Source code for padme

# Copyright 2015 Canonical Ltd.
# Written by:
#   Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
#
# This file is part of Padme.
#
# Padme is free software: you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License version 3,
# as published by the Free Software Foundation.
#
# Padme is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with Padme.  If not, see <http://www.gnu.org/licenses/>.
"""
:mod:`padme` -- a mostly transparent proxy class for Python
===========================================================

Padme, named after the Star Wars (tm) character, is a library for creating
proxy objects out of any other python object.

The resulting object is as close to mimicking the original as possible. Some
things are impossible to fake in CPython so those are highlighted below. All
other operations are silently forwarded to the original.

Let's consider a simple example:

    >>> pets = ['cat', 'dog', 'fish']
    >>> pets_proxy = proxy(pets)
    >>> pets_proxy
    ['cat', 'dog', 'fish']
    >>> isinstance(pets_proxy, list)
    True
    >>> pets_proxy.append('rooster')
    >>> pets
    ['cat', 'dog', 'fish', 'rooster']

By default, a proxy object is not that interesting. What is more interesting is
the ability to create subclasses that change a subset of the behavior. For
implementation simplicity such methods need to be decorated with
``@unproxied``.

Let's consider a crazy proxy that overrides the ``__repr__()`` method to censor
the word 'cat'. This is how it can be implemented:

    >>> class censor_cat(proxy):
    ...     @unproxied
    ...     def __repr__(self):
    ...         return super(censor_cat, self).__repr__().replace('cat', '***')

Now let's create a proxy for our pets collection and see how it looks like:

    >>> pets_proxy = censor_cat(pets)
    >>> pets_proxy
    ['***', 'dog', 'fish', 'rooster']

As before, all other aspects of the proxy behave the same way. All of the
methods work and are forwarded to the original object. The type of the proxy
object is correct, event the meta-class of the object is correct (this matters
for ``issubclass()``, for instance).

There are only two things that that give our proxy away.

The ``type()`` function:

    >>> type(pets_proxy)  # doctest: +ELLIPSIS
    <class 'padme...boundproxy'>

And the ``id`` function (and anything that checks object identity):

    >>> pets_proxy is pets
    False
    >>> id(pets) == id(pets_proxy)
    False

That's it, enjoy. You can read the unit tests for additional interesting
details of how the proxy class works. Those are not covered in this short
introduction.

.. note::
    There are a number of classes and meta-classes but the only public
    interface is the :class:`proxy` class and the :func:`unproxied` decorator.
    See below for examples.
"""
import logging

_logger = logging.getLogger("padme")

__author__ = 'Zygmunt Krynicki'
__email__ = 'zygmunt.krynicki@canonical.com'
__version__ = '1.0'
__all__ = ['proxy', 'unproxied']


class proxy_meta(type):
    """
    Meta-class for all proxy types

    This meta-class is responsible for gathering the __unproxied__ attribute on
    each created class. The attribute is a frosenset of names that will not be
    forwarded to the ``proxxie`` but instead will be looked up on the proxy
    itself.
    """

    def __new__(mcls, name, bases, ns):
        _logger.debug(
            "__new__ on proxy_meta with name: %r, bases: %r", name, bases)
        unproxied_set = set()
        for base in bases:
            if hasattr(base, '__unproxied__'):
                unproxied_set.update(base.__unproxied__)
        for ns_attr, ns_value in ns.items():
            if getattr(ns_value, 'unproxied', False):
                unproxied_set.add(ns_attr)
        if unproxied_set:
            _logger.debug(
                "proxy type %r will pass-thru %r", name, unproxied_set)
        ns['__unproxied__'] = frozenset(unproxied_set)
        return super().__new__(mcls, name, bases, ns)


def make_boundproxy_meta(proxiee):
    """
    Make a new bound proxy meta-class for the specified object

    :param proxiee:
        The object that will be proxied
    :returns:
        A new meta-class that lexically wraps ``proxiee`` and subclasses
        :class:`proxy_meta`.
    """

    class boundproxy_meta(proxy_meta):
        """
        Meta-class for all bound proxies.

        This meta-class is responsible for setting the setting the
        ``__proxiee__`` attribute to the proxiee object itself.

        In addition, it implements two methods that participate in instance and
        class checks: ``__instancecheck__`` and ``__subclasscheck__``.
        """

        def __new__(mcls, name, bases, ns):
            _logger.debug(
                "__new__ on boundproxy_meta with name %r and bases %r",
                name, bases)
            ns['__proxiee__'] = proxiee
            return super().__new__(mcls, name, bases, ns)

        def __instancecheck__(mcls, instance):
            # NOTE: this is never called in practice since
            # proxy(obj).__class__ is really obj.__class__.
            _logger.debug("__instancecheck__ %r on %r", instance, proxiee)
            return isinstance(instance, type(proxiee))

        def __subclasscheck__(mcls, subclass):
            # This is still called though since type(proxy(obj)) is
            # something else
            _logger.debug("__subclasscheck__ %r on %r", subclass, proxiee)
            return issubclass(type(proxiee), subclass)

    return boundproxy_meta


class proxy_base:
    """
    Base class for all proxies.

    This class implements the bulk of the proxy work by having a lot of dunder
    methods that delegate their work to a ``proxiee`` object. The ``proxiee``
    object must be available as the ``__proxiee__`` attribute on a class
    deriving from ``base_proxy``. Apart from ``__proxiee__`, the
    ``__unproxied__`` attribute, which should be a frozenset, must also be
    present in all derived classes.

    In practice, the two special attributes are injected via
    ``boundproxy_meta`` created by :func:`make_boundproxy_meta()`. This class
    is also used as a base class for the tricky :class:`proxy` below.

    NOTE: Look at ``pydoc3 SPECIALMETHODS`` section titled ``Special method
    lookup`` for a rationale of why we have all those dunder methods while
    still having __getattribute__()
    """
    # NOTE: the order of methods below matches that of ``pydoc3
    # SPECIALMETHODS``. The "N/A to instances" text means that it makes no
    # sense to add proxy support to the specified method because that method
    # makes no sense on instances. Proxy is designed to intercept access to
    # *objects*, not construction of such objects in the first place.

    # N/A to instances: __new__

    # N/A to instances: __init__

    def __del__(self):
        """
        NOTE: this method is handled specially since it must be called
        after an object becomes unreachable. As long as the proxy object
        itself exits, it holds a strong reference to the original object.
        """

    def __repr__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__repr__ on proxiee (%r)", proxiee)
        return repr(proxiee)

    def __str__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__str__ on proxiee (%r)", proxiee)
        return str(proxiee)

    def __bytes__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__bytes__ on proxiee (%r)", proxiee)
        return bytes(proxiee)

    def __format__(self, format_spec):
        proxiee = type(self).__proxiee__
        _logger.debug("__format__ on proxiee (%r)", proxiee)
        return format(proxiee, format_spec)

    def __lt__(self, other):
        proxiee = type(self).__proxiee__
        _logger.debug("__lt__ on proxiee (%r)", proxiee)
        return proxiee < other

    def __le__(self, other):
        proxiee = type(self).__proxiee__
        _logger.debug("__le__ on proxiee (%r)", proxiee)
        return proxiee <= other

    def __eq__(self, other):
        proxiee = type(self).__proxiee__
        _logger.debug("__eq__ on proxiee (%r)", proxiee)
        return proxiee == other

    def __ne__(self, other):
        proxiee = type(self).__proxiee__
        _logger.debug("__ne__ on proxiee (%r)", proxiee)
        return proxiee != other

    def __gt__(self, other):
        proxiee = type(self).__proxiee__
        _logger.debug("__gt__ on proxiee (%r)", proxiee)
        return proxiee > other

    def __ge__(self, other):
        proxiee = type(self).__proxiee__
        _logger.debug("__ge__ on proxiee (%r)", proxiee)
        return proxiee >= other

    def __hash__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__hash__ on proxiee (%r)", proxiee)
        return hash(proxiee)

    def __bool__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__bool__ on proxiee (%r)", proxiee)
        return bool(proxiee)

    def __getattr__(self, name):
        proxiee = type(self).__proxiee__
        _logger.debug("__getattr__ %r on proxiee (%r)", name, proxiee)
        return getattr(proxiee, name)

    def __getattribute__(self, name):
        cls = type(self)
        if name not in cls.__unproxied__:
            proxiee = cls.__proxiee__
            _logger.debug("__getattribute__ %r on proxiee (%r)", name, proxiee)
            return getattr(proxiee, name)
        else:
            _logger.debug("__getattribute__ %r on proxy itself", name)
            return object.__getattribute__(self, name)

    def __setattr__(self, attr, value):
        proxiee = type(self).__proxiee__
        _logger.debug("__setattr__ %r on proxiee (%r)", attr, proxiee)
        setattr(proxiee, attr, value)

    def __delattr__(self, attr):
        proxiee = type(self).__proxiee__
        _logger.debug("__delattr__ %r on proxiee (%r)", attr, proxiee)
        delattr(proxiee, attr)

    def __dir__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__dir__ on proxiee (%r)", proxiee)
        return dir(proxiee)

    def __get__(self, instance, owner):
        proxiee = type(self).__proxiee__
        _logger.debug("__get__ on proxiee (%r)", proxiee)
        return proxiee.__get__(instance, owner)

    def __set__(self, instance, value):
        proxiee = type(self).__proxiee__
        _logger.debug("__set__ on proxiee (%r)", proxiee)
        proxiee.__set__(instance, value)

    def __delete__(self, instance):
        proxiee = type(self).__proxiee__
        _logger.debug("__delete__ on proxiee (%r)", proxiee)
        proxiee.__delete__(instance)

    def __call__(self, *args, **kwargs):
        proxiee = type(self).__proxiee__
        _logger.debug("call on proxiee (%r)", proxiee)
        return proxiee(*args, **kwargs)

    def __len__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__len__ on proxiee (%r)", proxiee)
        return len(proxiee)

    def __length_hint__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__length_hint__ on proxiee (%r)", proxiee)
        return proxiee.__length_hint__()

    def __getitem__(self, item):
        proxiee = type(self).__proxiee__
        _logger.debug("__getitem__ on proxiee (%r)", proxiee)
        return proxiee[item]

    def __setitem__(self, item, value):
        proxiee = type(self).__proxiee__
        _logger.debug("__setitem__ on proxiee (%r)", proxiee)
        proxiee[item] = value

    def __delitem__(self, item):
        proxiee = type(self).__proxiee__
        _logger.debug("__delitem__ on proxiee (%r)", proxiee)
        del proxiee[item]

    def __iter__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__iter__ on proxiee (%r)", proxiee)
        return iter(proxiee)

    def __reversed__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__reversed__ on proxiee (%r)", proxiee)
        return reversed(proxiee)

    def __contains__(self, item):
        proxiee = type(self).__proxiee__
        _logger.debug("__contains__ on proxiee (%r)", proxiee)
        return item in proxiee

    # TODO: all numeric methods

    def __enter__(self):
        proxiee = type(self).__proxiee__
        _logger.debug("__enter__ on proxiee (%r)", proxiee)
        return proxiee.__enter__()

    def __exit__(self, exc_type, exc_value, traceback):
        proxiee = type(self).__proxiee__
        _logger.debug("__exit__ on proxiee (%r)", proxiee)
        return proxiee.__exit__(exc_type, exc_value, traceback)


class proxy(proxy_base, metaclass=proxy_meta):
    """
    A mostly transparent proxy type

    The proxy class can be used in two different ways. First, as a callable
    ``proxy(obj)``. This simply returns a proxy for a single object.

        >>> truth = ['trust no one']
        >>> lie = proxy(truth)

    This will return an instance of a new ``proxy`` sub-class which for all
    intents and purposes, to the extent possible in CPython, forwards all
    requests to the original object.

    One can still examine the proxy with some ways::

        >>> lie is truth
        False
        >>> type(lie) is type(truth)
        False

    Having said that, the vast majority of stuff will make the proxy behave
    identically to the original object.

        >>> lie[0]
        'trust no one'
        >>> lie[0] = 'trust the government'
        >>> truth[0]
        'trust the government'

    The second way of using the ``proxy`` class is as a base class. In this
    way, one can actually override certain methods. To ensure that all the
    dunder methods work correctly please use the ``@unproxied`` decorator on
    them.

        >>> import codecs
        >>> class crypto(proxy):
        ...
        ...     @unproxied
        ...     def __repr__(self):
        ...         return codecs.encode(super().__repr__(), "rot_13")

    With this weird class, we can change the repr() of any object we want to be
    ROT-13 encoded. Let's see:

        >>> orig = ['ala ma kota', 'a kot ma ale']
        >>> prox = crypto(orig)

    We can sill access all of the data through the proxy:

        >>> prox[0]
        'ala ma kota'

    But the whole repr() is now a bit different than usual:

        >>> prox
        ['nyn zn xbgn', 'n xbg zn nyr']
    """

    def __new__(proxy_cls, proxiee):
        """
        Create a new instance of ``proxy()`` wrapping ``proxiee``

        :param proxiee:
            The object to proxy
        :returns:
            An instance of new subclass of ``proxy``, called ``boundproxy``
            that uses a new meta-class that lexically bounds the ``proxiee``
            argument. The new sub-class has a different implementation of
            ``__new__`` and can be instantiated without additional arguments.
        """
        _logger.debug("__new__ on proxy with proxiee: %r", proxiee)
        boundproxy_meta = make_boundproxy_meta(proxiee)

        class boundproxy(proxy_cls, metaclass=boundproxy_meta):

            def __new__(boundproxy_cls):
                _logger.debug("__new__ on boundproxy %r", boundproxy_cls)
                return object.__new__(boundproxy_cls)
        return boundproxy()


def unproxied(fn):
    """
    Mark an object (attribute) as not-to-be-proxied.

    This decorator can be used inside :class:`proxy` sub-classes. Please
    consult the documentation of ``proxy`` for details.
    """
    fn.unproxied = True
    return fn