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# 5. Built-in Exceptions

In Python, all exceptions must be instances of a class that derives from `BaseException`. In a `try` statement with an `except` clause that mentions a particular class, that clause also handles any exception classes derived from that class (but not exception classes from which it is derived). Two exception classes that are not related via subclassing are never equivalent, even if they have the same name.

The built-in exceptions listed below can be generated by the interpreter or built-in functions. Except where mentioned, they have an “associated value” indicating the detailed cause of the error. This may be a string or a tuple of several items of information (e.g., an error code and a string explaining the code). The associated value is usually passed as arguments to the exception class’s constructor.

User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition “just like” the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error.

The built-in exception classes can be sub-classed to define new exceptions; programmers are encouraged to at least derive new exceptions from the `Exception` class and not `BaseException`. More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions.

When raising (or re-raising) an exception in an `except` clause `__context__` is automatically set to the last exception caught; if the new exception is not handled the traceback that is eventually displayed will include the originating exception(s) and the final exception.

This implicit exception chain can be made explicit by using `from` with `raise`. The single argument to `from` must be an exception or `None`. It will be set as `__cause__` on the raised exception. Setting `__cause__` implicitly sets the `__suppress_context__` to `True`. If `__cause__` is an exception, it will be displayed. If `__cause__` is present or `__suppress_context__` has a true value, `__context__` will not be displayed.

In either case, the default exception handling code will not display any of the remaining links in the `__context__` chain if `__cause__` has been set.

## 5.1. Base classes

The following exceptions are used mostly as base classes for other exceptions.

exception `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`). If `str()` is called on an instance of this class, the representation of the argument(s) to the instance are returned, or the empty string when there were no arguments.

  `args`

    The tuple of arguments given to the exception constructor. Some built-in exceptions (like `IOError`) expect a certain number of arguments and assign a special meaning to the elements of this tuple, while others are usually called only with a single string giving an error message.

  `with_traceback`(tb)

    This method sets tb as the new traceback for the exception and returns the exception object. It is usually used in exception handling code like this:

        try:
            ...
        except SomeException:
            tb = sys.exc_info()[2]
            raise OtherException(...).with_traceback(tb)

exception `Exception`

  All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.

exception `ArithmeticError`

  The base class for those built-in exceptions that are raised for various arithmetic errors: `OverflowError`, `ZeroDivisionError`, `FloatingPointError`.

exception `BufferError`

  Raised when a buffer related operation cannot be performed.

exception `LookupError`

  The base class for the exceptions that are raised when a key or index used on a mapping or sequence is invalid: `IndexError`, `KeyError`. This can be raised directly by `codecs.lookup()`.

## 5.2. Concrete exceptions

The following exceptions are the exceptions that are usually raised.

exception `AssertionError`

  Raised when an `assert` statement fails.

exception `AttributeError`

  Raised when an attribute reference (see Attribute references) or assignment fails. (When an object does not support attribute references or attribute assignments at all, `TypeError` is raised.)

exception `EOFError`

  Raised when one of the built-in functions (`input()` or `raw_input()`) hits an end-of-file condition (EOF) without reading any data. (N.B.: the `file.read()` and `file.readline()` methods return an empty string when they hit EOF.)

exception `FloatingPointError`

  Raised when a floating point operation fails. This exception is always defined, but can only be raised when Python is configured with the `--with-fpectl` option, or the `WANT_SIGFPE_HANDLER` symbol is defined in the `pyconfig.h` file.

exception `GeneratorExit`

  Raise when a generator‘s `close()` method is called. It directly inherits from `BaseException` instead of `Exception` since it is technically not an error.

exception `ImportError`

  Raised when an `import` statement fails to find the module definition or when a `from ... import` fails to find a name that is to be imported.

  The `name` and `path` attributes can be set using keyword-only arguments to the constructor. When set they represent the name of the module that was attempted to be imported and the path to any file which triggered the exception, respectively.

  Changed in version 3.3: Added the `name` and `path` attributes.

exception `IndexError`

  Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is not an integer, `TypeError` is raised.)

exception `KeyError`

  Raised when a mapping (dictionary) key is not found in the set of existing keys.

exception `KeyboardInterrupt`

  Raised when the user hits the interrupt key (normally `Control-C` or `Delete`). During execution, a check for interrupts is made regularly. The exception inherits from `BaseException` so as to not be accidentally caught by code that catches `Exception` and thus prevent the interpreter from exiting.

exception `MemoryError`

  Raised when an operation runs out of memory but the situation may still be rescued (by deleting some objects). The associated value is a string indicating what kind of (internal) operation ran out of memory. Note that because of the underlying memory management architecture (C’s `malloc()` function), the interpreter may not always be able to completely recover from this situation; it nevertheless raises an exception so that a stack traceback can be printed, in case a run-away program was the cause.

exception `NameError`

  Raised when a local or global name is not found. This applies only to unqualified names. The associated value is an error message that includes the name that could not be found.

exception `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.

exception `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” (not for illegal argument types or other incidental errors). Often a subclass of `OSError` will actually be raised as described in OS exceptions below. The `errno` attribute is a numeric error code from the C variable `errno`.

  Under Windows, the `winerror` attribute gives you the native Windows error code. The `errno` attribute is then an approximate translation, in POSIX terms, of that native error code.

  Under all platforms, the `strerror` attribute is the corresponding error message as provided by the operating system (as formatted by the C functions `perror()` under POSIX, and `FormatMessage()` Windows).

  For exceptions that involve a file system path (such as `open()` or `os.unlink()`), the exception instance will contain an additional attribute, `filename`, which is the file name passed to the function.

  Changed in version 3.3: `EnvironmentError`, `IOError`, `WindowsError`, `VMSError`, `socket.error`, `select.error` and `mmap.error` have been merged into `OSError`.

exception `OverflowError`

  Raised when the result of an arithmetic operation is too large to be represented. This cannot occur for integers (which would rather raise `MemoryError` than give up). Because of the lack of standardization of floating point exception handling in C, most floating point operations also aren’t checked.

exception `ReferenceError`

  This exception is 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. For more information on weak references, see the `weakref` module.

exception `RuntimeError`

  Raised when an error is detected that doesn’t fall in any of the other categories. The associated value is a string indicating what precisely went wrong. (This exception is mostly a relic from a previous version of the interpreter; it is not used very much any more.)

exception `StopIteration`

  Raised by built-in function `next()` and an iterator‘s `__next__()` method to signal that there are no further items to be produced by the iterator.

  The exception object has a single attribute `value`, which is given as an argument when constructing the exception, and defaults to `None`.

  When a generator function returns, a new `StopIteration` instance is raised, and the value returned by the function is used as the `value` parameter to the constructor of the exception.

  Changed in version 3.3: Added `value` attribute and the ability for generator functions to use it to return a value.

exception `SyntaxError`

  Raised when the parser encounters a syntax error. This may occur in an `import` statement, in a call to the built-in functions `exec()` or `eval()`, or when reading the initial script or standard input (also interactively).

  Instances of this class have attributes `filename`, `lineno`, `offset` and `text` for easier access to the details. `str()` of the exception instance returns only the message.

exception `IndentationError`

  Base class for syntax errors related to incorrect indentation. This is a subclass of `SyntaxError`.

exception `TabError`

  Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of `IndentationError`.

exception `SystemError`

  Raised when the interpreter finds an internal error, but the situation does not look so serious to cause it to abandon all hope. The associated value is a string indicating what went wrong (in low-level terms).

  You should report this to the author or maintainer of your Python interpreter. Be sure to report the version of the Python interpreter (`sys.version`; it is also printed at the start of an interactive Python session), the exact error message (the exception’s associated value) and if possible the source of the program that triggered the error.

exception `SystemExit`

  This exception is raised by the `sys.exit()` function. When it is not handled, the Python interpreter exits; no stack traceback is printed. If the associated value is an integer, it specifies the system exit status (passed to C’s `exit()` function); if it is `None`, the exit status is zero; if it has another type (such as a string), the object’s value is printed and the exit status is one.

  Instances have an attribute `code` which is set to the proposed exit status or error message (defaulting to `None`). Also, this exception derives directly from `BaseException` and not `Exception`, since it is not technically an error.

  A call to `sys.exit()` is translated into an exception so that clean-up handlers (`finally` clauses of `try` statements) can be executed, and so that a debugger can execute a script without running the risk of losing control. The `os._exit()` function can be used if it is absolutely positively necessary to exit immediately (for example, in the child process after a call to `fork()`).

  The exception inherits from `BaseException` instead of `Exception` so that it is not accidentally caught by code that catches `Exception`. This allows the exception to properly propagate up and cause the interpreter to exit.

exception `TypeError`

  Raised when an operation or function is applied to an object of inappropriate type. The associated value is a string giving details about the type mismatch.

exception `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`.

exception `UnicodeError`

  Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of `ValueError`.

exception `UnicodeEncodeError`

  Raised when a Unicode-related error occurs during encoding. It is a subclass of `UnicodeError`.

exception `UnicodeDecodeError`

  Raised when a Unicode-related error occurs during decoding. It is a subclass of `UnicodeError`.

exception `UnicodeTranslateError`

  Raised when a Unicode-related error occurs during translating. It is a subclass of `UnicodeError`.

exception `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 such as `IndexError`.

exception `ZeroDivisionError`

  Raised when the second argument of a division or modulo operation is zero. The associated value is a string indicating the type of the operands and the operation.

The following exceptions are kept for compatibility with previous versions; starting from Python 3.3, they are aliases of `OSError`.

exception `EnvironmentError`

exception `IOError`

exception `VMSError`

  Only available on VMS.

exception `WindowsError`

  Only available on Windows.

### 5.2.1. OS exceptions

The following exceptions are subclasses of `OSError`, they get raised depending on the system error code.

exception `BlockingIOError`

  Raised when an operation would block on an object (e.g. socket) set for non-blocking operation. Corresponds to `errno` `EAGAIN`, `EALREADY`, `EWOULDBLOCK` and `EINPROGRESS`.

  In addition to those of `OSError`, `BlockingIOError` can have one more attribute:

  `characters_written`

    An integer containing the number of characters written to the stream before it blocked. This attribute is available when using the buffered I/O classes from the `io` module.

exception `ChildProcessError`

  Raised when an operation on a child process failed. Corresponds to `errno` `ECHILD`.

exception `ConnectionError`

  A base class for connection-related issues. Subclasses are `BrokenPipeError`, `ConnectionAbortedError`, `ConnectionRefusedError` and `ConnectionResetError`.

  exception `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. Corresponds to `errno` `EPIPE` and `ESHUTDOWN`.

  exception `ConnectionAbortedError`

    A subclass of `ConnectionError`, raised when a connection attempt is aborted by the peer. Corresponds to `errno` `ECONNABORTED`.

  exception `ConnectionRefusedError`

    A subclass of `ConnectionError`, raised when a connection attempt is refused by the peer. Corresponds to `errno` `ECONNREFUSED`.

  exception `ConnectionResetError`

    A subclass of `ConnectionError`, raised when a connection is reset by the peer. Corresponds to `errno` `ECONNRESET`.

exception `FileExistsError`

  Raised when trying to create a file or directory which already exists. Corresponds to `errno` `EEXIST`.

exception `FileNotFoundError`

  Raised when a file or directory is requested but doesn’t exist. Corresponds to `errno` `ENOENT`.

exception `InterruptedError`

  Raised when a system call is interrupted by an incoming signal. Corresponds to `errno` `EEINTR`.

exception `IsADirectoryError`

  Raised when a file operation (such as `os.remove()`) is requested on a directory. Corresponds to `errno` `EISDIR`.

exception `NotADirectoryError`

  Raised when a directory operation (such as `os.listdir()`) is requested on something which is not a directory. Corresponds to `errno` `ENOTDIR`.

exception `PermissionError`

  Raised when trying to run an operation without the adequate access rights - for example filesystem permissions. Corresponds to `errno` `EACCES` and `EPERM`.

exception `ProcessLookupError`

  Raised when a given process doesn’t exist. Corresponds to `errno` `ESRCH`.

exception `TimeoutError`

  Raised when a system function timed out at the system level. Corresponds to `errno` `ETIMEDOUT`.

New in version 3.3: All the above `OSError` subclasses were added.

See also

PEP 3151 - Reworking the OS and IO exception hierarchy

## 5.3. Warnings

The following exceptions are used as warning categories; see the `warnings` module for more information.

exception `Warning`

  Base class for warning categories.

exception `UserWarning`

  Base class for warnings generated by user code.

exception `DeprecationWarning`

  Base class for warnings about deprecated features.

exception `PendingDeprecationWarning`

  Base class for warnings about features which will be deprecated in the future.

exception `SyntaxWarning`

  Base class for warnings about dubious syntax

exception `RuntimeWarning`

  Base class for warnings about dubious runtime behavior.

exception `FutureWarning`

  Base class for warnings about constructs that will change semantically in the future.

exception `ImportWarning`

  Base class for warnings about probable mistakes in module imports.

exception `UnicodeWarning`

  Base class for warnings related to Unicode.

exception `BytesWarning`

  Base class for warnings related to `bytes` and `buffer`.

exception `ResourceWarning`

  Base class for warnings related to resource usage.

  New in version 3.2.

## 5.4. Exception hierarchy

The class hierarchy for built-in exceptions is:

    BaseException
     +-- SystemExit
     +-- KeyboardInterrupt
     +-- GeneratorExit
     +-- Exception
          +-- StopIteration
          +-- ArithmeticError
          |    +-- FloatingPointError
          |    +-- OverflowError
          |    +-- ZeroDivisionError
          +-- AssertionError
          +-- AttributeError
          +-- BufferError
          +-- EOFError
          +-- ImportError
          +-- LookupError
          |    +-- IndexError
          |    +-- KeyError
          +-- MemoryError
          +-- NameError
          |    +-- UnboundLocalError
          +-- OSError
          |    +-- BlockingIOError
          |    +-- ChildProcessError
          |    +-- ConnectionError
          |    |    +-- BrokenPipeError
          |    |    +-- ConnectionAbortedError
          |    |    +-- ConnectionRefusedError
          |    |    +-- ConnectionResetError
          |    +-- FileExistsError
          |    +-- FileNotFoundError
          |    +-- InterruptedError
          |    +-- IsADirectoryError
          |    +-- NotADirectoryError
          |    +-- PermissionError
          |    +-- ProcessLookupError
          |    +-- TimeoutError
          +-- ReferenceError
          +-- RuntimeError
          |    +-- NotImplementedError
          +-- SyntaxError
          |    +-- IndentationError
          |         +-- TabError
          +-- SystemError
          +-- TypeError
          +-- ValueError
          |    +-- UnicodeError
          |         +-- UnicodeDecodeError
          |         +-- UnicodeEncodeError
          |         +-- UnicodeTranslateError
          +-- Warning
               +-- DeprecationWarning
               +-- PendingDeprecationWarning
               +-- RuntimeWarning
               +-- SyntaxWarning
               +-- UserWarning
               +-- FutureWarning
               +-- ImportWarning
               +-- UnicodeWarning
               +-- BytesWarning
               +-- ResourceWarning

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