PEP: 416
Title: Add a frozendict builtin type
Version: $Revision$
Last-Modified: $Date$
Author: Victor Stinner <victor.stinner@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 29-February-2012
Python-Version: 3.3


Abstract
========

Add a new frozendict builtin type.


Rationale
=========

A frozendict is a read-only mapping: a key cannot be added nor removed, and a
key is always mapped to the same value. However, frozendict values can be
mutable (not hashable). A frozendict is hashable and so immutable if and only
if all values are hashable (immutable).

Use cases:

 * frozendict lookup can be done at compile time instead of runtime because the
   mapping is read-only. frozendict can be used instead of a preprocessor to
   remove conditional code at compilation, like code specific to a debug build.
 * hashable frozendict can be used as a key of a mapping or as a member of set.
   frozendict can be used to implement a cache.
 * frozendict avoids the need of a lock when the frozendict is shared
   by multiple threads or processes, especially hashable frozendict. It would
   also help to prohibe coroutines (generators + greenlets) to modify the
   global state.
 * frozendict helps to implement read-only object proxies for security modules.
   For example, it would be possible to use frozendict type for __builtins__
   mapping or type.__dict__. This is possible because frozendict is compatible
   with the PyDict C API.
 * frozendict avoids the need of a read-only proxy in some cases. frozendict is
   faster than a proxy because getting an item in a frozendict is a fast lookup
   whereas a proxy requires a function call.
 * use a frozendict as the default value of function argument: avoid the
   problem of mutable default argument.


Constraints
===========

 * frozendict has to implement the Mapping abstract base class
 * frozendict keys and values can be unorderable
 * a frozendict is hashable if all keys and values are hashable
 * frozendict hash does not depend on the items creation order


Implementation
==============

 * Add a PyFrozenDictObject structure based on PyDictObject with an extra
   "Py_hash_t hash;" field
 * frozendict.__hash__() is implemented using hash(frozenset(self.items())) and
   caches the result in its private hash attribute
 * Register frozendict as a collections.abc.Mapping
 * frozendict can be used with PyDict_GetItem(), but PyDict_SetItem() and
   PyDict_DelItem() raise a TypeError


Recipe: hashable dict
======================

To ensure that a a frozendict is hashable, values can be checked
before creating the frozendict::

    import itertools

    def hashabledict(*args, **kw):
        # ensure that all values are hashable
        for key, value in itertools.chain(args, kw.items()):
            if isinstance(value, (int, str, bytes, float, frozenset, complex)):
                # avoid the compute the hash (which may be slow) for builtin
                # types known to be hashable for any value
                continue
            hash(value)
            # don't check the key: frozendict already checks the key
        return frozendict.__new__(cls, *args, **kw)


Objections
==========

*namedtuple may fit the requiements of a frozendict.*

A namedtuple is not a mapping, it does not implement the Mapping abstract base
class.

*frozendict can be implemented in Python using descriptors" and "frozendict
just need to be practically constant.*

If frozendict is used to harden Python (security purpose), it must be
implemented in C. A type implemented in C is also faster.

*The PEP 351 was rejected.*

The PEP 351 tries to freeze an object and so may convert a mutable object to an
immutable object (using a different type). frozendict doesn't convert anything:
hash(frozendict) raises a TypeError if a value is not hashable. Freezing an
object is not the purpose of this PEP.


Links
=====

 * PEP 412: Key-Sharing Dictionary
   (`issue #13903 <http://bugs.python.org/issue13903>`_)
 * PEP 351: The freeze protocol
 * `The case for immutable dictionaries; and the central misunderstanding of PEP 351 <http://www.cs.toronto.edu/~tijmen/programming/immutableDictionaries.html>`_
 * `Frozen dictionaries (Python recipe 414283) <http://code.activestate.com/recipes/414283-frozen-dictionaries/>`_
   by Oren Tirosh
 * Python security modules implementing read-only object proxies using a C
   extension:

   * `pysandbox <https://github.com/haypo/pysandbox/>`_
   * `mxProxy <http://www.egenix.com/products/python/mxBase/mxProxy/>`_
   * `zope.proxy <http://pypi.python.org/pypi/zope.proxy>`_
   * `zope.security <http://pypi.python.org/pypi/zope.security>`_


Copyright
=========

This document has been placed in the public domain.