How to use Assert in this instance?

Question

I'm trying to use assert to check if another function is working correctly.

In this case the function I am trying to test is the following:

def factorial(n):
    if type(n) != int or n < 0:
        raise ValueError, "\nThis error has been raised because the value inputted was either not an integer or negative."
    else:   
        result = 1
        for x in range(n):
            result = result*(x+1)
        print "\n", n, "factorial is:", result

n = input("\n\n Insert a non-negative integer to take the factorial of: ")

factorial(n)

As far as I was aware if I was using assert to check the result of when n=1 I would use:

assert factorial(1) == 1

However this returns an assertion error although I have checked the function manually with n=1 and it DOES return 1, so shouldn't it not raise the assertion error as the conditon is true?

Could anyone point out why I am getting an assertion error raised?

Also is it possible to use assert to check for the whether raise ValueError segment is correct?

Answer 1

Your factorial function prints the result, but it does not return it. This is why the assertion fails. Try this:

def factorial(n):
    if not isinstance(n, int):
        raise ValueError('value must be an int')
    if n < 0:
        raise ValueError('n must be >= 0')

    result = 1
    for x in range(n):
        result = result * (x + 1)

    return result  # <-- This is the magic line :)

As for the second part of the question, it looks like you've stumbled on unit testing.

If you want to test if your factorial works or not, I'd recommend a proper unit testing library such as doctest or pytest. Both are very easy to use, and it's good to get into the habit of writing tests early on in your code.

Here's an example of doctest:

def factorial(n):
    """Calculate the factorial of a non-negative integer ``n``.

    >>> factorial(1)
    1
    >>> factorial(4)
    24

    Negative numbers are not supported and will result in a ValueError:

    >>> factorial(-1)
    Traceback (most recent call last):
        ...
    ValueError: n must be >= 0

    """

    # (Insert original code here)


if __name__ == '__main__':
    import doctest
    doctest.testmod()