PEP: 416 Title: Add a frozendict builtin type Version: $Revision$
Last-Modified: $Date$ Author: Victor Stinner <vstinner@python.org>
Status: Rejected Type: Standards Track Content-Type: text/x-rst Created:
29-Feb-2012 Python-Version: 3.3

Rejection Notice

I'm rejecting this PEP. A number of reasons (not exhaustive):

-   According to Raymond Hettinger, use of frozendict is low. Those that
    do use it tend to use it as a hint only, such as declaring global or
    class-level "constants": they aren't really immutable, since anyone
    can still assign to the name.
-   There are existing idioms for avoiding mutable default values.
-   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.
-   Multiple threads can agree by convention not to mutate a shared
    dict, there's no great need for enforcement. Multiple processes
    can't share dicts.
-   Adding a security sandbox written in Python, even with a limited
    scope, is frowned upon by many, due to the inherent difficulty with
    ever proving that the sandbox is actually secure. Because of this we
    won't be adding one to the stdlib any time soon, so this use case
    falls outside the scope of a PEP.

On the other hand, exposing the existing read-only dict proxy as a
built-in type sounds good to me. (It would need to be changed to allow
calling the constructor.) GvR.

Update (2012-04-15): A new MappingProxyType type was added to the types
module of Python 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 not hashable. A frozendict is hashable if and only if all values
are hashable.

Use cases:

-   Immutable global variable like a default configuration.
-   Default value of a function parameter. Avoid the issue of mutable
    default arguments.
-   Implement a cache: frozendict can be used to store function
    keywords. frozendict can be used as a key of a mapping or as a
    member of set.
-   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 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.
-   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.

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 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 requirements 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.

Alternative: dictproxy

Python has a builtin dictproxy type used by type.__dict__ getter
descriptor. This type is not public. dictproxy is a read-only view of a
dictionary, but it is not read-only mapping. If a dictionary is
modified, the dictproxy is also modified.

dictproxy can be used using ctypes and the Python C API, see for example
the make dictproxy object via ctypes.pythonapi and type() (Python recipe
576540) by Ikkei Shimomura. The recipe contains a test checking that a
dictproxy is "mutable" (modify the dictionary linked to the dictproxy).

However dictproxy can be useful in some cases, where its mutable
property is not an issue, to avoid a copy of the dictionary.

Existing implementations

Whitelist approach.

-   Implementing an Immutable Dictionary (Python recipe 498072) 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.
-   PyWebmail contains an ImmutableDict type:
    webmail.utils.ImmutableDict. It is hashable if keys and values are
    hashable. It is not truly read-only: its internal dict is a public
    attribute.
-   remember project: remember.dicts.FrozenDict. 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.

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.

-   brownie: brownie.datastructures.ImmutableDict. It is hashable if
    keys and values are hashable. werkzeug project has the same code:
    werkzeug.datastructures.ImmutableDict. ImmutableDict is used for
    global constant (configuration options). The Flask project uses
    ImmutableDict of werkzeug for its default configuration.
-   SQLAlchemy project: sqlalchemy.util.immutabledict. 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().
-   Frozen dictionaries (Python recipe 414283) by Oren Tirosh. It is
    hashable if keys and values are hashable. Included in the following
    projects:
    -   lingospot: frozendict/frozendict.py
    -   factor-graphics: frozendict type in
        python/fglib/util_ext_frozendict.py
-   The gsakkis-utils project written by George Sakkis includes a
    frozendict type: datastructs.frozendict
-   characters: scripts/python/frozendict.py. It is hashable. __init__()
    sets __init__ to None.
-   Old NLTK (1.x): nltk.util.frozendict. Keys and values must be
    hashable. __init__() can be called twice to modify the mapping.
    frozendict is used to "freeze" an object.

Hashable dict: inherit from dict and just add an __hash__ method.

-   pypy.rpython.lltypesystem.lltype.frozendict. It is hashable but
    don't deny modification of the mapping.
-   factor-graphics: hashabledict type in
    python/fglib/util_ext_frozendict.py

Links

-   Issue #14162: PEP 416: Add a builtin frozendict type
-   PEP 412: Key-Sharing Dictionary (issue #13903)
-   PEP 351: The freeze protocol
-   The case for immutable dictionaries; and the central
    misunderstanding of PEP 351
-   make dictproxy object via ctypes.pythonapi and type() (Python
    recipe 576540) <http://code.activestate.com/recipes/576540/> by
    Ikkei Shimomura.
-   Python security modules implementing read-only object proxies using
    a C extension:
    -   pysandbox
    -   mxProxy
    -   zope.proxy
    -   zope.security

Copyright

This document has been placed in the public domain.