Inspired Python: Assertions and Exceptions are not the same

Assertions and Exceptions are not the same

What’s wrong with the withdraw_money function in the code below?

# bank.py
from dataclasses import dataclass

@dataclass
class Client:

    balance: float

def withdraw_money(client: Client, amount: float):
    assert client.balance >= amount, f'Insufficient funds: {client.balance}'
    # Withdraw amount from the client's bank account.
    client.balance -= amount
    return client.balance

client = Client(balance=10.0)
new_balance = withdraw_money(client=client, amount=20.0)
print(f'The new balance is {new_balance}')

If you run it with python bank.py you’re given an error message:

$ python bank.py
Traceback (most recent call last):
  File "bank.py", line 20, in <module>
    new_balance = withdraw_money(client=client, amount=20.0)
  File "bank.py", line 14, in withdraw_money
    assert client.balance >= amount, f'Insufficient funds: {client.balance}'
AssertionError: Insufficient funds: 10.0

Which, at first sight, is correct: I am withdrawing 20.0 from a client account with a balance of 10.0, and the code is specifically written to exit if the withdrawal would put the client account into the red. So what’s the problem, then?

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