Reading Errors Without Fear 🚨

An error is a bug report written for you, by the machine, instantly, for free

Nobody writes correct code first time. Not you, not anyone. What separates people is that experienced programmers glance at the error, mutter something, and fix it in four seconds, while beginners feel a jolt of dread and start changing things at random.

The dread is unnecessary. The message contains the answer. Here is how to read it.

Anatomy of a traceback

Traceback (most recent call last):
  File "adventure.py", line 12, in <module>
    show_room(rooms[3])
              ~~~~~^^^
IndexError: list index out of range

The eight you will actually meet

1. SyntaxError: it is not valid Python

print("hello"

  File "hello.py", line 1
    print("hello"
         ^
SyntaxError: '(' was never closed

Caught before anything runs. Almost always a missing bracket, quote or colon. Key insight: look at the line above the one Python names. An unclosed bracket on line 8 is often only noticed on line 9.

2. IndentationError: your spacing is wrong

  File "game.py", line 4
    print("inside")
    ^
IndentationError: expected an indented block after 'if' statement on line 3

You wrote a colon and then did not indent, or indented inconsistently. Four spaces, no tabs.

3. NameError: you used a name that does not exist

score = 10
print(scroe)

NameError: name 'scroe' is not defined. Did you mean: 'score'?

Ninety percent of the time: a typo. Python 3.12 even suggests the correction. The other ten percent: you used a variable before creating it, or created it inside a function and tried to use it outside (Lesson 19).

4. TypeError: right idea, wrong kind of thing

print("Total: " + 42)

TypeError: can only concatenate str (not "int") to str

The fix is usually a conversion or an f-string:

print("Total: " + str(42))
print(f"Total: {42}")

Total: 42
Total: 42

5. ValueError: right type, impossible value

int("twelve")

ValueError: invalid literal for int() with base 10: 'twelve'

A string is exactly what int() wants; that particular string is not a number. This is the error that user input causes, constantly.

6. IndexError: past the end

crew = ["Guybrush", "Elaine", "Otis"]
print(crew[3])

IndexError: list index out of range

Three items means positions 0, 1 and 2. The last is always len(x) - 1, or simply x[-1]. This is the classic off-by-one.

7. KeyError: no such dictionary key

KeyError: 'captain'

You asked a dictionary for something it does not have. Lesson 13 shows you .get(), which returns a default instead of exploding.

8. AttributeError: that thing cannot do that

number = 42
number.upper()

AttributeError: 'int' object has no attribute 'upper'

.upper() is a string thing. Numbers do not have it. This one usually means a variable is not holding the type you thought it was, which makes it a good moment to print(type(x)) and find out.

A method that always works

When the message alone is not enough, do not start changing lines at random. Do this instead. It is slower for thirty seconds and faster for the next two hours.

1. Read the last line. Out loud if necessary. It names the problem.
2. Go to the file and line number. Look at that line, and the one above it.
3. Print what you assumed. The bug is always in the gap between what you believe and what is true. Make the belief visible:

   row = "12,Guybrush,pirate"
   parts = row.split(",")
   
   print(f"{parts=}")
   print(f"{len(parts)=}")
   print(f"{type(parts[0])=}")

   parts=['12', 'Guybrush', 'pirate']
   len(parts)=3
   type(parts[0])=<class 'str'>

4. Cut the program in half. Does the first half do what you expect? Then the bug is in the second half. Repeat. Ten halvings finds a bug in a thousand lines.
5. Explain it to a duck. Out loud, line by line, to a rubber duck or a houseplant. You will very often catch it yourself mid-sentence. This is a real, named, widely used technique.

When you genuinely need help

A good question gets an answer in minutes. A bad one gets silence. The difference:

The act of writing that up solves the problem outright often enough to have a name: it is why Stack Overflow's "ask a question" page is the world's most effective debugger.

# A
print("Result: " + 10)

# B
name = input("Name: ")
print(nmae)

# C
numbers = [1, 2, 3]
print(numbers[3])

# D
print(int("3.5"))

crew = ["Guybrush", "Elaine", "Otis"]

print("Crew size: " + len(crew))

for i in range(4):
    print(crew[i])

if len(crew) > 2
    print("A full crew")

crew = ["Guybrush", "Elaine", "Otis"]

print(f"Crew size: {len(crew)}")

for name in crew:
    print(name)

if len(crew) > 2:
    print("A full crew")

Crew size: 3
Guybrush
Elaine
Otis
A full crew