Catching Specific Exceptions

Lesson

Catching Specific Exceptions

A try can have several except blocks — one per exception type. The block matching the raised exception runs:

def read_score(scores, position_text):
    try:
        position = int(position_text)
        return scores[position]
    except ValueError:
        return "not a number"
    except IndexError:
        return "no such position"

scores = [80, 92, 75]
print(read_score(scores, "1"))
print(read_score(scores, "one"))
print(read_score(scores, "9"))

Output:

92
not a number
no such position

Each failure gets its own response: int("one") raised ValueError, scores[9] raised IndexError.

Catch the narrowest exception that covers the situation. A bare except: catches everything — including typos and genuine bugs — and turns them into silent wrong behavior. This course never uses it.

Exercise

Define a function named price_lookup with two parameters: prices (a dictionary) and item.
It returns prices[item] divided by 100 (the price is stored in cents; return dollars).
When the item is missing, return the string "no such item". When the stored value is not divisible — a TypeError, for example the string "n/a" — return "bad price data".

price_lookup({"apple": 450}, "apple") returns 4.5.

Run your code to see the output, then press Submit.

Tests

import unittest


class TestPriceLookup(unittest.TestCase):
    def test_valid_item_returns_kroner(self):
        self.assertEqual(price_lookup({"apple": 450}, "apple"), 4.5)

    def test_missing_item(self):
        self.assertEqual(price_lookup({"apple": 450}, "pear"), "no such item")

    def test_unparseable_price(self):
        self.assertEqual(price_lookup({"apple": "n/a"}, "apple"), "bad price data")

Solution

def price_lookup(prices, item):
    try:
        return prices[item] / 100
    except KeyError:
        return "no such item"
    except TypeError:
        return "bad price data"


print(price_lookup({"apple": 450}, "apple"))
print(price_lookup({"apple": 450}, "pear"))
print(price_lookup({"apple": "n/a"}, "apple"))

Solution hidden. Give it a real try first.

Discussion

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main.py

Console

Press Run to execute your code, or Submit to test and complete this problem.