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<h1 class="page-title">PEP 416 – Add a frozendict builtin type</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Victor Stinner <vstinner at python.org></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Formally declined and will not be accepted">Rejected</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">29-Feb-2012</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.3</dd>
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</dl>
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<section id="contents">
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#rejection-notice">Rejection Notice</a></li>
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#constraints">Constraints</a></li>
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<li><a class="reference internal" href="#implementation">Implementation</a></li>
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<li><a class="reference internal" href="#recipe-hashable-dict">Recipe: hashable dict</a></li>
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<li><a class="reference internal" href="#objections">Objections</a></li>
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<li><a class="reference internal" href="#alternative-dictproxy">Alternative: dictproxy</a></li>
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<li><a class="reference internal" href="#existing-implementations">Existing implementations</a></li>
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<li><a class="reference internal" href="#links">Links</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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</ul>
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</details></section>
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<section id="rejection-notice">
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<h2><a class="toc-backref" href="#rejection-notice" role="doc-backlink">Rejection Notice</a></h2>
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<p>I’m rejecting this PEP. A number of reasons (not exhaustive):</p>
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<ul class="simple">
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<li>According to Raymond Hettinger, use of frozendict is low. Those
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that do use it tend to use it as a hint only, such as declaring
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global or class-level “constants”: they aren’t really immutable,
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||
since anyone can still assign to the name.</li>
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<li>There are existing idioms for avoiding mutable default values.</li>
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<li>The potential of optimizing code using frozendict in PyPy is
|
||
unsure; a lot of other things would have to change first. The same
|
||
holds for compile-time lookups in general.</li>
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<li>Multiple threads can agree by convention not to mutate a shared
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||
dict, there’s no great need for enforcement. Multiple processes
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can’t share dicts.</li>
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<li>Adding a security sandbox written in Python, even with a limited
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||
scope, is frowned upon by many, due to the inherent difficulty with
|
||
ever proving that the sandbox is actually secure. Because of this
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||
we won’t be adding one to the stdlib any time soon, so this use
|
||
case falls outside the scope of a PEP.</li>
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</ul>
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<p>On the other hand, exposing the existing read-only dict proxy as a
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built-in type sounds good to me. (It would need to be changed to
|
||
allow calling the constructor.) GvR.</p>
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<p><strong>Update</strong> (2012-04-15): A new <code class="docutils literal notranslate"><span class="pre">MappingProxyType</span></code> type was added to the types
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module of Python 3.3.</p>
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</section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
|
||
<p>Add a new frozendict builtin type.</p>
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</section>
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<section id="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<p>A frozendict is a read-only mapping: a key cannot be added nor removed, and a
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key is always mapped to the same value. However, frozendict values can be not
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hashable. A frozendict is hashable if and only if all values are hashable.</p>
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<p>Use cases:</p>
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<ul class="simple">
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||
<li>Immutable global variable like a default configuration.</li>
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||
<li>Default value of a function parameter. Avoid the issue of mutable default
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||
arguments.</li>
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<li>Implement a cache: frozendict can be used to store function keywords.
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||
frozendict can be used as a key of a mapping or as a member of set.</li>
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||
<li>frozendict avoids the need of a lock when the frozendict is shared
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||
by multiple threads or processes, especially hashable frozendict. It would
|
||
also help to prohibe coroutines (generators + greenlets) to modify the
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||
global state.</li>
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<li>frozendict lookup can be done at compile time instead of runtime because the
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||
mapping is read-only. frozendict can be used instead of a preprocessor to
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remove conditional code at compilation, like code specific to a debug build.</li>
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||
<li>frozendict helps to implement read-only object proxies for security modules.
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||
For example, it would be possible to use frozendict type for __builtins__
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||
mapping or type.__dict__. This is possible because frozendict is compatible
|
||
with the PyDict C API.</li>
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||
<li>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.</li>
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||
</ul>
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</section>
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<section id="constraints">
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||
<h2><a class="toc-backref" href="#constraints" role="doc-backlink">Constraints</a></h2>
|
||
<ul class="simple">
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||
<li>frozendict has to implement the Mapping abstract base class</li>
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<li>frozendict keys and values can be unorderable</li>
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<li>a frozendict is hashable if all keys and values are hashable</li>
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<li>frozendict hash does not depend on the items creation order</li>
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</ul>
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</section>
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<section id="implementation">
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<h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2>
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<ul class="simple">
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||
<li>Add a PyFrozenDictObject structure based on PyDictObject with an extra
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“Py_hash_t hash;” field</li>
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||
<li>frozendict.__hash__() is implemented using hash(frozenset(self.items())) and
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||
caches the result in its private hash attribute</li>
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||
<li>Register frozendict as a collections.abc.Mapping</li>
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||
<li>frozendict can be used with PyDict_GetItem(), but PyDict_SetItem() and
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||
PyDict_DelItem() raise a TypeError</li>
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||
</ul>
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||
</section>
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||
<section id="recipe-hashable-dict">
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||
<h2><a class="toc-backref" href="#recipe-hashable-dict" role="doc-backlink">Recipe: hashable dict</a></h2>
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||
<p>To ensure that a frozendict is hashable, values can be checked
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before creating the frozendict:</p>
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||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">itertools</span>
|
||
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||
<span class="k">def</span> <span class="nf">hashabledict</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kw</span><span class="p">):</span>
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<span class="c1"># ensure that all values are hashable</span>
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<span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">itertools</span><span class="o">.</span><span class="n">chain</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="n">kw</span><span class="o">.</span><span class="n">items</span><span class="p">()):</span>
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||
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">value</span><span class="p">,</span> <span class="p">(</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">,</span> <span class="nb">bytes</span><span class="p">,</span> <span class="nb">float</span><span class="p">,</span> <span class="nb">frozenset</span><span class="p">,</span> <span class="nb">complex</span><span class="p">)):</span>
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<span class="c1"># avoid the compute the hash (which may be slow) for builtin</span>
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<span class="c1"># types known to be hashable for any value</span>
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<span class="k">continue</span>
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<span class="nb">hash</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
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<span class="c1"># don't check the key: frozendict already checks the key</span>
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<span class="k">return</span> <span class="n">frozendict</span><span class="o">.</span><span class="fm">__new__</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kw</span><span class="p">)</span>
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||
</pre></div>
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</div>
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</section>
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<section id="objections">
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||
<h2><a class="toc-backref" href="#objections" role="doc-backlink">Objections</a></h2>
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||
<p><em>namedtuple may fit the requirements of a frozendict.</em></p>
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<p>A namedtuple is not a mapping, it does not implement the Mapping abstract base
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class.</p>
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<p><em>frozendict can be implemented in Python using descriptors” and “frozendict
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just need to be practically constant.</em></p>
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<p>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.</p>
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<p><em>The</em> <a class="pep reference internal" href="../pep-0351/" title="PEP 351 – The freeze protocol">PEP 351</a> <em>was rejected.</em></p>
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||
<p>The <a class="pep reference internal" href="../pep-0351/" title="PEP 351 – The freeze protocol">PEP 351</a> 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:
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||
hash(frozendict) raises a TypeError if a value is not hashable. Freezing an
|
||
object is not the purpose of this PEP.</p>
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||
</section>
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<section id="alternative-dictproxy">
|
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<h2><a class="toc-backref" href="#alternative-dictproxy" role="doc-backlink">Alternative: dictproxy</a></h2>
|
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<p>Python has a builtin dictproxy type used by type.__dict__ getter descriptor.
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This type is not public. dictproxy is a read-only view of a dictionary, but it
|
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is not read-only mapping. If a dictionary is modified, the dictproxy is also
|
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modified.</p>
|
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<p>dictproxy can be used using ctypes and the Python C API, see for example the
|
||
<a class="reference external" href="http://code.activestate.com/recipes/576540/">make dictproxy object via ctypes.pythonapi and type() (Python recipe 576540)</a>
|
||
by Ikkei Shimomura. The recipe contains a test checking that a dictproxy is
|
||
“mutable” (modify the dictionary linked to the dictproxy).</p>
|
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<p>However dictproxy can be useful in some cases, where its mutable property is
|
||
not an issue, to avoid a copy of the dictionary.</p>
|
||
</section>
|
||
<section id="existing-implementations">
|
||
<h2><a class="toc-backref" href="#existing-implementations" role="doc-backlink">Existing implementations</a></h2>
|
||
<p>Whitelist approach.</p>
|
||
<ul class="simple">
|
||
<li><a class="reference external" href="http://code.activestate.com/recipes/498072/">Implementing an Immutable Dictionary (Python recipe 498072)</a> by Aristotelis Mikropoulos.
|
||
Similar to frozendict except that it is not truly read-only: it is possible
|
||
to access to this private internal dict. It does not implement __hash__ and
|
||
has an implementation issue: it is possible to call again __init__() to
|
||
modify the mapping.</li>
|
||
<li>PyWebmail contains an ImmutableDict type: <a class="reference external" href="http://pywebmail.cvs.sourceforge.net/viewvc/pywebmail/webmail/webmail/utils/ImmutableDict.py?revision=1.2&view=markup">webmail.utils.ImmutableDict</a>.
|
||
It is hashable if keys and values are hashable. It is not truly read-only:
|
||
its internal dict is a public attribute.</li>
|
||
<li>remember project: <a class="reference external" href="https://bitbucket.org/mikegraham/remember/src/tip/remember/dicts.py">remember.dicts.FrozenDict</a>.
|
||
It is used to implement a cache: FrozenDict is used to store function callbacks.
|
||
FrozenDict may be hashable. It has an extra supply_dict() class method to
|
||
create a FrozenDict from a dict without copying the dict: store the dict as
|
||
the internal dict. Implementation issue: __init__() can be called to modify
|
||
the mapping and the hash may differ depending on item creation order. The
|
||
mapping is not truly read-only: the internal dict is accessible in Python.</li>
|
||
</ul>
|
||
<p>Blacklist approach: inherit from dict and override write methods to raise an
|
||
exception. It is not truly read-only: it is still possible to call dict methods
|
||
on such “frozen dictionary” to modify it.</p>
|
||
<ul class="simple">
|
||
<li>brownie: <a class="reference external" href="https://github.com/DasIch/brownie/blob/HEAD/brownie/datastructures/mappings.py">brownie.datastructures.ImmutableDict</a>.
|
||
It is hashable if keys and values are hashable. werkzeug project has the
|
||
same code: <a class="reference external" href="https://github.com/mitsuhiko/werkzeug/blob/master/werkzeug/datastructures.py">werkzeug.datastructures.ImmutableDict</a>.
|
||
ImmutableDict is used for global constant (configuration options). The Flask
|
||
project uses ImmutableDict of werkzeug for its default configuration.</li>
|
||
<li>SQLAlchemy project: <a class="reference external" href="http://hg.sqlalchemy.org/sqlalchemy/file/tip/lib/sqlalchemy/util/_collections.py">sqlalchemy.util.immutabledict</a>.
|
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It is not hashable and has an extra method: union(). immutabledict is used
|
||
for the default value of parameter of some functions expecting a mapping.
|
||
Example: mapper_args=immutabledict() in SqlSoup.map().</li>
|
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<li><a class="reference external" href="http://code.activestate.com/recipes/414283/">Frozen dictionaries (Python recipe 414283)</a>
|
||
by Oren Tirosh. It is hashable if keys and values are hashable. Included in
|
||
the following projects:<ul>
|
||
<li>lingospot: <a class="reference external" href="http://code.google.com/p/lingospot/source/browse/trunk/frozendict/frozendict.py">frozendict/frozendict.py</a></li>
|
||
<li>factor-graphics: frozendict type in <a class="reference external" href="https://github.com/ih/factor-graphics/blob/41006fb71a09377445cc140489da5ce8eeb9c8b1/python/fglib/util_ext_frozendict.py">python/fglib/util_ext_frozendict.py</a></li>
|
||
</ul>
|
||
</li>
|
||
<li>The gsakkis-utils project written by George Sakkis includes a frozendict
|
||
type: <a class="reference external" href="http://code.google.com/p/gsakkis-utils/source/browse/trunk/datastructs/frozendict.py">datastructs.frozendict</a></li>
|
||
<li>characters: <a class="reference external" href="https://github.com/JasonGross/characters/blob/15a2af5f7861cd33a0dbce70f1569cda74e9a1e3/scripts/python/frozendict.py#L1">scripts/python/frozendict.py</a>.
|
||
It is hashable. __init__() sets __init__ to None.</li>
|
||
<li>Old NLTK (1.x): <a class="reference external" href="http://nltk.googlecode.com/svn/trunk/nltk-old/src/nltk/util.py">nltk.util.frozendict</a>. Keys and
|
||
values must be hashable. __init__() can be called twice to modify the
|
||
mapping. frozendict is used to “freeze” an object.</li>
|
||
</ul>
|
||
<p>Hashable dict: inherit from dict and just add an __hash__ method.</p>
|
||
<ul class="simple">
|
||
<li><a class="reference external" href="https://bitbucket.org/pypy/pypy/src/1f49987cc2fe/pypy/rpython/lltypesystem/lltype.py#cl-86">pypy.rpython.lltypesystem.lltype.frozendict</a>.
|
||
It is hashable but don’t deny modification of the mapping.</li>
|
||
<li>factor-graphics: hashabledict type in <a class="reference external" href="https://github.com/ih/factor-graphics/blob/41006fb71a09377445cc140489da5ce8eeb9c8b1/python/fglib/util_ext_frozendict.py">python/fglib/util_ext_frozendict.py</a></li>
|
||
</ul>
|
||
</section>
|
||
<section id="links">
|
||
<h2><a class="toc-backref" href="#links" role="doc-backlink">Links</a></h2>
|
||
<ul class="simple">
|
||
<li><a class="reference external" href="http://bugs.python.org/issue14162">Issue #14162: PEP 416: Add a builtin frozendict type</a></li>
|
||
<li>PEP 412: Key-Sharing Dictionary
|
||
(<a class="reference external" href="http://bugs.python.org/issue13903">issue #13903</a>)</li>
|
||
<li><a class="pep reference internal" href="../pep-0351/" title="PEP 351 – The freeze protocol">PEP 351</a>: The freeze protocol</li>
|
||
<li><a class="reference external" href="http://www.cs.toronto.edu/~tijmen/programming/immutableDictionaries.html">The case for immutable dictionaries; and the central misunderstanding of
|
||
PEP 351</a></li>
|
||
<li><a class="reference external" href="http://code.activestate.com/recipes/576540/">make dictproxy object via ctypes.pythonapi and type() (Python recipe
|
||
576540)</a> by Ikkei Shimomura.</li>
|
||
<li>Python security modules implementing read-only object proxies using a C
|
||
extension:<ul>
|
||
<li><a class="reference external" href="https://github.com/vstinner/pysandbox/">pysandbox</a></li>
|
||
<li><a class="reference external" href="http://www.egenix.com/products/python/mxBase/mxProxy/">mxProxy</a></li>
|
||
<li><a class="reference external" href="http://pypi.python.org/pypi/zope.proxy">zope.proxy</a></li>
|
||
<li><a class="reference external" href="http://pypi.python.org/pypi/zope.security">zope.security</a></li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
</section>
|
||
<section id="copyright">
|
||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document has been placed in the public domain.</p>
|
||
</section>
|
||
</section>
|
||
<hr class="docutils" />
|
||
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0416.rst">https://github.com/python/peps/blob/main/peps/pep-0416.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0416.rst">2023-09-09 17:39:29 GMT</a></p>
|
||
|
||
</article>
|
||
<nav id="pep-sidebar">
|
||
<h2>Contents</h2>
|
||
<ul>
|
||
<li><a class="reference internal" href="#rejection-notice">Rejection Notice</a></li>
|
||
<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#rationale">Rationale</a></li>
|
||
<li><a class="reference internal" href="#constraints">Constraints</a></li>
|
||
<li><a class="reference internal" href="#implementation">Implementation</a></li>
|
||
<li><a class="reference internal" href="#recipe-hashable-dict">Recipe: hashable dict</a></li>
|
||
<li><a class="reference internal" href="#objections">Objections</a></li>
|
||
<li><a class="reference internal" href="#alternative-dictproxy">Alternative: dictproxy</a></li>
|
||
<li><a class="reference internal" href="#existing-implementations">Existing implementations</a></li>
|
||
<li><a class="reference internal" href="#links">Links</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
|
||
<br>
|
||
<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0416.rst">Page Source (GitHub)</a>
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