Week 3: Exceptions

Exceptions

#

That means that something went wrong in the program. It is an error that happens during the execution of the program.

Syntax Error

#

This is just and introductory example. The error is in the syntax of the code. It is a mistake in the code that the python interpreter can not execute.

print("Hello World)

This error is a error that you need to fix, you can’t create a code to handle this error.

ValueError

#

This error happens when you try to convert a string to a number, but the string is not a number.

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

This programs works fine if you input a number, but if you input a string, it will raise a ValueError.

try, except

#

You can handle exceptions with the try and except blocks.

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

NameError

#

This error happens when you try to use a variable that is not defined.

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

print(f"x is {x}")

else

#

You can use the else block to run a code if the try block does not raise an exception.

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

Reprompting, break

#

You can use a while loop to reprompt the user if the input is not valid.

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

print(f"x is {x}")

get_int

#

You can create a function to get an integer from the user.

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

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

main()

pass

#

You can use the pass statement to do nothing.

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

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

main()

Note: In this case I wouln’t use the pass argument, I think as a user would be weird to just be asked again and again for the same input.

function arguments

#

This is a refined version of the get_int function.

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

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

main()

raise

#

This was just a quick mention of the raise statement.