NameError: name 'x' is not defined
Meaning, common causes, and fixes with examples.
Errors & Exceptions
What each error means, why it happens, and how to fix it—with short, runnable examples. Quick debugging for real code.
When Python stops your program and prints an error message, it is not trying to confuse you—it is telling you exactly where and why something went wrong. This section is a reference for the most common errors you will see as a beginner or as you move into real projects. Each article explains the error in plain English, shows a typical cause with runnable code, and gives one or more ways to fix it.
Errors fall into a few broad groups. Name errors mean you used a variable or name that Python does not know (for example a typo or a name used before it is defined)—see NameError and UnboundLocalError. Type errors mean you used a value of the wrong type: calling something that is not a function (e.g. int not callable, list not callable), mixing numbers and strings (int and str), or using None where a list or dict was expected (NoneType has no len(), NoneType not subscriptable, NoneType not iterable). Index and key errors happen when you access a position or key that does not exist—IndexError: list index out of range, string index out of range, KeyError, and KeyError with nested dicts. Value errors usually mean the value you passed is invalid for the operation, such as invalid literal for int() or could not convert string to float.
Syntax and indentation errors (SyntaxError, IndentationError, TabError) come from the shape of your code rather than runtime values. File and import problems (FileNotFoundError, ModuleNotFoundError, ImportError, I/O operation on closed file, PermissionError) are about the environment and how you open or load resources. Understanding these categories helps you know which link to open when you see a traceback.
Use the list below to jump to the exact error. If you are new to Python, start with Getting Started and Data types so that type and name errors make more sense in context.
TypeError: 'NoneType' object is not iterable
When you get None instead of a sequence, and how to avoid it.
IndexError: list index out of range
Why it happens and simple ways to fix or prevent it.
KeyError: 'key'
When a dictionary key doesn't exist, and how to handle it.
AttributeError: 'NoneType' object has no attribute 'x'
Trying to use a method or attribute on None.
ValueError: invalid literal for int() with base 10
When you try to convert a string that isn't a number to an integer.
TypeError: unsupported operand type(s) for +: 'int' and 'str'
Trying to add a number and a string together.
IndentationError: expected an indented block
Python uses indentation to show code blocks—missing indentation causes this error.
SyntaxError: invalid syntax
Your code doesn't follow Python's grammar rules.
FileNotFoundError: [Errno 2] No such file or directory
Python can't find the file you're trying to open.
ImportError: cannot import name 'x' from 'y'
The name you're trying to import doesn't exist in that module.
ModuleNotFoundError: No module named 'x'
Python can't find the module you're trying to import.
ZeroDivisionError: division by zero
You can't divide a number by zero—it's mathematically undefined.
TypeError: 'int' object is not callable
Trying to call an integer like a function.
UnboundLocalError: local variable 'x' referenced before assignment
Using a variable before assigning it in a function.
RecursionError: maximum recursion depth exceeded
A function called itself too many times without stopping.
TypeError: 'list' object is not callable
Trying to call a list like a function.
AttributeError: 'str' object has no attribute 'append'
Trying to use a list method on a string.
ValueError: too many values to unpack (expected 2)
Trying to unpack more values than you have variables.
TypeError: can't multiply sequence by non-int of type 'float'
Trying to multiply a string or list by a float.
TypeError: 'NoneType' object is not subscriptable
Trying to use indexing or slicing on None.
IndexError: string index out of range
Trying to access a character position that doesn't exist in a string.
ValueError: I/O operation on closed file
Trying to read or write to a file that's already been closed.
TypeError: 'builtin_function_or_method' object is not subscriptable
Trying to use square brackets on a function instead of calling it.
AttributeError: 'tuple' object has no attribute 'append'
Trying to modify an immutable tuple.
TypeError: object of type 'NoneType' has no len()
Trying to get the length of None.
ValueError: could not convert string to float
Trying to convert a string that isn't a valid number to a float.
TypeError: 'dict' object is not callable
Trying to call a dictionary like a function.
KeyError when accessing nested dictionary
A key doesn't exist in a nested dictionary structure.
TypeError: 'set' object is not subscriptable
Trying to use indexing on a set, which doesn't support it.