pytest.raises(ValueError) passes for any ValueError, including one raised by a bug on a different line. Two more tools make a test say which error it means.

import pytest

class OutOfStock(Exception):
    def __init__(self, item, short):
        self.short = short
        super().__init__(f"{item} short")

def take(stock, item, n):
    if stock[item] < n:
        short = n - stock[item]
        raise OutOfStock(item, short)
    stock[item] -= n

stock = {"tea": 2}
with pytest.raises(OutOfStock,
                   match="tea") as info:
    take(stock, "tea", 5)
print(info.value.short, stock)

• match is a regular expression, searched for in the message. Wrap literal text in re.escape if it has special characters.
• as info keeps the exception. After the block, info.value is the exception object, with its attributes.

A refused operation should usually leave things as they were, so check the state after the block too: here stock must still hold 2. Keep only the raising call inside the with, because nothing after it in the block runs.

withdraw and InsufficientFunds are written for you. Write test_rejects_zero_amount, using match to check it is the ValueError about a positive amount, and test_insufficient_funds, which withdraws 50 from {"ann": 30} and checks that the exception's needed is 20 and the balance is still 30.

The explanation opens here once every check passes.