Reading an Error Message

Every programmer sees error messages all day long. The difference between a stuck beginner and everyone else is not that the others make fewer mistakes. It is that they read the message. A Python error report is called a traceback, and it is a precise account of what went wrong and where. It has one rule you need to remember: read it from the bottom up.

Think of the warning light on a car's dashboard. It does not fix the engine, but it names the system that has a problem and saves you guessing. A traceback does the same for your program, and it is far more specific than any dashboard light.

The Last Line First

The last line of a traceback says what kind of problem happened and describes it in a sentence. The kind is a word ending in Error: SyntaxError, NameError, TypeError. Everything above that line is there to tell you where. So start at the bottom, understand the problem, and only then look up.

Then the Location

Above the last line, Python names the file and the line number, and shows the line itself. Under it, a row of carets or tildes marks the exact part that failed. Go to that line in your editor. Most of the time the mistake is right there; sometimes, as the first example shows, the real cause is just before it.

SyntaxError: Python Could Not Read the Line

A SyntaxError means Python could not even understand the text, so nothing in the file ran. Here the opening bracket after print on line 1 is never closed. Python says exactly that, and puts the caret under the bracket that was left open. The line Python struggled with is the next one, but the message points back to where the problem began.

print("Total:", 5
print("done")

  File "/Users/you/python-course/syntax.py", line 1
    print("Total:", 5
         ^
SyntaxError: '(' was never closed

IndentationError: the Blocks Do Not Line Up

An IndentationError means the indentation does not form sensible blocks. In the program below the last line is indented by two spaces, which matches neither the if's block nor the level outside it. The message says the unindent does not match any outer indentation level: Python could not tell which block that line belongs to.

temp = 30
if temp > 25:
    print("It is hot today.")
  print("Drink water.")

  File "/Users/you/python-course/indent.py", line 4
    print("Drink water.")
                         ^
IndentationError: unindent does not match any outer indentation level

NameError: a Name That Does Not Exist

A NameError means the program used a name that was never given a value. Most often it is a typo. Python compares the unknown name with the names it does know, and when one is close it suggests it. Here total was typed as totl, and the message asks: did you mean total?

total = 10
print(totl)

Traceback (most recent call last):
  File "/Users/you/python-course/total.py", line 2, in <module>
    print(totl)
          ^^^^
NameError: name 'totl' is not defined. Did you mean: 'total'?

Notice the first line of this one: Traceback, most recent call last. SyntaxError and IndentationError do not have it, because they stop the file before it starts. A NameError happens while the program is running, and Python reports the path it took to get there. Chapter 8 shows longer paths through several functions; the rule stays the same.

TypeError: the Right Operation on the Wrong Type

A TypeError means an operation was given a type of value it cannot work with. The program below forgot the lesson of Chapter 2: input returns text. Comparing that text with the number 18 fails, and the message says greater-than-or-equal is not supported between a str and an int. The fix is to convert the answer with int first.

age = input("Age: ")
if age >= 18:
    print("adult")

Age: 20
Traceback (most recent call last):
  File "/Users/you/python-course/adult.py", line 2, in <module>
    if age >= 18:
       ^^^^^^^^^
TypeError: '>=' not supported between instances of 'str' and 'int'

What to Do Next

The method is the same for every error. Read the last line and say in plain words what went wrong. Go to the line number. Look at that line, and at the line above it. Change one thing. Run again. If the message changes, you have made progress, even if there is a new error: it means the old problem is gone and Python got further.