lecture3.md

CS50P W3 - Exceptions

try and except

x = int(input("What's x? "))
print(f"x is {x}")

If user input is a str, we will get the runtime error ValueError (the value of variable is not the one expected).

try:
    x = int(input("What's x? "))
    print(f"x is {x}")
except ValueError:
    print("x is not an integer")

2. Best practice is we should try the fewest lines of code possible

try:
    x = int(input("What's x? "))
except ValueError:
    print("x is not an integer")

print(f"x is {x}")

Now if user input is a str, we will get the runtime error NameError: name ‘x’ is not defined. This is because the wrong input triggers the Error handling except before ‘x’ gets to be defined.

3. try and except with else block

try:
    x = int(input("What's x? "))
except ValueError:
    print("x is not an integer")
else:
    print(f"x is {x}")

Now if the except block does not get triggered, the else block will.

4. try and except with while loop and else block

while True:
    try:
        x = int(input("What's x? "))
    except ValueError:
        print("x is not an integer")
    else:
        break
print(f"x is {x}")

Now, with the while loop, if the except block gets triggered, the program will prompt the user again infinitely until the right value is entered.

5. Create the function get_int()

def main():
    x = get_int()
        print(f"x is {x}")

# Version 1
def get_int():
    while True:
        try:
            x = int(input("What's x? "))
        except ValueError:
            print("x is not an integer")
        else:
            break
    return x

main()

# Version 2
def get_int():
    while True:
        try:
            x = int(input("What's x? "))
        except ValueError:
            print("x is not an integer")
        else:
            return x

The break is not needed, return x will stop the loop while returning the value.

# Version 3
def get_int():
    while True:
        try:
            x = int(input("What's x? "))
	        return x
        except ValueError:
            print("x is not an integer")

The else block can be removed. Placing return x in the try statement

# Version 4
def get_int():
    while True:
        try:
            return int(input("What's x? "))
        except ValueError:
            print("x is not an integer")

We can even avoid an extra line for the return x statement and avoid defining x explicitly by returning the result of int(input(...))

6. try and except with pass

def get_int():
    while True:
        try:
            return int(input("What's x? "))
        except ValueError:
            pass

We can use the pass keyword to not act on the error and simply prompt again.

7. Final refined version of get_int() adding a function parameter

def main():
    x = get_int("What's x? ")
    print(f"x is {x}")


def get_int(prompt):
    while True:
        try:
            return int(input(prompt))
        except ValueError:
	        pass

main()

We can improve the implementation of the get_int() function. It is a good idea for main() to show the prompt argument ("What's x? ") and the get_int() to accept a parameter (prompt).

distances.py

distances = {
    "Voyager 1": "163",
    "Voyager 2": "136",
    "Pioneer 10": "80 AU",
    "New Horizons": "58",
    "Pioneer 11": "44 AU"
}

def main():
    spacecraft = input("Enter a spacecraft: ")
    m = convert(distances[spacecraft])
    print(f"{m} m away")

def convert(au):
    return au * 149597870700

main()

Because passing a string to the convert() function, the function will attempt to multiply the string by the huge number and this code will result in a MemoryError. We need to convert the string before converting the units.

def main():
    spacecraft = input("Enter a spacecraft: ")
    au = float(distances[spacecraft])
    m = convert(au)
    print(f"{m} m away")

Now we fixed the string problem converting the strings to floats before they are passed in as arguments to convert(). But we still have messy data in some of the key-value pairs, where the value is a string composed of letters and digits = ValueError

2. try and except

def main():
    spacecraft = input("Enter a spacecraft: ")
    try:
        au = float(distances[spacecraft])
    except ValueError: 
	    print(f"Can’t convert '{distances[spacecraft]}' to a float")
        return

        m = convert(au)
    print(f"{m} m away")

The try and except block will now handle the ValueError.

3. Adding except blocks for different errors

def main():
    spacecraft = input("Enter a spacecraft: ")
    
    try:
        au = float(distances[spacecraft])
    except KeyError:
	    print(f" '{spacecraft}' is not in dictionary")
        return
    except ValueError: 
	    print(f"Can't convert '{distances[spacecraft]}' to a float")
        return

        m = convert(au)
    print(f"{m} m away")

We now have an except block for each ValueErrors and KeyErrors. It is important to try to anticipate potential errors when writing code and include them in the try and except block.