I hope CS50 community can help why I got expected exit code 0, not 1

Question

If I "return None" explicitly for exceptions in a regular .py file, when I do the test.py file, is it correct to test with a "assert function(argument) is None?" I did a pytest and it worked fine, but when I did a CS50 check, it gave me:

":( correct fuel.py passes all test_fuel checks
expected exit code 0, not 1"

If I remove the test cases under test_convert_invalid(), CS50 did not give me the exist code 0 prompt, but it gave errors on checks for "ValueError" and "ZeroDivisionError".

Is my assert wrong under def test_convert_invalid()? If it is, why does the test file passed with pytest? How can I fix the issue?

I returned "None" explicitly because return False will be equivalent to 0, which will result in "E" (Empty tank).

`fuel.py`

import sys


def main():
    while True:
        try:
            # convert() expects a str and returns an int
            percentage = convert(input("Fraction: "))

            # gauge() expects an int and returns a str
            print(gauge(percentage))
        except:
            continue
        else:
            sys.exit()


def convert(fraction):
    try:
        x, y = fraction.split("/")

        if not x[:].isdigit() or not y[:].isdigit():
            raise TypeError

        if y == "0":
            raise ZeroDivisionError

        if int(x) > int(y) and int(y) != 0:
            raise ValueError

    except TypeError:
        return None
    except ZeroDivisionError:
        return None
    except ValueError:
        return None
    else:
        return int((float(x) / float(y)) * 100)


def gauge(percentage):
    if 0 <= percentage <= 1:
        return "E"
    elif percentage >= 99:
        return "F"
    else:
        return str(percentage) + "%"


if __name__ == "__main__":
    main()



`test_fuel.py`

from fuel import convert, gauge

def test_convert_valid():
    assert convert("3/4") == 75
    assert convert("100/100") == 100
    assert convert("99/100") == 99
    assert convert("0/100") == 0
    assert convert("1/100") == 1


def test_convert_invalid():
    assert convert("1/0") is None
    assert convert("4/3") is None
    assert convert("test") is None
    assert convert("t/1") is None
    assert convert("1/t") is None


def test_gauge():
    assert gauge(0) == "E"
    assert gauge(1) == "E"
    assert gauge(100) == "F"
    assert gauge(99) == "F"
    assert gauge(75) == "75%"

Answer 1

Please have A look at my code, please upvote if helps,your code a kind of complicated. I tested with check50 as well, All Passed. fuel.py

def main():
    fraction = input("Fraction: ")
    result = gauge(convert(fraction))
    print(result)

def convert(fraction):
    x,y = fraction.split("/")
    try:
        return (int(x)/int(y)) * 100
    except (ValueError,ZeroDivisionError):
        raise

def gauge(percentage):
    if 0 <= percentage <= 100:
        if percentage >= 99:
            return "F"
        elif percentage <= 1:
            return "E"
        else:
            return f"{percentage:.0f}%"

if __name__ == "__main__":
    main()

test_fuel.py

from fuel import convert,gauge
import pytest

def test_convert():
    assert convert("1/2") == 50
    assert convert("1/4") == 25
    with pytest.raises(ValueError):
        convert("cat/dog")
    with pytest.raises(ZeroDivisionError):
        convert("2/0")
def test_gauge():
    assert gauge(50) == "50%"
    assert gauge(100) == "F"
    assert gauge(0) == "E"
    assert gauge(1) == "E"
    assert gauge(99) == "F"

Answer 2

@souttawther has a small fallacy in his answer, It does not take into consideration the case when y > x and prints None, which is wrong as per the problem description in the course website, ideally it must raise a value error, though this isn't covered by the check50. So here is my version of the code which covers that corner case as well.

fuel.py

def main():
    inp = input("Fraction:")
    percentage = convert(inp)

    print(gauge(percentage))

def convert(fraction):


    x,y = fraction.split("/")

    try:
        x = int(x)
        y = int(y)
        print(x,y)
    except ValueError:
            raise

    try:
        if x <= y and y!=0:
            try:
                return round((x/y)*100)
            except (ZeroDivisionError, ValueError):
                raise
        else:
            if y == 0:
                raise ZeroDivisionError
            else:
                raise ValueError
    except (ZeroDivisionError, ValueError):
        raise

def gauge(percentage):

    if percentage >= 99:
        return("F")
    elif percentage <= 1:
        return("E")
    else:
        return(f"{percentage}%")


if __name__ == "__main__":
    main()

test_fuel.py

from fuel import convert
from fuel import gauge
import pytest

def test_gauge():
    assert gauge(100) == "F"
    assert gauge(99) == "F"
    assert gauge(98) == "98%"
    assert gauge(1) == "E"
    assert gauge(2) == "2%"
    assert gauge(0) == "E"

def test_convert():
    assert convert("10/10") == 100
    assert convert("1/100") == 1
    assert convert("1/1000") == 0
    with pytest.raises(ValueError):
        convert("5/3")
    with pytest.raises(ZeroDivisionError):
        convert("1/0")
    with pytest.raises(ZeroDivisionError):
        convert("7/0")
    with pytest.raises(ValueError):
        convert("3/cat")
    with pytest.raises(ValueError):
        convert("bat/cat")

Please feel free to optimize this code as I'm not very good at it :) Thanks.

Answer 3

When there is an Exception, you want to return the exception, so your code can handle it properly. You don't want to return None; that is meaningless to the calling function. Google try/except/raise. You should find examples like this:

try:
    1 or more statements
except:
    raise # passes the exception back

A bare raise statement re-raises the current exception in the exception handler, so it can handled further up the call stack (e.g. the calling function). See these SO articles:

• How to use "raise" keyword in Python (1)
• How to use "raise" keyword in Python (2)
• Manually raising (throwing) an exception in Python Scroll to Best Practices: except clause

When trying to learn a new code feature, I write simple examples to understand the behavior. Take a look at this. It shows how a bare raise statement in function() is re-raised to main(). Once you understand how it works, you can adapt to your project. Good luck!

def main():
    test_list = [('1', '2'), ('e', '2'), ('1', '0'), ('4.4', '2')  ]
    for x,y in test_list:
        try:
            z = function(x,y)
            print(f'For x={x} and y={y}; z={z}\n')
        except ValueError:
            print(f'For x={x} and y={y}; ValueError raised by function\n')
        except ZeroDivisionError:
            print(f'For x={x} and y={y}; ZeroDivisionError raised by function\n')       
            
def function(x,y):
    try:
        z = int(x)/int(y)
        return z
    except Exception as e:
        print(f'Exception raised in function: {e}')
        raise # raises the exception to calling function -> main()

if __name__ == "__main__":
    main()