在这种情况下如何使用Assert？

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()

Answer 2

All your function seems to do is print out the result so what the actual function is returning is None so assert factorial(1) == 1 is actually being evaluated as assert None == 1 , which fails. Return a value from the function and the assert will no longer fail.

Answer 3

The problem here is you are not returning a value for the assert function to check what is returned by factorial function.

This is what I did: Just to test I simply added a return statement to return the results.

>>> assert factorial(2)==1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
>>> assert factorial(2)==2
>>> assert factorial(1)==1

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)
        return result

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