Using If Statements To Check If Something Raises An Error

Question

I'm trying to write a program that will solve questions about parametric equations for me. I'm trying to do the following:

I'm trying to find 2 answers to a parametric equation. The first answer will be the positive square root. The second answer will be the negative square root. If the first square root raises a math domain error, don't find the second answer. This is what I have so far:

def hitsGround(vertical_velocity, y_coordinate):
    h = vertical_velocity/-16/-2
    k = -16*(h)**2 + vertical_velocity*(h) + y_coordinate
    time = float(input("At what height do you want me to solve for time?: "))
    try:
        hits_height1 = math.sqrt((time - k)/-16) + h
    except ValueError:
        print("It won't reach this height.")
    else:
        print(f"It will take {hits_height1} seconds to hit said height.")

    try:
        hits_height2 = -math.sqrt((time - k)/16) + h
    except ValueError:
        print("There is no second time it will reach this height.")
    else:     
        print(f"It will take {hits_height2} seconds to hit said height.")

Is there any way to use an if statement to check if the first equation raises a math domain error so I can make it so it doesn't find the second answer? Thanks!

score 0 · Accepted Answer · answered May 15 '20 at 00:25

You cannot test for a run-time exception with if; that's exactly what try-except does. However, when the illegal operation is so directly defined, you can test for that condition before you try the sqrt opertaion:

if (time - k)/-16 < 0:
    # no roots
else:
    # proceed to find roots.

score 0 · Answer 2 · answered May 15 '20 at 00:37

In general, the way to make exception handling easier to is to do everything you need to do to handle the exception within the except. For example, if you don't want to find the second answer after you hit the first exception, just return:

    try:
        hits_height1 = math.sqrt((time - k)/-16) + h
    except ValueError:
        print("It won't reach this height.")
        return

    print(f"It will take {hits_height1} seconds to hit said height.")

If you want to make life even easier, just allow the exception to raise (without catching it at all), and catch it in the calling function!

def hitsGround(vertical_velocity, y_coordinate):
    h = vertical_velocity/-16/-2
    k = -16*(h)**2 + vertical_velocity*(h) + y_coordinate
    time = float(input("At what height do you want me to solve for time?: "))

    hits_height1 = math.sqrt((time - k)/-16) + h
    print(f"It will take {hits_height1} seconds to hit said height.")

    hits_height2 = -math.sqrt((time - k)/16) + h
    print(f"It will take {hits_height2} seconds to hit said height.")

try:
    hitGround(1.0, 10.0)
except ValueError:
    print("It won't reach that height again.")

Wherever the exception is raised in hitsGround, it will immediately stop whatever that function is doing, and it will hit the except in the calling scope. That way you only need a single try/except to cover both cases.

Using If Statements To Check If Something Raises An Error

2 Answers2