Is there a way to autogenerate valid arithmetic expressions?

I'm currently trying to create a Python script that will autogenerate space-delimited arithmetic expressions which are valid. However, I get sample output that looks like this: ( 32 - 42 / 95 + 24 ( ) ( 53 ) + ) 21

While the empty parentheses are perfectly OK by me, I can't use this autogenerated expression in calculations since there's no operator between the 24 and the 53, and the + before the 21 at the end has no second argument.

What I want to know is, is there a way to account for/fix these errors using a Pythonic solution? (And before anyone points it out, I'll be the first to acknowledge that the code I posted below is probably the worst code I've pushed and conforms to...well, very few of Python's core tenets.)

import random
parentheses = ['(',')']
ops = ['+','-','*','/'] + parentheses

lines = 0

while lines < 1000:
    fname = open('test.txt','a')
    expr = []
    numExpr = lines
    if (numExpr % 2 == 0):
        numExpr += 1
    isDiv = False # Boolean var, makes sure there's no Div by 0

    # isNumber, isParentheses, isOp determine whether next element is a number, parentheses, or operator, respectively
    isNumber = random.randint(0,1) == 0 # determines whether to start sequence with number or parentheses
    isParentheses = not isNumber
    isOp = False
    # Counts parentheses to ensure parentheses are matching
    numParentheses = 0
    while (numExpr > 0 or numParentheses > 0):
        if (numExpr < 0 and numParentheses > 0):
            isDiv = False
            expr.append(')')
            numParentheses -= 1
        elif (isOp and numParentheses > 0):
            rand = random.randint(0,5)
            expr.append(ops[rand])
            isDiv = (rand == 3) # True if div op was just appended
            # Checks to see if ')' was appended
            if (rand == 5):
                isNumber = False
                isOp = True
                numParentheses -= 1
            # Checks to see if '(' was appended
            elif (rand == 4):
                isNumber = True
                isOp = False
                numParentheses += 1
            # All other operations go here
            else:
                isNumber = True
                isOp = False
        # Didn't add parentheses possibility here in case expression in parentheses somehow reaches 0
        elif (isNumber and isDiv):
            expr.append(str(random.randint(1,100)))
            isDiv = False
            isNumber = False
            isOp = True
        # If a number's up, decides whether to append parentheses or a number
        elif (isNumber):
            rand = random.randint(0,1)
            if (rand == 0):
                expr.append(str(random.randint(0,100)))
                isNumber = False
                isOp = True
            elif (rand == 1):
                if (numParentheses == 0):
                    expr.append('(')
                    numParentheses += 1
                else:
                    rand = random.randint(0,1)
                    expr.append(parentheses[rand])
                    if rand == 0:
                        numParentheses += 1
                    else:
                        numParentheses -= 1
            isDiv = False
        numExpr -= 1

    fname.write(' '.join(expr) + '\n')
    fname.close()
    lines += 1