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PEP 417's dictproxy (immutable dicts) for everyone

Project Description

Exposes the immutable dictproxy type to all (?) versions of Python.

PEP416 asked for a frozendict type, but was rejected. The alternative was to publicly expose dictproxy, the type used for and immutable object’s __dict__, which wraps an existing dict and provides a read-only interface to it. The type has existed since 2.2, but it’s never had a Python-land constructor.

Until now. But only in 3.3+. Which is not all that helpful to some of us.

This module clumsily exposes the same type to previous versions of Python.


from dictproxyhack import dictproxy

myproxy = dictproxy(dict(foo="bar"))
myproxy['baz'] = "quux"  # TypeError

Since the proxy holds a reference to the underlying dict (but doesn’t provide any way to get it back), you can trivially implement frozendict:

def frozendict(*args, **kwargs):
    return dictproxy(dict(*args, **kwargs))

Might as well just inline that where you need it, really.


Python. Should work anywhere. Maybe.

On Python 3.3+, you get the real mappingproxy type, which lives in the types module as MappingProxyType.

On CPython 2.5+, you get a fake class that forcibly instantiates dictproxy objects via ctypes shenanigans.

On nearly anything else, you get a regular class that wraps a dict and doesn’t implement any mutating parts of the mapping interface. Not a fabulous solution, but good enough, and only applies until your favorite port catches up with 3.3’s standard library.

On non-C Python ports that predate 2.6, you get pretty much a dict, because the Mapping ABC doesn’t even exist. Sorry.

The shim classes also fool isinstance and issubclass, so your dirty typechecking should work equally poorly anywhere.

I’ve only actually tried this library on CPython 2.6, 2.7, 3.2, 3.3, and PyPy 2.1, but I’m interested in hearing whether it works elsewhere.


Don’t subclass dictproxy. Python 3.3+ won’t let you, and the shims for other versions are of extremely dubious use.

dictproxy has been renamed to mappingproxy in 3.3+, so don’t rely on repr to match across versions or anything. (It seemed apropos to use the older name for this module.)

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