Reading Error Messages

Why this matters

Everyone who writes code sees errors, constantly. Beginners panic; experienced programmers read them. An error message is Python telling you exactly what it could not do and where. Learning to read them is the single biggest time-saver you will pick up.

The idea

When your program breaks, Python prints a traceback. Here is one:

Traceback (most recent call last):
  File "exercise.py", line 3, in <module>
    print(nme)
NameError: name 'nme' is not defined

Read it from the bottom up:

1. The last line says what kind of error it is (NameError) and what went wrong (name 'nme' is not defined).
2. The line above shows the code that failed.
3. line 3 tells you where to look.

The kinds you will meet most often early on:

Error	What it means	Typical cause
SyntaxError	Python cannot understand how the code is written	a missing quote or bracket, a missing :
NameError	You used a name Python does not know	a typo in a variable name, or using it before creating it
TypeError	You used the wrong kind of value	adding text to a number
ZeroDivisionError	You divided by zero	10 / 0
IndentationError	The spacing at the start of a line is wrong	you will meet this with if soon

Two habits to build: fix one error at a time, and run again after every fix. Fixing one problem often changes the next message.

Try it

Make a mistake on purpose in the editor: type print(hello) without quotes, press Run, and read the last line. Then fix it and try a different mistake.

Your turn

The program in the editor has three bugs. Each one produces a different error. Press Run, read the error, fix that bug, and repeat until the program prints exactly:

Starting the program
Hello, Ada
Half of 10 is 5.0

Recap

• Read a traceback from the bottom up: error type and message first, then the line number.
• SyntaxError means Python could not read the code; NameError means an unknown name.
• Fix one error at a time and run again.

Common mistakes

• Fixing the line Python points at when the real problem is the line before. A missing ) on one line often gets reported on the next line.
• Changing many things at once. You will not know which change helped.
• Ignoring the message. It is usually accurate. Read the actual words.