Reading an Error Message

Every programmer sees errors, every day. The skill is not to avoid them. The skill is to read them.

How to read an error message

1. Read the last line first. It names the type of the error and says what went wrong.
2. Find the line number above it. That is where Python noticed the problem.
3. Look at that line, and the line before it. The cause is often just before.
4. Fix one thing, then run again. One mistake can hide another.

The common errors

Try it

This program has a mistake on purpose. Run it and read the last line of the message.

A different kind of problem

Sometimes the program runs without an error, but the answer is wrong. This is a logic error. Python cannot find it for you, because it does what you wrote, not what you meant. To find it, print the values in the middle of the program and compare them with what you expect.

Mistakes to avoid

• Changing many things at once. Then you do not know which change helped.
• Reading from the top. The top of the message is only the route. The answer is at the bottom.
• Panicking when a new error appears. A new error often means the first one is fixed, and Python can now see the next problem.

Think it through

Write your answers in full sentences. Try each one before you read the pointers.

1. For each line, say which error you expect and how you would fix it: (a) print("Total: " + 5). (b) print(nam) when the variable is called name. (c) a for i in range(3) line with no colon at the end.
2. Why is it better to read an error message from the bottom?
3. A student fixes one error and a new error appears. The student says, "My fix broke the program." Explain what probably happened.
4. A program divides by a number typed in by the user. Using only an if, how can you stop a ZeroDivisionError?
5. A program for the area of a rectangle runs without any error, but gives 12 for a 3 by 5 rectangle. What kind of error is this? How would you find it?

Pointers for your answers

1. (a) TypeError. Turn the 5 into text with str(5), or use a comma in print. (b) NameError. Use name. (c) SyntaxError. Add a colon.
2. The last line says what the problem is. The lines above only show the route that the program took.
3. Python stops at the first error. When it is fixed, Python goes on and finds the next one. The program was not broken by the fix.
4. Check if divisor == 0: first, and print a message. Only divide in the else part.
5. A logic error. The program probably added instead of multiplying. Print the values of the two sides and the result, and compare them with a hand calculation.