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# DirDBM: Directory-based Storage

## dirdbm.DirDBM

twisted.persisted.dirdbm.DirDBM is a DBM-like storage system. That is, it stores mappings between keys and values, like a Python dictionary, except that it stores the values in files in a directory - each entry is a different file. The keys must always be strings, as are the values. Other than that, DirDBM objects act just like Python dictionaries.

DirDBM is useful for cases when you want to store small amounts of data in an organized fashion, without having to deal with the complexity of a RDBMS or other sophisticated database. It is simple, easy to use, cross-platform, and doesn’t require any external C libraries, unlike Python’s built-in DBM modules.

    >>> from twisted.persisted import dirdbm
    >>> d = dirdbm.DirDBM("/tmp/dir")
    >>> d["librarian"] = "ook"
    >>> d["librarian"]
    'ook'
    >>> d.keys()
    ['librarian']
    >>> del d["librarian"]
    >>> d.items()
    []

## dirdbm.Shelf

Sometimes it is neccessary to persist more complicated objects than strings. With some care, dirdbm.Shelf can transparently persist them. `Shelf` works exactly like `DirDBM` , except that the values (but not the keys) can be arbitrary picklable objects. However, notice that mutating an object after it has been stored in the `Shelf` has no effect on the Shelf. When mutating objects, it is neccessary to explicitly store them back in the `Shelf` afterwards:

    >>> from twisted.persisted import dirdbm
    >>> d = dirdbm.Shelf("/tmp/dir2")
    >>> d["key"] = [1, 2]
    >>> d["key"]
    [1, 2]
    >>> l = d["key"]
    >>> l.append(3)
    >>> d["key"]
    [1, 2]
    >>> d["key"] = l
    >>> d["key"]
    [1, 2, 3]