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.. Automatically generated by code2rst.py
Edit src/blob.c not this file!
.. currentmodule:: apsw
.. _blobio:
Blob Input/Output
*****************
A `blob <https://en.wikipedia.org/wiki/Binary_large_object>`_ is a
SQLite `datatype <https://sqlite.org/datatype3.html>`_ representing
a sequence of bytes. It can be zero or more bytes in size. Blobs
cannot be resized, but you can read and overwrite parts of them.
SQLite blobs have an absolute maximum size of 2GB and a `default
maximum size <https://sqlite.org/c3ref/c_limit_attached.html>`_ of
1GB.
An alternate approach to using blobs is to store the data in files and
store the filename in the database. Doing so loses the `ACID
<https://sqlite.org/transactional.html>`_ properties of SQLite.
There are `benchmarks <https://www.sqlite.org/fasterthanfs.html>`__.
zeroblob class
==============
.. class:: zeroblob(size: int)
If you want to insert a blob into a row, you need to
supply the entire blob in one go. Using this class or
`function <https://www.sqlite.org/lang_corefunc.html#zeroblob>`__
allocates the space in the database filling it with zeroes.
You can then overwrite parts in smaller chunks, without having
to do it all at once. The :ref:`example <example_blob_io>` shows
how to use it.
:param size: Number of zeroed bytes to create
.. method:: zeroblob.length() -> int
Size of zero blob in bytes.
Blob class
==========
.. class:: Blob
This object is created by :meth:`Connection.blob_open` and provides
access to a blob in the database. It behaves like a Python file.
It wraps a `sqlite3_blob
<https://sqlite.org/c3ref/blob.html>`_.
.. note::
You cannot change the size of a blob using this object. You should
create it with the correct size in advance either by using
:class:`zeroblob` or the `zeroblob()
<https://sqlite.org/lang_corefunc.html>`_ function.
See the :ref:`example <example_blob_io>`.
.. method:: Blob.__enter__() -> Blob
You can use a blob as a `context manager
<https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers>`_
as defined in :pep:`0343`. When you use *with* statement,
the blob is always :meth:`closed <Blob.close>` on exit from the block, even if an
exception occurred in the block.
For example::
with connection.blob_open() as blob:
blob.write("...")
res=blob.read(1024)
.. method:: Blob.__exit__(etype: Optional[type[BaseException]], evalue: Optional[BaseException], etraceback: Optional[types.TracebackType]) -> Optional[bool]
Implements context manager in conjunction with
:meth:`~Blob.__enter__`. Any exception that happened in the
*with* block is raised after closing the blob.
.. index:: sqlite3_blob_close
.. method:: Blob.close(force: bool = False) -> None
Closes the blob. Note that even if an error occurs the blob is
still closed.
.. note::
In some cases errors that technically occurred in the
:meth:`~Blob.read` and :meth:`~Blob.write` routines may not be
reported until close is called. Similarly errors that occurred
in those methods (eg calling :meth:`~Blob.write` on a read-only
blob) may also be re-reported in :meth:`~Blob.close`. (This
behaviour is what the underlying SQLite APIs do - it is not APSW
doing it.)
It is okay to call :meth:`~Blob.close` multiple times.
:param force: Ignores any errors during close.
Calls: `sqlite3_blob_close <https://sqlite.org/c3ref/blob_close.html>`__
.. index:: sqlite3_blob_bytes
.. method:: Blob.length() -> int
Returns the size of the blob in bytes.
Calls: `sqlite3_blob_bytes <https://sqlite.org/c3ref/blob_bytes.html>`__
.. index:: sqlite3_blob_read
.. method:: Blob.read(length: int = -1) -> bytes
Reads amount of data requested, or till end of file, whichever is
earlier. Attempting to read beyond the end of the blob returns an
empty bytes in the same manner as end of file on normal file
objects. Negative numbers read all remaining data.
Calls: `sqlite3_blob_read <https://sqlite.org/c3ref/blob_read.html>`__
.. index:: sqlite3_blob_read
.. method:: Blob.read_into(buffer: bytearray | array.array[Any] | memoryview, offset: int = 0, length: int = -1) -> None
Reads from the blob into a buffer you have supplied. This method is
useful if you already have a buffer like object that data is being
assembled in, and avoids allocating results in :meth:`Blob.read` and
then copying into buffer.
:param buffer: A writable buffer like object.
There is a :class:`bytearray` type that is very useful.
:mod:`Arrays <array>` also work.
:param offset: The position to start writing into the buffer
defaulting to the beginning.
:param length: How much of the blob to read. The default is the
remaining space left in the buffer. Note that if
there is more space available than blob left then you
will get a *ValueError* exception.
Calls: `sqlite3_blob_read <https://sqlite.org/c3ref/blob_read.html>`__
.. index:: sqlite3_blob_reopen
.. method:: Blob.reopen(rowid: int) -> None
Change this blob object to point to a different row. It can be
faster than closing an existing blob an opening a new one.
Calls: `sqlite3_blob_reopen <https://sqlite.org/c3ref/blob_reopen.html>`__
.. method:: Blob.seek(offset: int, whence: int = 0) -> None
Changes current position to *offset* biased by *whence*.
:param offset: New position to seek to. Can be positive or negative number.
:param whence: Use 0 if *offset* is relative to the beginning of the blob,
1 if *offset* is relative to the current position,
and 2 if *offset* is relative to the end of the blob.
:raises ValueError: If the resulting offset is before the beginning (less than zero) or beyond the end of the blob.
.. method:: Blob.tell() -> int
Returns the current offset.
.. index:: sqlite3_blob_write
.. method:: Blob.write(data: bytes) -> None
Writes the data to the blob.
:param data: bytes to write
:raises TypeError: Wrong data type
:raises ValueError: If the data would go beyond the end of the blob.
You cannot increase the size of a blob by writing beyond the end.
You need to use :class:`zeroblob` to set the desired size first when
inserting the blob.
Calls: `sqlite3_blob_write <https://sqlite.org/c3ref/blob_write.html>`__
|
