Built-in Exceptions — Python 3.14.3 documentation
Description: 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 excep...

# 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 in this chapter 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 subclassed to define new exceptions; programmers are encouraged to derive new exceptions from the `Exception` class or one of its subclasses, and not from `BaseException`. More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions.

## Exception context

Three attributes on exception objects provide information about the context in which the exception was raised:

BaseException.__context__
BaseException.__cause__
BaseException.__suppress_context__

  When raising a new exception while another exception is already being handled, the new exception’s `__context__` attribute is automatically set to the handled exception. An exception may be handled when an `except` or `finally` clause, or a `with` statement, is used.

  This implicit exception context can be supplemented with an explicit cause by using `from` with `raise`:

      raise new_exc from original_exc

  The expression following `from` must be an exception or `None`. It will be set as `__cause__` on the raised exception. Setting `__cause__` also implicitly sets the `__suppress_context__` attribute to `True`, so that using `raise new_exc from None` effectively replaces the old exception with the new one for display purposes (e.g. converting `KeyError` to `AttributeError`), while leaving the old exception available in `__context__` for introspection when debugging.

  The default traceback display code shows these chained exceptions in addition to the traceback for the exception itself. An explicitly chained exception in `__cause__` is always shown when present. An implicitly chained exception in `__context__` is shown only if `__cause__` is `None` and `__suppress_context__` is false.

  In either case, the exception itself is always shown after any chained exceptions so that the final line of the traceback always shows the last exception that was raised.

## Inheriting from built-in exceptions

User code can create subclasses that inherit from an exception type. It’s recommended to only subclass one exception type at a time to avoid any possible conflicts between how the bases handle the `args` attribute, as well as due to possible memory layout incompatibilities.

CPython implementation detail: Most built-in exceptions are implemented in C for efficiency, see: Objects/exceptions.c. Some have custom memory layouts which makes it impossible to create a subclass that inherits from multiple exception types. The memory layout of a type is an implementation detail and might change between Python versions, leading to new conflicts in the future. Therefore, it’s recommended to avoid subclassing multiple exception types altogether.

## 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 `OSError`) 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 was more commonly used before the exception chaining features of PEP 3134 became available. The following example shows how we can convert an instance of `SomeException` into an instance of `OtherException` while preserving the traceback. Once raised, the current frame is pushed onto the traceback of the `OtherException`, as would have happened to the traceback of the original `SomeException` had we allowed it to propagate to the caller.

        try:
            ...
        except SomeException:
            tb = sys.exception().__traceback__
            raise OtherException(...).with_traceback(tb)

  __traceback__

    A writable field that holds the traceback object associated with this exception. See also: The raise statement.

  add_note(note)

    Add the string `note` to the exception’s notes which appear in the standard traceback after the exception string. A `TypeError` is raised if `note` is not a string.

    Added in version 3.11.

  __notes__

    A list of the notes of this exception, which were added with `add_note()`. This attribute is created when `add_note()` is called.

    Added in version 3.11.

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()`.

## 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.)

  The optional name and obj keyword-only arguments set the corresponding attributes:

  name

    The name of the attribute that was attempted to be accessed.

  obj

    The object that was accessed for the named attribute.

  Changed in version 3.10: Added the `name` and `obj` attributes.

exception EOFError

  Raised when the `input()` function hits an end-of-file condition (EOF) without reading any data. (Note: the `io.IOBase.read()` and `io.IOBase.readline()` methods return an empty string when they hit EOF.)

exception FloatingPointError

  Not currently used.

exception GeneratorExit

  Raised when a generator or coroutine is closed; see `generator.close()` and `coroutine.close()`. It directly inherits from `BaseException` instead of `Exception` since it is technically not an error.

exception ImportError

  Raised when the `import` statement has troubles trying to load a module. Also raised when the “from list” in `from ... import` has a name that cannot be found.

  The optional name and path keyword-only arguments set the corresponding attributes:

  name

    The name of the module that was attempted to be imported.

  path

    The path to any file which triggered the exception.

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

exception ModuleNotFoundError

  A subclass of `ImportError` which is raised by `import` when a module could not be located. It is also raised when `None` is found in `sys.modules`.

  Added in version 3.6.

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.

  Note

  Catching a `KeyboardInterrupt` requires special consideration. Because it can be raised at unpredictable points, it may, in some circumstances, leave the running program in an inconsistent state. It is generally best to allow `KeyboardInterrupt` to end the program as quickly as possible or avoid raising it entirely. (See Note on Signal Handlers and Exceptions.)

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.

  The optional name keyword-only argument sets the attribute:

  name

    The name of the variable that was attempted to be accessed.

  Changed in version 3.10: Added the `name` attribute.

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, or while the class is being developed to indicate that the real implementation still needs to be added.

  Note

  It should not be used to indicate that an operator or method is not meant to be supported at all – in that case either leave the operator / method undefined or, if a subclass, set it to `None`.

  Caution

  `NotImplementedError` and `NotImplemented` are not interchangeable. This exception should only be used as described above; see `NotImplemented` for details on correct usage of the built-in constant.

exception OSError([arg])
exception OSError(errno, strerror[, filename[, winerror[, filename2]]])

  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).

  The second form of the constructor sets the corresponding attributes, described below. The attributes default to `None` if not specified. For backwards compatibility, if three arguments are passed, the `args` attribute contains only a 2-tuple of the first two constructor arguments.

  The constructor often actually returns a subclass of `OSError`, as described in OS exceptions below. The particular subclass depends on the final `errno` value. This behaviour only occurs when constructing `OSError` directly or via an alias, and is not inherited when subclassing.

  errno

    A numeric error code from the C variable `errno`.

  winerror

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

    Under Windows, if the winerror constructor argument is an integer, the `errno` attribute is determined from the Windows error code, and the errno argument is ignored. On other platforms, the winerror argument is ignored, and the `winerror` attribute does not exist.

  strerror

    The corresponding error message, as provided by the operating system. It is formatted by the C functions `perror()` under POSIX, and `FormatMessage()` under Windows.

  filename
  filename2

    For exceptions that involve a file system path (such as `open()` or `os.unlink()`), `filename` is the file name passed to the function. For functions that involve two file system paths (such as `os.rename()`), `filename2` corresponds to the second file name passed to the function.

  Changed in version 3.3: `EnvironmentError`, `IOError`, `WindowsError`, `socket.error`, `select.error` and `mmap.error` have been merged into `OSError`, and the constructor may return a subclass.

  Changed in version 3.4: The `filename` attribute is now the original file name passed to the function, instead of the name encoded to or decoded from the filesystem encoding and error handler. Also, the filename2 constructor argument and attribute was added.

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). However, for historical reasons, OverflowError is sometimes raised for integers that are outside a required range. Because of the lack of standardization of floating-point exception handling in C, most floating-point operations are not checked.

exception PythonFinalizationError

  This exception is derived from `RuntimeError`. It is raised when an operation is blocked during interpreter shutdown also known as Python finalization.

  Examples of operations which can be blocked with a `PythonFinalizationError` during the Python finalization:

  - Creating a new Python thread.

  - `Joining` a running daemon thread.

  - `os.fork()`.

  See also the `sys.is_finalizing()` function.

  Added in version 3.13: Previously, a plain `RuntimeError` was raised.

  Changed in version 3.14: `threading.Thread.join()` can now raise this exception.

exception RecursionError

  This exception is derived from `RuntimeError`. It is raised when the interpreter detects that the maximum recursion depth (see `sys.getrecursionlimit()`) is exceeded.

  Added in version 3.5: Previously, a plain `RuntimeError` was raised.

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.

exception StopIteration

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

  value

    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 or coroutine 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.

  If a generator code directly or indirectly raises `StopIteration`, it is converted into a `RuntimeError` (retaining the `StopIteration` as the new exception’s cause).

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

  Changed in version 3.5: Introduced the RuntimeError transformation via `from __future__ import generator_stop`, see PEP 479.

  Changed in version 3.7: Enable PEP 479 for all code by default: a `StopIteration` error raised in a generator is transformed into a `RuntimeError`.

exception StopAsyncIteration

  Must be raised by `__anext__()` method of an asynchronous iterator object to stop the iteration.

  Added in version 3.5.

exception SyntaxError(message, details)

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

  The `str()` of the exception instance returns only the error message. Details is a tuple whose members are also available as separate attributes.

  filename

    The name of the file the syntax error occurred in.

  lineno

    Which line number in the file the error occurred in. This is 1-indexed: the first line in the file has a `lineno` of 1.

  offset

    The column in the line where the error occurred. This is 1-indexed: the first character in the line has an `offset` of 1.

  text

    The source code text involved in the error.

  end_lineno

    Which line number in the file the error occurred ends in. This is 1-indexed: the first line in the file has a `lineno` of 1.

  end_offset

    The column in the end line where the error occurred finishes. This is 1-indexed: the first character in the line has an `offset` of 1.

  For errors in f-string fields, the message is prefixed by “f-string: ” and the offsets are offsets in a text constructed from the replacement expression. For example, compiling f’Bad {a b} field’ results in this args attribute: (‘f-string: …’, (‘’, 1, 2, ‘(a b)n’, 1, 5)).

  Changed in version 3.10: Added the `end_lineno` and `end_offset` attributes.

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). In CPython, this could be raised by incorrectly using Python’s C API, such as returning a `NULL` value without an exception set.

  If you’re confident that this exception wasn’t your fault, or the fault of a package you’re using, 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. It 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. When it is not handled, the Python interpreter exits; no stack traceback is printed. The constructor accepts the same optional argument passed to `sys.exit()`. If the 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.

  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 `os.fork()`).

  code

    The exit status or error message that is passed to the constructor. (Defaults to `None`.)

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.

  This exception may be raised by user code to indicate that an attempted operation on an object is not supported, and is not meant to be. If an object is meant to support a given operation but has not yet provided an implementation, `NotImplementedError` is the proper exception to raise.

  Passing arguments of the wrong type (e.g. passing a `list` when an `int` is expected) should result in a `TypeError`, but passing arguments with the wrong value (e.g. a number outside expected boundaries) should result in a `ValueError`.

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`.

  `UnicodeError` has attributes that describe the encoding or decoding error. For example, `err.object[err.start:err.end]` gives the particular invalid input that the codec failed on.

  encoding

    The name of the encoding that raised the error.

  reason

    A string describing the specific codec error.

  object

    The object the codec was attempting to encode or decode.

  start

    The first index of invalid data in `object`.

    This value should not be negative as it is interpreted as an absolute offset but this constraint is not enforced at runtime.

  end

    The index after the last invalid data in `object`.

    This value should not be negative as it is interpreted as an absolute offset but this constraint is not enforced at runtime.

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 an 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 WindowsError

  Only available on Windows.

### 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 bytes 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` `EINTR`.

  Changed in version 3.5: Python now retries system calls when a syscall is interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising `InterruptedError`.

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. On most POSIX platforms, it may also be raised if an operation attempts to open or traverse a non-directory file as if it were 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`, `EPERM`, and `ENOTCAPABLE`.

  Changed in version 3.11.1: WASI’s `ENOTCAPABLE` is now mapped to `PermissionError`.

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`.

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

See also

PEP 3151 - Reworking the OS and IO exception hierarchy

## Warnings

The following exceptions are used as warning categories; see the Warning Categories documentation for more details.

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 when those warnings are intended for other Python developers.

  Ignored by the default warning filters, except in the `__main__` module (PEP 565). Enabling the Python Development Mode shows this warning.

  The deprecation policy is described in PEP 387.

exception PendingDeprecationWarning

  Base class for warnings about features which are obsolete and expected to be deprecated in the future, but are not deprecated at the moment.

  This class is rarely used as emitting a warning about a possible upcoming deprecation is unusual, and `DeprecationWarning` is preferred for already active deprecations.

  Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.

  The deprecation policy is described in PEP 387.

exception SyntaxWarning

  Base class for warnings about dubious syntax.

  This warning is typically emitted when compiling Python source code, and usually won’t be reported when running already compiled code.

exception RuntimeWarning

  Base class for warnings about dubious runtime behavior.

exception FutureWarning

  Base class for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.

exception ImportWarning

  Base class for warnings about probable mistakes in module imports.

  Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.

exception UnicodeWarning

  Base class for warnings related to Unicode.

exception EncodingWarning

  Base class for warnings related to encodings.

  See Opt-in EncodingWarning for details.

  Added in version 3.10.

exception BytesWarning

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

exception ResourceWarning

  Base class for warnings related to resource usage.

  Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.

  Added in version 3.2.

## Exception groups

The following are used when it is necessary to raise multiple unrelated exceptions. They are part of the exception hierarchy so they can be handled with `except` like all other exceptions. In addition, they are recognised by `except*`, which matches their subgroups based on the types of the contained exceptions.

exception ExceptionGroup(msg, excs)

exception BaseExceptionGroup(msg, excs)

  Both of these exception types wrap the exceptions in the sequence `excs`. The `msg` parameter must be a string. The difference between the two classes is that `BaseExceptionGroup` extends `BaseException` and it can wrap any exception, while `ExceptionGroup` extends `Exception` and it can only wrap subclasses of `Exception`. This design is so that `except Exception` catches an `ExceptionGroup` but not `BaseExceptionGroup`.

  The `BaseExceptionGroup` constructor returns an `ExceptionGroup` rather than a `BaseExceptionGroup` if all contained exceptions are `Exception` instances, so it can be used to make the selection automatic. The `ExceptionGroup` constructor, on the other hand, raises a `TypeError` if any contained exception is not an `Exception` subclass.

  message

    The `msg` argument to the constructor. This is a read-only attribute.

  exceptions

    A tuple of the exceptions in the `excs` sequence given to the constructor. This is a read-only attribute.

  subgroup(condition)

    Returns an exception group that contains only the exceptions from the current group that match condition, or `None` if the result is empty.

    The condition can be an exception type or tuple of exception types, in which case each exception is checked for a match using the same check that is used in an `except` clause. The condition can also be a callable (other than a type object) that accepts an exception as its single argument and returns true for the exceptions that should be in the subgroup.

    The nesting structure of the current exception is preserved in the result, as are the values of its `message`, `__traceback__`, `__cause__`, `__context__` and `__notes__` fields. Empty nested groups are omitted from the result.

    The condition is checked for all exceptions in the nested exception group, including the top-level and any nested exception groups. If the condition is true for such an exception group, it is included in the result in full.

    Added in version 3.13: `condition` can be any callable which is not a type object.

  split(condition)

    Like `subgroup()`, but returns the pair `(match, rest)` where `match` is `subgroup(condition)` and `rest` is the remaining non-matching part.

  derive(excs)

    Returns an exception group with the same `message`, but which wraps the exceptions in `excs`.

    This method is used by `subgroup()` and `split()`, which are used in various contexts to break up an exception group. A subclass needs to override it in order to make `subgroup()` and `split()` return instances of the subclass rather than `ExceptionGroup`.

    `subgroup()` and `split()` copy the `__traceback__`, `__cause__`, `__context__` and `__notes__` fields from the original exception group to the one returned by `derive()`, so these fields do not need to be updated by `derive()`.

        >>> class MyGroup(ExceptionGroup):
        ...     def derive(self, excs):
        ...         return MyGroup(self.message, excs)
        ...
        >>> e = MyGroup("eg", [ValueError(1), TypeError(2)])
        >>> e.add_note("a note")
        >>> e.__context__ = Exception("context")
        >>> e.__cause__ = Exception("cause")
        >>> try:
        ...    raise e
        ... except Exception as e:
        ...    exc = e
        ...
        >>> match, rest = exc.split(ValueError)
        >>> exc, exc.__context__, exc.__cause__, exc.__notes__
        (MyGroup('eg', [ValueError(1), TypeError(2)]), Exception('context'), Exception('cause'), ['a note'])
        >>> match, match.__context__, match.__cause__, match.__notes__
        (MyGroup('eg', [ValueError(1)]), Exception('context'), Exception('cause'), ['a note'])
        >>> rest, rest.__context__, rest.__cause__, rest.__notes__
        (MyGroup('eg', [TypeError(2)]), Exception('context'), Exception('cause'), ['a note'])
        >>> exc.__traceback__ is match.__traceback__ is rest.__traceback__
        True

  Note that `BaseExceptionGroup` defines `__new__()`, so subclasses that need a different constructor signature need to override that rather than `__init__()`. For example, the following defines an exception group subclass which accepts an exit_code and constructs the group’s message from it.

      class Errors(ExceptionGroup):
         def __new__(cls, errors, exit_code):
            self = super().__new__(Errors, f"exit code: {exit_code}", errors)
            self.exit_code = exit_code
            return self

         def derive(self, excs):
            return Errors(excs, self.exit_code)

  Like `ExceptionGroup`, any subclass of `BaseExceptionGroup` which is also a subclass of `Exception` can only wrap instances of `Exception`.

  Added in version 3.11.

## Exception hierarchy

The class hierarchy for built-in exceptions is:

    BaseException
     ├── BaseExceptionGroup
     ├── GeneratorExit
     ├── KeyboardInterrupt
     ├── SystemExit
     └── Exception
          ├── ArithmeticError
          │    ├── FloatingPointError
          │    ├── OverflowError
          │    └── ZeroDivisionError
          ├── AssertionError
          ├── AttributeError
          ├── BufferError
          ├── EOFError
          ├── ExceptionGroup [BaseExceptionGroup]
          ├── ImportError
          │    └── ModuleNotFoundError
          ├── 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
          │    ├── PythonFinalizationError
          │    └── RecursionError
          ├── StopAsyncIteration
          ├── StopIteration
          ├── SyntaxError
          │    └── IndentationError
          │         └── TabError
          ├── SystemError
          ├── TypeError
          ├── ValueError
          │    └── UnicodeError
          │         ├── UnicodeDecodeError
          │         ├── UnicodeEncodeError
          │         └── UnicodeTranslateError
          └── Warning
               ├── BytesWarning
               ├── DeprecationWarning
               ├── EncodingWarning
               ├── FutureWarning
               ├── ImportWarning
               ├── PendingDeprecationWarning
               ├── ResourceWarning
               ├── RuntimeWarning
               ├── SyntaxWarning
               ├── UnicodeWarning
               └── UserWarning