Errors and Exceptions in Python in Python

def spend(self, amount):
    if amount > self.money:
        raise ValueError("not enough money")

    self.money -= amount

try:
    wallet.spend(80)
except ValueError as e:
    print("error:", e)

Full lesson:

Some requests can't be done. A wallet with 50 can't spend 80.

The method could return a special value, like -1. But the caller may forget to check it. Then the program goes on with a wrong number.

An exception is better. It is an error that the program can't miss.

Raising an error

• raise ValueError("not enough money") stops the method at once.
• The lines after it don't run. So the wallet keeps its money.
• ValueError is Python's built-in error for a bad value.
• The text in the parentheses is the error's message.

Catching it

• Put the call inside a try block.
• When the call raises an error, the rest of the try block is skipped.
• Python runs the except ValueError as e block instead. e is the error.
• Printing e shows its message.
• Then the program goes on below the except block.

An error that nobody catches stops the whole program. Python prints the error and the line where it happened.

Your task

A Wallet starts with 50. Its spend method takes money out and returns what is left.

• In spend, raise a ValueError when amount is more than self.money. Its message is "not enough money".
• In the loop, call spend inside a try block. Print what was spent and what is left.
• Catch the ValueError. Print error: and its message.

The loop tries to spend 20, then 40, then 30.

Expected output:

spent 20, 30 left
error: not enough money
spent 30, 0 left