development version of persistentdicts
Project description
persistentdicts
This python library exposes various databases through dict-like objects.
Key difference with competing projects
If you search on pypi, you will find a few competing projects: pdicts, durabledicts, etc. The key differences between persistentdicts and those are:
Implementations of persistentdicts do not keep a local cache dictionary: changes are immediately written to database. Iterators are also proceeding directly on the database. It allows to interact with datasets that would not fit in RAM otherwise.
The test suite is significantly bigger than in all the other implementation I have seen. When relevant, tests have been backported from the CPython test suite of dict.
Serialization is done in json rather than using pickle.
Gotchas
You can’t modify a value of the dictionary in place. For example:
>>> import persistentdicts >>> d = persistentdicts.SqliteDict() >>> d["a"] = [] >>> d["a"].append(1) >>> d["a"] # with a normal dict, you would get [1] []
That is because d["a"] returns a copy of the database entry for the key "a", and not a reference to a python object. Modifying this copy (with append) does not affect the database itself.
To circumvent this, you should do:
>>> import persistentdicts >>> d = persistentdicts.SqliteDict() >>> d["a"] = [] >>> d["a"] = d["a"] + [1] >>> d["a"] [1]
Similarly, setdefault will not work as expected since it does not return a reference to the stored value, but a copy of this value.
>>> import persistentdicts >>> d = persistentdicts.SqliteDict() >>> d.setdefault("a", []).append(1) >>> d["a"] []
Databases supported so far
Done:
sqlite
kyotocabinet
Planned:
cassandra
leveldb
You can request new formats on the bug tracker.
Short documentation
persistentdicts.SqliteDict
persistentdicts.SqliteDict(path=":memory:", table="dict", isolation_level="DEFERRED", *args, **kwargs)
path is the path to the file where you wish to store the data
dict is the table to use in this file
isolation_level is the isolation level used for all transactions. See the sqlite documentation for more details.
the remaining arguments *args, **kwargs are used to fill the dictionary (like a normal dict)
persistentdicts.KyotoCabinetDict
persistentdicts.KyotoCabinetDict(path, *args, **kwargs)
path is the path to the file where you wish to store the data. The file extension matters and will determine which format is going to be used internally (must be one of .kch, .kct, .kcd, .kcf or .kcx). See the kyotocabinet documentation for more details.
the remaining arguments *args, **kwargs are used to fill the dictionary (like a normal dict)
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