How can I randomly choose a maths operator and ask recurring maths questions with it?

How about you make a dictionary that maps the operator's character (e.g. '+') to the operator (e.g. operator.add). Then sample that, format you string, and perform the operation.

import random
import operator

Generating a random mathematical expression

def randomCalc():
    ops = {'+':operator.add,
           '-':operator.sub,
           '*':operator.mul,
           '/':operator.truediv}
    num1 = random.randint(0,12)
    num2 = random.randint(1,10)   # I don't sample 0's to protect against divide-by-zero
    op = random.choice(list(ops.keys()))
    answer = ops.get(op)(num1,num2)
    print('What is {} {} {}?\n'.format(num1, op, num2))
    return answer

Asking the user

def askQuestion():
    answer = randomCalc()
    guess = float(input())
    return guess == answer

Finally making a multi-question quiz

def quiz():
    print('Welcome. This is a 10 question math quiz\n')
    score = 0
    for i in range(10):
        correct = askQuestion()
        if correct:
            score += 1
            print('Correct!\n')
        else:
            print('Incorrect!\n')
    return 'Your score was {}/10'.format(score)

Some testing

>>> quiz()
Welcome. This is a 10 question math quiz

What is 8 - 6?
2
Correct!

What is 10 + 6?
16
Correct!

What is 12 - 1?
11
Correct!

What is 9 + 4?
13
Correct!

What is 0 - 8?
-8
Correct!

What is 1 * 1?
5
Incorrect!

What is 5 * 8?
40
Correct!

What is 11 / 1?
11
Correct!

What is 1 / 4?
0.25
Correct!

What is 1 * 1?
1
Correct!

'Your score was 9/10'