The kinds of error

What you will be able to do

After this lesson you can

• read an error message and name the kind of error,
• read a traceback from the bottom,
• tell a SyntaxError from an error while running, and
• match the six common errors to the lines that cause them.

An error is a message, not a failure

You have seen red messages when something went wrong. They are not a punishment. They tell you what went wrong and where. A good programmer reads them with interest, because they are free help.

The most important part is the name of the error, on the last line.

The six to know

Predict first. Each line below breaks a rule. Write down the error name that you expect for each.

The program catches every error with try, so it does not stop, and it can show the name and the message of each. (You learn how try works in the next lesson.)

For example, int("abc") is text, which is the right type to give to int, but the content abc cannot become a number. So it is a ValueError. And "5" + 5 mixes text with a number, so it is a TypeError.

Reading a traceback

When an error stops a program, Python shows a message of several lines. It is called a traceback. It lists the chain of calls that led to the error.

Traceback (most recent call last):
  File "main.py", line 9, in <module>
    report([])
  File "main.py", line 6, in report
    avg = average(marks)
  File "main.py", line 3, in average
    return sum(numbers) / len(numbers)
ZeroDivisionError: division by zero

Read it from the bottom.

1. The last line is the kind of error and what went wrong: ZeroDivisionError: division by zero.
2. The line above it shows the line of code where it happened: return sum(numbers) / len(numbers), inside average.
3. Each pair of lines above that is a step back in the chain: average was called by report, and report was called by the main program.

So the cause: report([]) was called with an empty list, and the average of nothing divides by len([]), which is 0. The fault is not in average, it is in the call, or in a missing check for the empty list.

The name of the error is the first clue. It tells you what kind of thing went wrong before you read any further.

Two families of mistakes

• SyntaxError. The code is written wrongly, for example a missing bracket. Python finds it before the program runs, so not a single line runs.
• Errors while running. The code is written correctly, but something goes wrong with the values: a zero, a missing key, a bad text. These are the ones that we can catch.

Here is a program with a syntax error.

There is a third family that has no message at all: the logic fault. The program runs, and the answer is wrong. No tool can find it for you. You find it by comparing with the answer that you expected.

Common mistakes

• Reading the top of the message first. Read the bottom.
• Fixing the line in the traceback when the real fault is in the call above it, as in the empty list.
• Confusing ValueError and TypeError. Wrong content or wrong type?
• Giving up on a long traceback. It is only a list of calls. The last line and the line above it tell most of the story.
• Not saving the error text when you ask someone for help. Copy the whole message.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the problem Name the error. For each line you say the name of the error, before you run it.
2. Build. Open the lab Which error? The checker confirms the names that you give.
3. Stretch. Make the program in this lesson stop with an IndexError by changing one number, then fix it again.

Remember these

• The name of the error is on the last line of the message.
• The six to know: ValueError, ZeroDivisionError, IndexError, KeyError, TypeError, NameError.
• Read a traceback from the bottom. The cause may be in the call, not in the last line of code.
• A SyntaxError is found before the program runs. A logic fault has no message.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which error does [1, 2, 3][5] give?

2. Which error does 10 / 0 give?

3. Why did the first line of the syntax error program not run?

4. Where do you start reading a traceback?