# Exceptions and Errors

Python uses `exceptions` to indicate an error has happened. The SQLite library uses integer error codes. 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

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 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 `VFS.excepthook()` or `VFSFile.excepthook()` (more info). In other code sys.unraisablehook is called, and if that is not present then sys.excepthook is called.

sqlite3_log 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.

Error.result

  For exceptions corresponding to SQLite error codes codes this attribute is the numeric error code.

Error.extendedresult

  APSW runs with extended result codes 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 `result` would have the value SQLITE_IOERR while `extendedresult` would have the value SQLITE_IOERR_SHORT_READ.

Error.error_offset

  The location of the error in the SQL when encoded in UTF-8. The value is from sqlite3_error_offset, 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 `Cursor.executemany()`). Cursors can only be used for one thing at a time.

exception ForkingViolationError

  See `apsw.fork_checker()`.

exception IncompleteExecutionError

  You have tried to start a new SQL execute call before executing all the previous ones. See the execution model 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 `Connection.close()` and then continued to use the `Connection` or associated `cursors`.

exception CursorClosedError

  You have called `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 execution tracer returned False so execution was aborted.

exception ExtensionLoadingError

  An error happened loading an extension.

exception VFSNotImplementedError

  A call cannot be made to an inherited Virtual File System (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.

#### 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 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. `Authorization` denied.

#### Abort/Busy Etc

exception AbortError

  SQLITE_ABORT. Callback routine requested an abort.

exception BusyError

  SQLITE_BUSY. The database file is locked. Use `Connection.setbusytimeout()` to change how long SQLite waits for the database to be unlocked or `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 - use `Connection.interrupt()`.

exception SchemaChangeError

  SQLITE_SCHEMA. The database schema changed. A `prepared statement` becomes invalid if the database schema was changed. Behind the scenes SQLite reprepares the statement. Another or the same `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 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 extended error code 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 `Connection.limit()`.

exception NoLFSError

  SQLITE_NOLFS. SQLite has attempted to use a feature not supported by the operating system such as large file support.

exception EmptyError

  SQLITE_EMPTY. Database is completely empty.

exception FormatError

  SQLITE_FORMAT. (No longer used) Auxiliary database 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.

### 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 `apsw.ext.print_augmented_traceback()` to print an exception with the local variables.

Here is a far more complex example from some virtual tables 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