.. currentmodule:: apsw

.. _exceptions:

Exceptions and Errors
*********************

Python uses :class:`exceptions <Exception>` to indicate an error has
happened.  The SQLite library uses `integer error codes
<https://www.sqlite.org/rescode.html>`__.  APSW maps between the two
systems as needed.  Exceptions raised in Python code called by SQLite
will have that exception present when control returns to Python, and
SQLite will understand that an error occurred.

.. _unraisable:

Unraisable
----------

There are a few places where it is not possible for a Python exception
to be reported to SQLite as an error, typically because SQLite does
not allow an error to be signalled in that context.  Another example
would be in :doc:`VFS <vfs>` code, because SQLite takes actions to
recover from errors (eg it may try to rollback a transaction on a
write error).  Python wants to return to callers, not continue
execution while the exception is pending.  (Also only one exception
can be active at a time.)

Unraisable exceptions in VFS code are handled by calling
:meth:`VFS.excepthook` or :meth:`VFSFile.excepthook` (:ref:`more info
<vfserrors>`).  In other code `sys.unraisablehook
<https://docs.python.org/3/library/sys.html#sys.unraisablehook>`__ is
called, and if that is not present then `sys.excepthook
<https://docs.python.org/3/library/sys.html#sys.excepthook>`__ is
called.

`sqlite3_log <https://www.sqlite.org/c3ref/log.html>`__ is also called
so that you will have the context of when the exception happened
relative to the errors SQLite is logging.

Exception Classes
-----------------


.. exception:: Error

  This is the base for APSW exceptions.

.. attribute:: Error.result

         For exceptions corresponding to `SQLite error codes
         <https://sqlite.org/c3ref/c_abort.html>`_ codes this attribute
         is the numeric error code.

.. attribute:: Error.extendedresult

         APSW runs with `extended result codes
         <https://sqlite.org/rescode.html>`_ turned on.
         This attribute includes the detailed code.

         As an example, if SQLite issued a read request and the system
         returned less data than expected then :attr:`~Error.result`
         would have the value *SQLITE_IOERR* while
         :attr:`~Error.extendedresult` would have the value
         *SQLITE_IOERR_SHORT_READ*.

.. attribute:: Error.error_offset

        The location of the error in the SQL when encoded in UTF-8.
        The value is from `sqlite3_error_offset
        <https://www.sqlite.org/c3ref/errcode.html>`__, and will be
        `-1` when a specific token in the input is not the cause.



APSW specific exceptions
========================

The following exceptions happen when APSW detects various problems.

.. exception:: ThreadingViolationError

  You have used an object concurrently in two threads. For example you
  may try to use the same cursor in two different threads at the same
  time, or tried to close the same connection in two threads at the
  same time.

  You can also get this exception by using a cursor as an argument to
  itself (eg as the input data for :meth:`Cursor.executemany`).
  Cursors can only be used for one thing at a time.

.. exception:: ForkingViolationError

  See :meth:`apsw.fork_checker`.

.. exception:: IncompleteExecutionError

  You have tried to start a new SQL execute call before executing all
  the previous ones. See the :ref:`execution model <executionmodel>`
  for more details.

.. exception:: ConnectionNotClosedError

  This exception is no longer generated.  It was required in earlier
  releases due to constraints in threading usage with SQLite.

.. exception:: ConnectionClosedError

  You have called :meth:`Connection.close` and then continued to use
  the :class:`Connection` or associated :class:`cursors <Cursor>`.

.. exception:: CursorClosedError

  You have called :meth:`Cursor.close` and then tried to use the cursor.

.. exception:: BindingsError

  There are several causes for this exception.  When using tuples, an incorrect number of bindings where supplied::

     cursor.execute("select ?,?,?", (1,2))     # too few bindings
     cursor.execute("select ?,?,?", (1,2,3,4)) # too many bindings

  You are using named bindings, but not all bindings are named.  You should either use entirely the
  named style or entirely numeric (unnamed) style::

     cursor.execute("select * from foo where x=:name and y=?")

.. exception:: ExecutionCompleteError

  A statement is complete but you try to run it more anyway!


.. exception:: ExecTraceAbort

  The :ref:`execution tracer <executiontracer>` returned False so
  execution was aborted.


.. exception:: ExtensionLoadingError

  An error happened loading an `extension
  <https://sqlite.org/cvstrac/wiki/wiki?p=LoadableExtensions>`_.

.. exception:: VFSNotImplementedError

  A call cannot be made to an inherited :ref:`VFS` method as the VFS
  does not implement the method.

.. exception:: VFSFileClosedError

  The VFS file is closed so the operation cannot be performed.

SQLite Exceptions
=================

The following lists which Exception classes correspond to which `SQLite
error codes <https://sqlite.org/c3ref/c_abort.html>`_.


General Errors
^^^^^^^^^^^^^^

.. exception:: SQLError

  *SQLITE_ERROR*.  This error is documented as a bad SQL query
  or missing database, but is also returned for a lot of other
  situations.  It is the default error code unless there is a more
  specific one.

.. exception:: MismatchError

  *SQLITE_MISMATCH*. Data type mismatch.  For example a rowid
  or integer primary key must be an integer.

.. exception:: NotFoundError

  *SQLITE_NOTFOUND*. Returned when various internal items were
  not found such as requests for non-existent system calls or file
  controls.

Internal Errors
^^^^^^^^^^^^^^^

.. exception:: InternalError

  *SQLITE_INTERNAL*. (No longer used) Internal logic error in SQLite.

.. exception:: ProtocolError

  *SQLITE_PROTOCOL*. (No longer used) Database lock protocol error.

.. exception:: MisuseError

  *SQLITE_MISUSE*.  SQLite library used incorrectly - typically similar to *ValueError* in Python.  Examples include not
  having enough flags when opening a connection (eg not including a READ or WRITE flag), or out of spec such as registering
  a function with more than 127 parameters.

.. exception:: RangeError

  *SQLITE_RANGE*.  (Cannot be generated using APSW).  2nd parameter to `sqlite3_bind <https://sqlite.org/c3ref/bind_blob.html>`_ out of range

Permissions Etc
^^^^^^^^^^^^^^^

.. exception:: PermissionsError

  *SQLITE_PERM*. Access permission denied by the operating system, or parts of the database are readonly such as a cursor.

.. exception:: ReadOnlyError

  *SQLITE_READONLY*. Attempt to write to a readonly database.

.. exception:: CantOpenError

  *SQLITE_CANTOPEN*.  Unable to open the database file.

.. exception:: AuthError

  *SQLITE_AUTH*.  :attr:`Authorization <Connection.authorizer>` denied.

Abort/Busy Etc
^^^^^^^^^^^^^^

.. exception:: AbortError

  *SQLITE_ABORT*. Callback routine requested an abort.

.. exception:: BusyError

  *SQLITE_BUSY*.  The database file is locked.  Use
  :meth:`Connection.setbusytimeout` to change how long SQLite waits
  for the database to be unlocked or :meth:`Connection.setbusyhandler`
  to use your own handler.

.. exception:: LockedError

  *SQLITE_LOCKED*.  A table in the database is locked.

.. exception:: InterruptError

  *SQLITE_INTERRUPT*.  Operation terminated by
  `sqlite3_interrupt <https://sqlite.org/c3ref/interrupt.html>`_ -
  use :meth:`Connection.interrupt`.

.. exception:: SchemaChangeError

  *SQLITE_SCHEMA*.  The database schema changed.  A
  :meth:`prepared statement <Cursor.execute>` becomes invalid
  if the database schema was changed.  Behind the scenes SQLite
  reprepares the statement.  Another or the same :class:`Connection`
  may change the schema again before the statement runs.  SQLite will
  attempt up to 5 times before giving up and returning this error.

.. exception:: ConstraintError

  *SQLITE_CONSTRAINT*. Abort due to `constraint
  <https://sqlite.org/lang_createtable.html>`_ violation.  This
  would happen if the schema required a column to be within a specific
  range.

Memory/Disk
^^^^^^^^^^^

.. exception:: NoMemError

  *SQLITE_NOMEM*.  A memory allocation failed.

.. exception:: IOError

  *SQLITE_IOERR*.  Some kind of disk I/O error occurred.  The
  :ref:`extended error code <exceptions>` will give more detail.

.. exception:: CorruptError

  *SQLITE_CORRUPT*.  The database disk image appears to be a
  SQLite database but the values inside are inconsistent.

.. exception:: FullError

  *SQLITE_FULL*.  The disk appears to be full.

.. exception:: TooBigError

  *SQLITE_TOOBIG*.  String or BLOB exceeds size limit.  You can
  change the limits using :meth:`Connection.limit`.

.. exception:: NoLFSError

  *SQLITE_NOLFS*.  SQLite has attempted to use a feature not
  supported by the operating system such as `large file support
  <http://en.wikipedia.org/wiki/Large_file_support>`_.

.. exception:: EmptyError

  *SQLITE_EMPTY*. Database is completely empty.

.. exception:: FormatError

  *SQLITE_FORMAT*. (No longer used) `Auxiliary database <https://sqlite.org/lang_attach.html>`_ format error.

.. exception:: NotADBError

  *SQLITE_NOTADB*.  File opened that is not a database file.
  SQLite has a header on database files to verify they are indeed
  SQLite databases.


.. _augmentedstacktraces:

Augmented stack traces
======================

When an exception occurs, Python does not include frames from
non-Python code (ie the C code called from Python).  This can make it
more difficult to work out what was going on when an exception
occurred for example when there are callbacks to collations, functions
or virtual tables, triggers firing etc.

This is an example showing the difference between the tracebacks you
would have got with earlier versions of apsw and the augmented
traceback::

  import apsw

  def myfunc(x):
    1/0

  con=apsw.Connection(":memory:")
  con.createscalarfunction("foo", myfunc)
  con.createscalarfunction("fam", myfunc)
  cursor=con.cursor()
  cursor.execute("create table bar(x,y,z);insert into bar values(1,2,3)")
  cursor.execute("select foo(1) from bar")


+-----------------------------------------------------------+
| Original Traceback                                        |
+===========================================================+
| ::                                                        |
|                                                           |
|   Traceback (most recent call last):                      |
|     File "t.py", line 11, in <module>                     |
|       cursor.execute("select foo(1) from bar")            |
|     File "t.py", line 4, in myfunc                        |
|       1/0                                                 |
|   ZeroDivisionError: integer division or modulo by zero   |
|                                                           |
|                                                           |
+-----------------------------------------------------------+

+----------------------------------------------------------+
|      Augmented Traceback                                 |
+==========================================================+
| ::                                                       |
|                                                          |
|   Traceback (most recent call last):                     |
|     File "t.py", line 11, in <module>                    |
|       cursor.execute("select foo(1) from bar")           |
|     File "apsw.c", line 3412, in resetcursor             |
|     File "apsw.c", line 1597, in user-defined-scalar-foo |
|     File "t.py", line 4, in myfunc                       |
|       1/0                                                |
|   ZeroDivisionError: integer division or modulo by zero  |
+----------------------------------------------------------+

In the original traceback you can't even see that code in apsw was
involved. The augmented traceback shows that there were indeed two
function calls within apsw and gives you line numbers should you need
to examine the code. Also note how you are told that the call was in
`user-defined-scalar-foo` (ie you can tell which function was called.)

*But wait, there is more!!!* In order to further aid troubleshooting,
the augmented stack traces make additional information available. Each
frame in the traceback has local variables defined with more
information. You can use :meth:`apsw.ext.print_augmented_traceback` to
print an exception with the local variables.

Here is a far more complex example from some :ref:`virtual tables
<Virtualtables>` code I was writing. The BestIndex method in my code
had returned an incorrect value. The augmented traceback includes
local variables. I can see what was passed in to my method, what I
returned and which item was erroneous. The original traceback is
almost completely useless!

Original traceback::

  Traceback (most recent call last):
    File "tests.py", line 1387, in testVtables
      cursor.execute(allconstraints)
  TypeError: Bad constraint (#2) - it should be one of None, an integer or a tuple of an integer and a boolean

Augmented traceback with local variables::

  Traceback (most recent call last):
    File "tests.py", line 1387, in testVtables
      cursor.execute(allconstraints)
                  VTable =  __main__.VTable
                     cur =  <apsw.Cursor object at 0x988f30>
                       i =  10
                    self =  testVtables (__main__.APSW)
          allconstraints =  select rowid,* from foo where rowid>-1000 ....

    File "apsw.c", line 4050, in Cursor_execute.sqlite3_prepare
              Connection =  <apsw.Connection object at 0x978800>
               statement =  select rowid,* from foo where rowid>-1000 ....

    File "apsw.c", line 2681, in VirtualTable.xBestIndex
                    self =  <__main__.VTable instance at 0x98d8c0>
                    args =  (((-1, 4), (0, 32), (1, 8), (2, 4), (3, 64)), ((2, False),))
                  result =  ([4, (3,), [2, False], [1], [0]], 997, u'\xea', False)

    File "apsw.c", line 2559, in VirtualTable.xBestIndex.result_constraint
                 indices =  [4, (3,), [2, False], [1], [0]]
                    self =  <__main__.VTable instance at 0x98d8c0>
                  result =  ([4, (3,), [2, False], [1], [0]], 997, u'\xea', False)
              constraint =  (3,)

  TypeError: Bad constraint (#2) - it should be one of None, an integer or a tuple of an integer and a boolean


