Unit test - start of reverse polish notation class

These are the two files, I made in class for unit test demo purposes.

My original class in the file: ReversePolishCalc.py

class ReversePolishCalc:
    def __init__(self):
        self.stack = list()

    def _checkstack(self, count):
        if len(self.stack) < count:
            raise IndexError("Stack does not contain enough elements to perform operaation")
    
    def push(self, vector):
        if isinstance(vector, (int, float, str)):
            vector = [vector]
        if not isinstance(vector, (list, tuple)):
            raise ValueError("Input can not be understood as numbers")
        for number in vector:
            if isinstance(number, (int, float)):
                self.stack.append(number)
            elif isinstance(number, str):
                try:
                    self.stack.append(int(number))
                except ValueError:
                    try:
                        self.stack.append(float(number))
                    except ValueError:
                        raise ValueError("Input can not be understood as numbers")
            else:
                raise ValueError("Input can not be understood as numbers")
    
    def pop(self):
        self._checkstack(1)
        return self.stack.pop()
        
    def add(self):
        self._checkstack(2)
        self.stack[-2] += self.stack[-1]
        del self.stack[-1]

    def subtract(self):
        self._checkstack(2)
        self.stack[-2] -= self.stack[-1]
        del self.stack[-1]

    def multiply(self):
        self._checkstack(2)
        self.stack[-2] *= self.stack[-1]
        del self.stack[-1]

    def divide(self):
        self._checkstack(2)
        if self.stack[-1] == 0:
            raise ZeroDivisionError
        self.stack[-2] /= self.stack[-1]
        del self.stack[-1]

    def factorial(self):
        self._checkstack(1)
        no = int(self.stack[-1])
        if no != self.stack[-1]:
            raise ValueError("Factorial with floats is invalid")
        if no < 0:
            raise ValueError("Factorial can not be calcuated with negatives")
        res = 1
        for i in range(2, no+1):
            res *= i        
        self.stack[-1] = res

My test file of the class: test_ReversePolishCalc.py

import pytest
from ReversePolishCalc import ReversePolishCalc as rpc

def test_push1():
    # Arrange
    calc = rpc()
    # Act
    calc.push(1)
    # Assert
    assert calc.stack[-1] == 1, "Simple push of integer 1"

def test_push2():
    # Arrange
    calc = rpc()
    # Act
    calc.push(1)
    # Assert
    assert calc.stack[0] == 1, "Simple push of integer 2"

def test_push3():
    # Arrange
    calc = rpc()
    # Act
    calc.push(1.2)
    # Assert
    assert calc.stack[0] == 1.2, "Push of float"

def test_push4():
    # Arrange
    calc = rpc()
    # Act
    calc.push("1")
    # Assert
    assert calc.stack[0] == 1, "Push of 1 as string"

def test_push5():
    # Arrange
    calc = rpc()
    # Act
    calc.push([1, 1.5, "2.5"])
    # Assert
    assert calc.stack == [1, 1.5, 2.5], "Advanced list push of ints, floats and strings"

@pytest.mark.parametrize("x,y", [(1,1), (1.2, 1.2), ("1.2", 1.2), ("1", 1)])
def test_push(x,y):
    # Arrange
    calc = rpc()
    # Act
    calc.push(x)
    # Assert
    assert calc.stack[0] == y, "Push of 1 as string"


def test_pop1():
    # Arrange
    calc = rpc()
    calc.push(1)
    # Act
    num = calc.pop()
    # Assert
    assert num == 1

def test_failpop1():
    # Arrange
    calc = rpc()
    # Act
    with pytest.raises(IndexError):
        num = calc.pop()
    # Assert