Blob Input/Output

A blob is a SQLite datatype representing a sequence of bytes. It can be zero or more bytes in size.

SQLite blobs have an absolute maximum size of 2GB and a default maximum size 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 properties of SQLite.

zeroblob class

class zeroblob(size)

If you want to insert a blob into a row, you previously needed to supply the entire blob in one go. To read just one byte also required retrieving the blob in its entirety. For example to insert a 100MB file you would have done:

largedata=open("largefile", "rb").read()
cur.execute("insert into foo values(?)", (buffer(largedata),))

SQLite 3.5 allowed for incremental Blob I/O so you can read and write blobs in small amounts. You cannot change the size of a blob so you need to reserve space which you do through zeroblob which creates a blob of the specified size but full of zero bytes. For example you would reserve space for your 100MB one of these two ways:

cur.execute("insert into foo values(zeroblob(100000000))")
cur.execute("insert into foo values(?),
             (apsw.zeroblob(100000000),))

This class is used for the second way. Once a blob exists in the database, you then use the blob class to read and write its contents.

zeroblob.length() → int

Size of zero blob in bytes.

blob class

class blob

This object is created by Connection.blobopen() and provides access to a blob in the database. It behaves like a Python file. At the C level it wraps a sqlite3_blob.

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 zeroblob or the zeroblob() function.

See the example.

blob.__enter__() → context

You can use a blob as a context manager as defined in PEP 0343. When you use with statement, the blob is always closed on exit from the block, even if an exception occurred in the block.

For example:

with connection.blobopen() as blob:
    blob.write("...")
    res=blob.read(1024)
blob.__exit__() → False

Implements context manager in conjunction with __enter__(). Any exception that happened in the with block is raised after closing the blob.

blob.close([force=False])

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 read() and write() routines may not be reported until close is called. Similarly errors that occurred in those methods (eg calling write() on a read-only blob) may also be re-reported in close(). (This behaviour is what the underlying SQLite APIs do - it is not APSW doing it.)

It is okay to call close() multiple times.

Parameters:force – Ignores any errors during close.

Calls: sqlite3_blob_close

blob.length() → int

Returns the size of the blob in bytes.

Calls: sqlite3_blob_bytes

blob.read([nbytes]) → bytes

Reads amount of data requested, or till end of file, whichever is earlier. Attempting to read beyond the end of the blob returns the empty string/bytes, in the same manner as end of file on normal file objects.

Return type:(Python 2) string (Python 3) bytes

Calls: sqlite3_blob_read

blob.readinto(buffer[, offset=0, length=remaining-buffer]) → 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 blob.read() and then copying into buffer.

Parameters:
  • buffer – A writable buffer like object. In Python 2.6 onwards there is a bytearray type that is very useful. array.array also works.
  • offset – The position to start writing into the buffer defaulting to the beginning.
  • 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

blob.reopen(rowid)

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

blob.seek(offset[, whence=0]) → None

Changes current position to offset biased by whence.

Parameters:
  • offset – New position to seek to. Can be positive or negative number.
  • 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.

blob.tell() → int

Returns the current offset.

blob.write(data) → None

Writes the data to the blob.

Parameters:

data – (Python 2) buffer or string. (Python 3) buffer or bytes.

Raises:
  • TypeError – Wrong data type
  • 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 zeroblob to set the desired size first when inserting the blob.

Calls: sqlite3_blob_write