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 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.

When raising (or re-raising) an exception in an "except" or "finally"
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.

When raising a new exception (rather than using a bare "raise" to re-
raise the exception currently being handled), the 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.


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 "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 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 the "input()" function hits an end-of-file condition
   (EOF) without reading any data. (N.B.: the "io.IOBase.read()" and
   "io.IOBase.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

   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 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([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.  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 RecursionError

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

   New 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.

   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 function defined in the presence of a "from
   __future__ import generator_stop" directive raises "StopIteration",
   it will be 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.

exception StopAsyncIteration

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

   New in version 3.5.

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.  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.

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".

   end

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

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 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" "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. 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 "bytearray".

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
         +-- StopAsyncIteration
         +-- 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
         |    +-- RecursionError
         +-- SyntaxError
         |    +-- IndentationError
         |         +-- TabError
         +-- SystemError
         +-- TypeError
         +-- ValueError
         |    +-- UnicodeError
         |         +-- UnicodeDecodeError
         |         +-- UnicodeEncodeError
         |         +-- UnicodeTranslateError
         +-- Warning
              +-- DeprecationWarning
              +-- PendingDeprecationWarning
              +-- RuntimeWarning
              +-- SyntaxWarning
              +-- UserWarning
              +-- FutureWarning
              +-- ImportWarning
              +-- UnicodeWarning
              +-- BytesWarning
              +-- ResourceWarning
