Reading Error Messages¶

Error messages are Python's way of telling you exactly what went wrong and where. This guide teaches you how to decode them.

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Anatomy of a traceback¶

Traceback (most recent call last):
  File "app.py", line 12, in main
    result = process(data)
  File "app.py", line 7, in process
    return data["name"].upper()
TypeError: 'NoneType' object has no attribute 'upper'

Read a traceback bottom to top:

Last line -- the error type and message: TypeError: 'NoneType' object has no attribute 'upper'
Lines above -- the call stack, showing each function that was called. The bottom frame is where it actually broke.
File and line number -- tells you exactly where to look: File "app.py", line 7
Code snippet -- the exact line that failed: return data["name"].upper()

The call stack shows the path Python took to reach the error. In this example: main() called process(), and process() broke on line 7.

The 10 most common errors¶

1. SyntaxError -- Python cannot parse your code¶

# Missing colon
if x > 5
    print("big")

  File "script.py", line 1
    if x > 5
           ^
SyntaxError: expected ':'

What it means: Your code has a grammar mistake. Python could not even begin running it. Common causes: Missing colons, unmatched parentheses/quotes, typos in keywords.

2. NameError -- a name does not exist¶

print(username)

NameError: name 'username' is not defined

What it means: You used a name Python has never seen. It does not exist in the current scope. Common causes: Typos in variable names, using a variable before defining it, forgetting an import.

3. TypeError -- wrong type for the operation¶

age = 25
print("Age: " + age)

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

What it means: You tried to do something with the wrong type of value. Common causes: Adding strings and numbers, calling something that is not a function, wrong number of arguments.

4. ValueError -- right type, wrong value¶

number = int("hello")

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

What it means: The type is correct (it is a string), but the specific value cannot be used. Common causes: Converting non-numeric strings to numbers, unpacking the wrong number of values.

5. IndexError -- list index out of range¶

items = ["a", "b", "c"]
print(items[5])

IndexError: list index out of range

What it means: You asked for a position that does not exist in the list. Common causes: Off-by-one errors, looping past the end, empty lists.

6. KeyError -- dict key does not exist¶

person = {"name": "Alice"}
print(person["email"])

KeyError: 'email'

What it means: You asked for a dictionary key that is not there. Fix: Use .get("email") for safe access, or check with if "email" in person: first.

7. AttributeError -- object does not have that attribute¶

name = None
print(name.upper())

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

What it means: You tried to access a method or property that does not exist on this type. Common causes: Calling methods on None (a function returned nothing), typos in method names.

8. ImportError / ModuleNotFoundError -- cannot find the module¶

import pandas

ModuleNotFoundError: No module named 'pandas'

What it means: Python cannot find the module you are trying to import. Common causes: Package not installed (pip install pandas), wrong virtual environment, file named the same as a module.

9. FileNotFoundError -- file does not exist¶

with open("data.csv") as f:
    data = f.read()

FileNotFoundError: [Errno 2] No such file or directory: 'data.csv'

What it means: The file path you gave does not point to an existing file. Common causes: Wrong filename, wrong directory, typo in the path.

10. IndentationError -- inconsistent indentation¶

def greet():
print("hello")

IndentationError: expected an indented block after function definition on line 1

What it means: Python expected indented code but found something at the wrong level. Common causes: Missing indentation after if/for/def/class, mixing tabs and spaces.

How to read each part of a traceback¶

Traceback (most recent call last):       ← Start of the call chain
  File "main.py", line 15, in <module>   ← Which file, which line, which scope
    run()                                 ← The code on that line
  File "main.py", line 10, in run        ← One level deeper
    total = add_prices(items)             ← The code that called the next level
  File "main.py", line 5, in add_prices  ← Where it actually broke
    total += item["price"]               ← The exact failing line
KeyError: 'price'                        ← What went wrong (THE ANSWER)

Step-by-step reading process:

Look at the very last line -- this is the error type and message
Look at the frame just above -- this is where the error actually happened
Note the file name and line number -- open your file and go to that line
Read the code snippet -- does the code match what you expected?
If needed, trace upward through the call stack to understand how you got there

The `friendly` library¶

The friendly library rewrites Python error messages in plain English:

pip install friendly

# Instead of running: python script.py
# Run: python -m friendly script.py

It turns cryptic messages into explanations like:

A NameError exception indicates that a variable or function name
is not known to Python. Most often, this is because there is a
spelling mistake. However, sometimes it is because the name is
used before being defined or given a value.

Did you mean 'username'?

This is especially helpful while you are still learning to read tracebacks on your own.

Practice¶

Quick check: Take the quiz

Review: Flashcard decks Practice reps: Coding challenges

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