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gh-131323: Add comprehensive key-value type documentation to dbm module #137972

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Add comprehensive key-value type documentation to dbm module
furkanonder Aug 19, 2025
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Merge branch 'main' into gh-131323
furkanonder Aug 19, 2025
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50 changes: 47 additions & 3 deletions Doc/library/dbm.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -100,7 +100,8 @@ The :meth:`!setdefault` method requires two arguments.

Key and values are always stored as :class:`bytes`. This means that when
strings are used they are implicitly converted to the default encoding before
being stored.
being stored. For detailed information about accepted types, see
:ref:`dbm-key-value-types`.

These objects also support being used in a :keyword:`with` statement, which
will automatically close them when done.
Expand All @@ -121,6 +122,48 @@ will automatically close them when done.
:meth:`!clear` methods are now available for all :mod:`dbm` backends.


.. _dbm-key-value-types:

Key and Value Types
-------------------

The accepted types for keys and values vary by backend:

**Keys:**

* **All backends**: Accept :class:`str` and :class:`bytes` objects
* **String keys** are automatically converted to bytes using the default
encoding
* **Bytes keys** are stored as-is

**Values:**

* **Traditional backends** (``dbm.gnu``, ``dbm.ndbm``, ``dbm.dumb``): Only
accept :class:`str` and :class:`bytes` objects
* **SQLite backend** (``dbm.sqlite3``): Accepts any object that can be
converted to bytes:

* **Accepted**: :class:`str`, :class:`bytes`, :class:`int`,
:class:`float`, :class:`bool`
* **Rejected**: :class:`None`, :class:`list`, :class:`dict`,
:class:`tuple`, custom objects

**Storage Format:**

All keys and values are stored as :class:`bytes` objects in the database.
When retrieving, you'll always get bytes back, regardless of the original
type stored.

**Type Conversion Examples:**

* ``db['key'] = 'string'`` stored as ``b'string'``
* ``db['key'] = 42`` stored as ``b'42'`` (sqlite3 only)
* ``db['key'] = 3.14`` stored as ``b'3.14'`` (sqlite3 only)
* ``db['key'] = True`` stored as ``b'True'`` (sqlite3 only)
* ``db['key'] = None`` fails on all backends
* ``db['key'] = [1, 2, 3]`` fails on all backends


The following example records some hostnames and a corresponding title, and
then prints out the contents of the database::

Expand All @@ -142,8 +185,9 @@ then prints out the contents of the database::
# Often-used methods of the dict interface work too.
print(db.get('python.org', b'not present'))

# Storing a non-string key or value will raise an exception (most
# likely a TypeError).
# Storing a non-string key or value behavior depends on the backend:
# - Traditional backends (ndbm, gnu, dumb) will raise a TypeError
# - The sqlite3 backend accepts it and converts to bytes
db['www.yahoo.com'] = 4

# db is automatically closed when leaving the with statement.
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