Errors and Modules: Quick Revision

A brief note on Errors and Modules. Read it before a test. Try each question at the end before you open its answer.

The kinds of error

• An error is a message, not a failure. Its name is on the last line. Read a traceback from the bottom up.
• The cause may be in the line that called a function, not in the last line of code shown.

• A SyntaxError is found before the program runs. A logic mistake gives no message at all, only a wrong answer.

try and except

• Put the risky line in try. Name the error you expect in except.
• else runs when there was no error. finally runs every time.
• except (ValueError, ZeroDivisionError) as error: catches two kinds, and error holds the message.
• Keep only the risky line in the try, and never write a bare except:.
• Asking again: while True, then try, except with continue, and break once the answer is good. try checks the type; an if checks the range.

Raising your own errors

• raise ValueError("the age must be from 0 to 120") makes an error of your own and stops the function at once.
• Check a value at the door, and refuse a bad one with a message that says what is wrong.
• Look before you leap with an if when a short check exists. Try and catch when the check is hard.

Modules: toolboxes that are ready

• import math, then math.sqrt(49). Or from math import sqrt, then sqrt(49).
• math: ceil rounds up, floor rounds down, trunc cuts off. sin and cos use radians, so convert with math.radians. math.isclose compares two decimal numbers.
• random: randint(a, b) includes both ends. choice picks one item, shuffle mixes the list itself, and sample picks different items. random.seed(n) makes the same numbers come again, for testing.
• statistics: mean, median and mode describe a list of numbers; pstdev gives the spread.
• datetime: subtracting two dates gives a gap, and .days is the number of days. timedelta adds days.
• dir(math) lists the tools of a module, and help(math.ceil) explains one.
• ModuleNotFoundError means a wrong module name. AttributeError means a wrong tool name.

A worked example

What does this program show?

Take the three pieces of text one at a time.

1. "4". int("4") is 4, and share(120, 4) is 30.0. There is no error, so else runs: 4 people get 30.0 rupees each.
2. "zero". int("zero") raises a ValueError at once, so share is never called. The except part prints zero is not usable: invalid literal for int() with base 10: 'zero'.
3. "0". int("0") works, but share raises its own ValueError. The same except catches it: 0 is not usable: people must be at least 1.

One except caught two different mistakes: Python's own error, and the one the function raised.

Practice with new numbers

A program to try

A spinner has four colours. The program asks how many spins to make, refuses a bad answer, counts each colour and names the most common one.

Mistakes to avoid

• A bare except:. It catches every error, even a spelling mistake in your own code, so the real fault hides.
• Too much in try. With ten lines inside, you cannot tell which one failed. Keep only the risky line there.
• A wrong module name. import maths stops with ModuleNotFoundError: No module named 'maths'.
• A wrong tool name. math.root(9) stops with AttributeError: module 'math' has no attribute 'root'.
• Degrees in sin. math.sin(30) is the sine of 30 radians. Write math.sin(math.radians(30)).
• Your own file called random.py. Then import random brings in your file, not the module.

Check yourself

1. Name the error that each of these gives: int("3.5"), [10, 20][2] and 10 / 0.

2. A try has an except, an else and a finally. The lines in try run with no error. Which of the four parts run?

3. Leela puts fifteen lines inside one try, with a bare except: that prints "Something went wrong". Give two reasons why this is a poor habit.

4. Pick a module and a tool for each job. (a) The buses for a trip, rounded up. (b) A winner picked at random from a list. (c) The middle mark of a class. (d) The days from one date to another.

5. Farhan's quiz program asks questions in a random order, so every test run is different. How can he make it repeat the same order while he fixes a fault?