Python’s error messages look scary at first, but they follow a pattern. The last line names the kind of error and gives a short reason. The lines above it say where it happened: the file, the line number, and the line itself. Read from the bottom up.

Output

Traceback (most recent call last):
  File "C:\Users\Sara\totals.py", line 4, in <module>
    average = total / count
              ~~~~~~^~~~~~~
ZeroDivisionError: division by zero

Here the last line says what went wrong (dividing by zero), and the line above it says where (line 4 of totals.py). That’s usually all you need. Chapter 8 explains tracebacks in detail, and Chapter 21 shows how to hunt down the harder bugs.

#Errors in your code

These are the errors you’ll meet most often while you’re writing programs. They’re listed alphabetically, so you can find yours quickly.

Error	What it usually means	What to try	More in
`AttributeError`	You asked an object for something it doesn’t have, such as `name.uper()`	Check the spelling. Use `dir(thing)` to list what it does have. In a class, check `__init__` creates the attribute.	Ch. 11
`FileNotFoundError`	The file isn’t where Python looked	Check the name and extension. Build the path from `Path(__file__).parent`. Open the project folder in VS Code.	Ch. 9
`ImportError`	The module exists, but not the name you asked for from it	Check the spelling after `from ... import`. Don’t name your own file after a module, such as `random.py`.	Ch. 10
`IndentationError`	The spaces at the start of a line don’t line up	Indent the block under `if`, `for`, `def` or `class` by four spaces, and be consistent.	Ch. 4
`IndexError`	You asked for a list position that doesn’t exist	Remember counting starts at 0, and the last item is `items[-1]` or `items[len(items) - 1]`.	Ch. 6
`JSONDecodeError`	The text you tried to read as JSON isn’t valid JSON	Print the text first. Look for single quotes, a trailing comma, or an error page instead of data.	Ch. 20
`KeyboardInterrupt`	You pressed Ctrl+C. Not really an error	Useful for stopping a loop that never ends.	Ch. 5
`KeyError`	The dictionary has no such key	Check the spelling and capitals. Use `.get(key)` when a key might be missing.	Ch. 6
`ModuleNotFoundError`	Python can’t find a module, often a package you haven’t installed	`python -m pip install` the package, in the same Python (and virtual environment) that runs your code.	Ch. 10, 23
`NameError`	You used a name Python hasn’t seen yet	Check the spelling. Make sure the variable is created before the line that uses it. Did you forget quotes around text?	Ch. 3, 8
`PermissionError`	The operating system won’t let Python touch that file	Close the file in Excel or Word, or choose a folder you own.	Ch. 14
`SyntaxError`	The line isn’t valid Python	Look for a missing colon, bracket or quote, on that line or the one above it.	Ch. 2, 4
`TypeError`	The operation doesn’t work with these types, like `"5" + 3`	Convert with `int()`, `float()` or `str()`. Check you’re passing the right number of arguments.	Ch. 3, 7
`UnboundLocalError`	Inside a function, you changed a variable that lives outside it	Pass the value in as a parameter and `return` the new value.	Ch. 7
`UnicodeDecodeError`	The file’s characters aren’t in the encoding Python expected	Open it with `encoding="utf-8"`, or try `encoding="utf-8-sig"` or `"latin-1"`.	Ch. 9, 21
`ValueError`	Right type, unusable value, like `int("hello ")`	Check input before converting, or catch it with `try` and `except`.	Ch. 3, 8
`ZeroDivisionError`	You divided by zero, often an average of an empty list	Check the count isn’t zero first.	Ch. 8

#Errors from classes and functions

A few TypeError messages are so common they deserve their own row:

You see	It means	Fix
`takes 0 positional arguments but 1 was given`	A method is missing `self`	`def volume_up(self):`
`missing 1 required positional argument`	You called a function or made an object without all the values it needs	Count the parameters in the `def` line and pass that many
`'NoneType' object is not subscriptable` (or has no attribute)	A function returned nothing, so you got `None`	Check the function ends with `return`
`'str' object is not callable`	You used a function’s name as a variable, such as `input = 5`	Rename the variable and restart
`unsupported operand type(s) for +: 'int' and 'str'`	You tried to add a number and text	Convert one of them first

#Errors from your computer, not your code

Some messages come from your terminal, your operating system or the internet. Your code may be perfectly fine.

You see	What’s going on	What to do
`'python' is not recognized as an internal or external command` (Windows)	Python isn’t on the PATH	Try `py`. Then see Appendix C.
`command not found: python` (Mac)	On a Mac the command is `python3`	Type `python3` instead.
`can't open file ... No such file or directory`	The terminal isn’t in the folder where your file is	Use `cd` to go to the folder, or run it with VS Code’s Run button.
`error: externally-managed-environment`	Your system’s Python doesn’t want packages installed into it	Make a virtual environment first (Chapter 23).
`running scripts is disabled on this system` (PowerShell)	A Windows security setting blocks the activate script	Use Command Prompt, or see Appendix C.
`CERTIFICATE_VERIFY_FAILED`	Python can’t check a website’s security certificate	On a Mac, run Install Certificates.command (Chapter 2).
`ConnectionError` or `Max retries exceeded`	No internet, a typo in the address, or the site is down	Check the address in a browser. Try again later.
`Address already in use`	Another program, often an old copy of your Flask app, is using that port	Stop the old one with Ctrl+C, or close the terminal it’s in.
`SMTPAuthenticationError`	The email server rejected your username or password	For Gmail, use an app password (Chapter 27).