Python 3 Built-in Exceptions

List of built-in exceptions in Python 3.

Base Classes

These exceptions are generally used as base classes for other exceptions.

Exception Description
BaseException The base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes (for that, use Exception below).
Exception All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.
ArithmeticError The base class for built-in exceptions that are raised for various arithmetic errors. These are: OverflowError, ZeroDivisionError, FloatingPointError.
BufferError Raised when a buffer related operation cannot be performed.
LookupError The base class for exceptions that are raised when a key or index used on a mapping or sequence is invalid. These are: IndexError and KeyError.

Concrete Exceptions

These are the exceptions that are usually raised.

Exception Description
AssertionError Raised when an assert statement fails.

Raised when an attribute reference or assignment fails.

EOFError Raised when the input() function hits an end-of-file condition (EOF) without reading any data.
FloatingPointError Raised when a floating point operation fails.
GeneratorExit Raised when a generator or coroutine is closed. Technically, this is not an error, and the GeneratorExit exception directly inherits from BaseException instead of Exception.
ImportError Raised when the import statement fails when trying to load a module. This exception is also raised when the from in from ... import has a name that cannot be found.
ModuleNotFoundError A subclass of ImportError which is raised by import when a module could not be located. This exception is also raised when None is found in sys.modules.
IndexError Raised when a sequence subscript is out of range.
KeyError Raised when a mapping (dictionary) key is not found in the set of existing keys.
KeyboardInterrupt Raised when the user hits the interrupt key (typically Control-C or Delete).
MemoryError Raised when an operation runs out of memory but the situation may still be rescued (by deleting some objects).
NameError Raised when a local or global name is not found. This applies only to unqualified names.
NotImplementedError This exception is derived from RuntimeError. In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method, or while the class is being developed to indicate that the real implementation still needs to be added.
OSError() This exception is raised when a system function returns a system-related error, including I/O failures such as "file not found" or "disk full".
OverflowError Raised when the result of an arithmetic operation is too large to be represented.
RecursionError This exception is derived from RuntimeError. It is raised when the interpreter detects that the maximum recursion depth is exceeded.
ReferenceError Raised when a weak reference proxy, created by the weakref.proxy() function, is used to access an attribute of the referent after it has been garbage collected.
RuntimeError Raised when an error is detected that doesn’t fall in any of the other categories.
StopIteration Raised by the next() function and an iterator's __next__() method to signal that there are no further items produced by the iterator.
StopAsyncIteration Must be raised by __anext__() method of an asynchronous iterator object to stop the iteration.
SyntaxError Raised when the parser encounters a syntax error.
IndentationError Base class for syntax errors related to incorrect indentation. This is a subclass of SyntaxError.
TabError Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of IndentationError.
SystemError Raised when the interpreter finds an internal error, but it's not serious enough to exit the interpreter.
SystemExit This exception is raised by the sys.exit() function, and (when it is not handled) causes the Python interpreter to exit.
TypeError Raised when an operation or function is applied to an object of inappropriate type.
UnboundLocalError Raised when a reference is made to a local variable in a function or method, but no value has been bound to that variable. This is a subclass of NameError.
UnicodeError Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of ValueError.
UnicodeEncodeError Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError.
UnicodeDecodeError Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError.
UnicodeTranslateError Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError.
ValueError Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception.
ZeroDivisionError Raised when the second argument of a division or modulo operation is zero.

OS Exceptions

These exceptions are subclasses of OSError. They get raised depending on the actual system error code.

Exception Description
BlockingIOError Raised when an operation would block on an object (e.g. socket) set for non-blocking operation.
ChildProcessError Raised when an operation on a child process failed.
ConnectionError A base class for connection-related issues.
BrokenPipeError A subclass of ConnectionError, raised when trying to write on a pipe while the other end has been closed, or trying to write on a socket which has been shutdown for writing.
ConnectionAbortedError A subclass of ConnectionError, raised when a connection attempt is aborted by the peer.
ConnectionRefusedError A subclass of ConnectionError, raised when a connection is reset by the peer.
FileExistsError Raised when trying to create a file or directory which already exists.
FileNotFoundError Raised when a file or directory is requested but doesn’t exist.
InterruptedError Raised when a system call is interrupted by an incoming signal.
IsADirectoryError Raised when a file operation is requested on a directory.
NotADirectoryError Raised when a directory operation is requested on something which is not a directory.
PermissionError Raised when trying to run an operation without the adequate access rights.
ProcessLookupError Raised when a given process doesn’t exist.
TimeoutError Raised when a system function timed out at the system level.


The following exceptions are used as warning categories.

Exception Description
Warning Base class for warning categories.
UserWarning Base class for warnings generated by user code.
DeprecationWarning Base class for warnings about deprecated features.
PendingDeprecationWarning Base class for warnings about features which will be deprecated in the future.
SyntaxWarning Base class for warnings about dubious syntax.
RuntimeWarning Base class for warnings about dubious runtime behavior.
FutureWarning Base class for warnings about constructs that will change semantically in the future.
ImportWarning Base class for warnings about probable mistakes in module imports.
UnicodeWarning Base class for warnings related to Unicode.
BytesWarning Base class for warnings related to bytes and bytearray.
ResourceWarning Base class for warnings related to resource usage.