try / except

Some errors are expected, such as bad user input or dividing by zero. try lets you catch them instead of crashing:

try:
    n = int(text)
except ValueError:
    n = 0

Catch the specific error you expect, not everything.

A try can also have else and finally blocks, and as e gives you the error object:

try:
    n = int(text)
except ValueError as e:    # e holds the error; str(e) is its message
    print("Bad input:", e)
else:                      # runs only when no error happened
    print("Got", n)
finally:                   # always runs last, error or not
    print("Done")

See it run

def read(text):
    try:
        n = int(text)
    except ValueError:
        n = 0
    else:
        print("parsed", n)
    finally:
        print("done")
    return n

read("12")
read("hi")

Watch which lines light up on each call: some are skipped.

Practice try / except

Exercise 1

safe_divide(a, b) returns a / b, or None if b is 0 (catch ZeroDivisionError).
to_int(text) returns int(text), or 0 if the text cannot be converted to an integer, such as "hi" or "3.5" (catch ValueError).

Exercise 2

Write parse_age(text): return an integer age from 0 through 120 inclusive. Return None for non-integer text or an integer outside that range. Catch ValueError for bad numeric text; do not turn a valid zero into a missing age.

Exercise 3

Write readings_average(texts). Try converting each string with float; skip ValueError entries. Return the mean of valid readings, including zero and negative readings, or None when none are valid. All supplied numeric readings are finite.

Exercise 4

Write parse_qty(text) that returns a list of log messages:

try int(text); catch ValueError as e and add "error: " followed by str(e)
in an else block, add "ok: N" with the number
in a finally block, always add "checked" last

So parse_qty("4") returns ["ok: 4", "checked"] and parse_qty("x") returns ["error: invalid literal for int() with base 10: 'x'", "checked"].

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Practice

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Memory · names → values

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Output & plots