An Overview of Semidbm

The easiest way to think of semidbm is as an improvement over python’s dumbdbm module.

While the standard library has faster dbms based on well established C libraries (GNU dbm, Berkeley DB, ndbm), dumbdbm is the only pure python portable dbm in the standard library.

Semidbm offers a few improvements over dumbdbm including:

  • Better overall performance (more on this later).
  • Only a single file is used (no separate index and data file).
  • Data file compaction. Free space can be reclaimed (though this only happens whenever explicitly asked to do so using the compact() method).
  • Get/set/delete are require O(1) IO.

Like dumbdbm, semidbm is cross platform. It has been tested on:

  • Linux (Ubuntu 11.10, debian)
  • Mac OS X (Lion/Mountain Lion)
  • Windows 7/8.

There are also a few caveats to consider when using semidbm:

  • The entire index must fit in memory, this means all keys must fit in memory.
  • Not thread safe; can only be accessed by a single process.
  • While the performance is reasonable, it still will not beat one of the standard dbms (GNU dbm, Berkeley DB, etc).

Using Semidbm

To create a new db, specify the name of the directory:

import semidbm
db ='mydb', 'c')

This will create a mydb directory. This directory is where semidbm will place all the files it needs to keep track of the keys and values stored in the db. If the directory does not exist, it will be created.

Once the db has been created, you can get and set values:

db['key1'] = 'value1'
print db['key1']

Keys and values can be either str or bytes.

str types will be encoded to utf-8 before writing to disk. You can avoid this encoding step by providing a byte string directly:

db[b'key1'] = b'value1'

Otherwise, semidbm will do the equivalent of:

db['key1'.encode('utf-8')] = 'value1'.encode('utf-8')

It is recommended that you handle the encoding of your strings in your application, and only use bytes when working with semidbm. The reason for this is that when a value is retrieved, it is returned as a bytestring (semidbm can’t know the encoding of the bytes it retrieved). For example (this is with python 3.3):

>>> db['foo'] = 'value'
>>> db['foo']
>>> db['kēy'] = 'valueē'
>>> db['kēy']

To avoid this confusion, encode your strings before storing with with semidbm.

The reason this automatic conversion is supported is that this is what is done with the DBMs in the python standard library (including dumbdbm which this module was intended to be a drop in replacement for). In order to be able to be a drop in replacement, this automatic encoding process needs to be supported (but not recommended).