I have so far

def findNextOpr(s): #DONE
#this function is used to find the next operator in the string s

if len(s)<=0 or not isinstance(s,str):
    print("type mimatch error: findNextOpr")
    return "type mimatch error: findNextOpr"

s = list(s) #converts s to a list
operator = ["+", "-", "/", "*"]

for i in s:
    if i in operator:
        return i
else:
    return -1

def isNumber(s): #DONE
#this function is used to check if it's a number

#checks if string s meets the requirements
if len(s)==0 or not isinstance(s, str):
    print("type mismatch error: isNumber")
    return "type mismatch error: isNumber"

#strip string of all whitespace. 
s = s.strip(" ")
s = s[0:] #removes minus signs but only from the front of string

try: #checks to make sure s2 is a number and doesn't have more than 1 period
    s=float(s)
    return True
except:
    return False


def getNextNumber(expr, pos): #DONE

#checks to make sure expr and pos match the requirements. returns error if they don't
if len(expr)==0 or not isinstance(expr, str) or pos<0 or pos>=len(expr) or not isinstance(pos, int):
    print("type mismatch error: getNextNumber")
    return None, None, "type mismatch error: getNextNumber"

s = expr[pos:] #sets string1 equal to position that was passed
op=['-','+','*','/']

newOpr = findNextOpr(s) #newOpr equals left-most operator, calls findNext to achieve this

if newOpr in op:
    if expr.find(newOpr,pos)>=pos:
        oprPos=expr.find(newOpr,pos)
else:
    newOpr=None
if newOpr==None:
    oprPos = None

if isNumber(expr[pos:oprPos]): #checks to make sure if string is actually a #
    newNumber = float(expr[pos:oprPos])
else:
    newNumber = None #if it is a #, assigns value to newNumber


return newNumber, newOpr, oprPos


def exeOpr(num1, opr, num2):
#This is a simple utility function skipping type check
if opr=="+":
    return num1+num2
elif opr=="-":
    return num1-num2
elif opr=="*":
    return num1*num2
elif opr=="/":
    return num1/num2
else:
    return None

def calc(expr):

#the below line checks if expr is a string
if not isinstance(expr, str) or len(expr) <= 0:
    print("argument error: line A in eval_expr")
    return "argument error: line A in eval_expr"

#below line  creates three variables
#sets them equal to whatever is returned by the getNextNumber function
#the getNextNumber function is called. It passes the expr and integer 0
newNumber, newOpr, oprPos = getNextNumber(expr, 0)

#if newNumber is none, return error
#if newOpr is none, return newNumber
#if newOpr is add/sub set mode=add, create addResult=newNumber and mulResult = None
#if newOpr is mult/div set mode=mul, crate addResult=0 and mulResult=newNumber
if newNumber is None:
    print("input formula error: line B in eval_expr")
    return "input formula error: line B in eval_expr"
elif newOpr is None:
    return newNumber
elif newOpr=="+" or newOpr=="-":
    mode="add" 
    addResult=newNumber #saves # at first index to addResult if 1st operator is + or -    
    mulResult=None          
elif newOpr=="*" or newOpr=="/":
    mode="mul"
    addResult=0
    lastOpr = "+"
    mulResult=newNumber #saves # at first index to mulResult if 1st operator is + or -

#pos and opr are created
pos=oprPos+1 #current positon
opr=newOpr #current operator

while True:
    newNumber, newOpr, oprPos = getNextNumber(expr, pos)

    #--- code while loop ---#

    while True:
        newNumber, newOpr, oprPos=getNextNumber(expr, pos)
        #if expr[pos:] == " 3- 2*1":
        # pdb.set_trace()
        if newNumber== None and pos>=len(expr):
            return "Expression error"
        elif newOpr== None and mode=='add':
            return exeOpr(addResult, opr, newNumber)
        elif newOpr== None and mode=='mul':
            return exeOpr(mulResult, opr, newNumber)
        elif (newOpr== "+" or newOpr=='-') and mode=='add' :
            addResult= exeOpr(addResult,opr,newNumber)
            pos= oprPos+1
            opr=newOpr
        elif (newOpr=="*" or newOpr=='/') and mode=='add':
            addResult= newNumber
            lastOpr= opr
            mulResult= exeOpr(addResult,newOpr, newNumber)
            mode='mul'
            pos= oprPos+1
            opr=newOpr
        elif (newOpr=="*" or newOpr=="/") and mode=="mul":
            mulResult= exeOpr(mulResult, opr, newNumber)
            pos= oprPos+1
            opr=newOpr
        elif (newOpr=="-" or newOpr=='+') and mode=="mul":
            mulResult=exeOpr(mulResult,opr, newNumber)
            addResult= exeOpr(mulResult, lastOpr, newNumber)
            mode="add"
            pos= oprPos+1
            opr=newOpr



    #--- end of function ---#


expr = "2 / 4 - 2 * 3 + 4"
print(calc(expr)) 

Everything works except the output. When I run the program I get an answer of 13.0

I know the error is in the calc function. Can anyone figure out what I'm doing wrong.

Thanks.

Answer 1

Your calc is not working as you expected

HINT :

def exeOpr(num1, opr, num2):
    #This is a simple utility function skipping type check
    if opr=="+":
        print "{} + {}".format(num1, num2)
        return num1+num2
    elif opr=="-":
        print "{} - {}".format(num1, num2)
        return num1-num2
    elif opr=="*":
        print "{} * {}".format(num1, num2)  
        return num1*num2
    elif opr=="/":
        print "{} / {}".format(num1, num2)
        return num1/num2
    else:
        return None

OUTPUT

2.0 / 4.0
0.5 + 4.0
2.0 * 2.0
4.0 * 3.0
12.0 - 3.0
9.0 + 4.0
13.0