113 lines
3.3 KiB
Plaintext
113 lines
3.3 KiB
Plaintext
PEP: 416
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Title: Add a frozendict builtin type
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Version: $Revision$
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Last-Modified: $Date$
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Author: Victor Stinner <victor.stinner@gmail.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 29-February-2012
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Python-Version: 3.3
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Abstract
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========
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Add a new frozendict builtin type.
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Rationale
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=========
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A frozendict mapping cannot be changed, but its values can be mutable (not
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hashable). A frozendict is hashable and so immutable if all values are hashable
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(immutable).
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Use cases of frozendict:
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* hashable frozendict can be used as a key of a mapping or as a member of set
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* frozendict helps optimization because the mapping is constant
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* frozendict avoids the need of a lock when the frozendict is shared
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by multiple threads or processes, especially hashable frozendict
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Constraints
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===========
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* frozendict has to implement the Mapping abstract base class
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* frozendict keys and values can be unorderable
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* a frozendict is hashable if all keys and values are hashable
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* frozendict hash does not depend on the items creation order
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Implementation
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==============
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* Add a PyFrozenDictObject structure based on PyDictObject with an extra
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"Py_hash_t hash;" field
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* frozendict.__hash__() is implemented using hash(frozenset(self.items())) and
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caches the result in its private hash attribute
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* Register frozendict has a collections.abc.Mapping
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* frozendict can be used with PyDict_GetItem(), but PyDict_SetItem() and
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PyDict_DelItem() raise a TypeError
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Recipe: immutable dict
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======================
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An immutable mapping can be implemented using frozendict::
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import itertools
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class immutabledict(frozendict):
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def __new__(cls, *args, **kw):
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# ensure that all values are immutable
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for key, value in itertools.chain(args, kw.items()):
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if not isinstance(value, (int, float, complex, str, bytes)):
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hash(value)
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# frozendict ensures that all keys are immutable
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return frozendict.__new__(cls, *args, **kw)
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def __repr__(self):
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return 'immutabledict' + frozendict.__repr__(self)[10:]
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Objections
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==========
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*namedtuple may fit the requiements of a frozendict.*
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A namedtuple is not a mapping, it does not implement the Mapping abstract base
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class.
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*frozendict can be implemented in Python using descriptors" and "frozendict
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just need to be practically constant.*
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If frozendict is used to harden Python (security purpose), it must be
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implemented in C. A type implemented in C is also faster.
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*The PEP 351 was rejected.*
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The PEP 351 tries to freeze an object and so may convert a mutable object to an
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immutable object (using a different type). frozendict doesn't convert anything:
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hash(frozendict) raises a TypeError if a value is not hashable. Freezing an
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object is not the purpose of this PEP.
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Links
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=====
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* PEP 412: Key-Sharing Dictionary
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(`issue #13903 <http://bugs.python.org/issue13903>`_)
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* PEP 351: The freeze protocol
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* `The case for immutable dictionaries; and the central misunderstanding of PEP 351 <http://www.cs.toronto.edu/~tijmen/programming/immutableDictionaries.html>`_
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* `Frozen dictionaries (Python recipe 414283) <http://code.activestate.com/recipes/414283-frozen-dictionaries/>`_
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by Oren Tirosh
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Copyright
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=========
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This document has been placed in the public domain.
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