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PEP 231 -- __findattr__()

PEP:231
Title:__findattr__()
Author:barry at python.org (Barry Warsaw)
Status:Rejected
Type:Standards Track
Created:30-Nov-2000
Python-Version:2.1
Post-History:

Introduction

This PEP describes an extension to instance attribute lookup and modification machinery, which allows pure-Python implementations of many interesting programming models. This PEP tracks the status and ownership of this feature. It contains a description of the feature and outlines changes necessary to support the feature. This PEP summarizes discussions held in mailing list forums, and provides URLs for further information, where appropriate. The CVS revision history of this file contains the definitive historical record.

Background

The semantics for Python instances allow the programmer to customize some aspects of attribute lookup and attribute modification, through the special methods __getattr__() and __setattr__() [1].

However, because of certain restrictions imposed by these methods, there are useful programming techniques that can not be written in Python alone, e.g. strict Java Bean-like [2] interfaces and Zope style acquisitions [3]. In the latter case, Zope solves this by including a C extension called ExtensionClass [5] which modifies the standard class semantics, and uses a metaclass hook in Python's class model called alternatively the "Don Beaudry Hook" or "Don Beaudry Hack" [6].

While Zope's approach works, it has several disadvantages. First, it requires a C extension. Second it employs a very arcane, but truck-sized loophole in the Python machinery. Third, it can be difficult for other programmers to use and understand (the metaclass has well-known brain exploding properties). And fourth, because ExtensionClass instances aren't "real" Python instances, some aspects of the Python runtime system don't work with ExtensionClass instances.

Proposals for fixing this problem have often been lumped under the rubric of fixing the "class/type dichotomy"; that is, eliminating the difference between built-in types and classes [7]. While a laudable goal itself, repairing this rift is not necessary in order to achieve the types of programming constructs described above. This proposal provides an 80% solution with a minimum of modification to Python's class and instance objects. It does nothing to address the type/class dichotomy.

Proposal

This proposal adds a new special method called __findattr__() with the following semantics:

  • If defined in a class, it will be called on all instance attribute resolutions instead of __getattr__() and __setattr__().
  • __findattr__() is never called recursively. That is, when a specific instance's __findattr__() is on the call stack, further attribute accesses for that instance will use the standard __getattr__() and __setattr__() methods.
  • __findattr__() is called for both attribute access ('getting') and attribute modification ('setting'). It is not called for attribute deletion.
  • When called for getting, it is passed a single argument (not counting 'self'): the name of the attribute being accessed.
  • When called for setting, it is called with third argument, which is the value to set the attribute to.
  • __findattr__() methods have the same caching semantics as __getattr__() and __setattr__(); i.e. if they are present in the class at class definition time, they are used, but if they are subsequently added to a class later they are not.

Key Differences with the Existing Protocol

__findattr__()'s semantics are different from the existing protocol in key ways:

First, __getattr__() is never called if the attribute is found in the instance's __dict__. This is done for efficiency reasons, and because otherwise, __setattr__() would have no way to get to the instance's attributes.

Second, __setattr__() cannot use "normal" syntax for setting instance attributes, e.g. "self.name = foo" because that would cause recursive calls to __setattr__().

__findattr__() is always called regardless of whether the attribute is in __dict__ or not, and a flag in the instance object prevents recursive calls to __findattr__(). This gives the class a chance to perform some action for every attribute access. And because it is called for both gets and sets, it is easy to write similar policy for all attribute access. Further, efficiency is not a problem because it is only paid when the extended mechanism is used.

Examples

One programming style that this proposal allows is a Java Bean-like interface to objects, where unadorned attribute access and modification is transparently mapped to a functional interface. E.g.

class Bean:
    def __init__(self, x):
        self.__myfoo = x

    def __findattr__(self, name, *args):
        if name.startswith('_'):
            # Private names
            if args: setattr(self, name, args[0])
            else:    return getattr(self, name)
        else:
            # Public names
            if args: name = '_set_' + name
            else:    name = '_get_' + name
            return getattr(self, name)(*args)

    def _set_foo(self, x):
        self.__myfoo = x

    def _get_foo(self):
        return self.__myfoo


b = Bean(3)
print b.foo
b.foo = 9
print b.foo

A second, more elaborate example is the implementation of both implicit and explicit acquisition in pure Python:

import types

class MethodWrapper:
    def __init__(self, container, method):
        self.__container = container
        self.__method = method

    def __call__(self, *args, **kws):
        return self.__method.im_func(self.__container, *args, **kws)


class WrapperImplicit:
    def __init__(self, contained, container):
        self.__contained = contained
        self.__container = container

    def __repr__(self):
        return '<Wrapper: [%s | %s]>' % (self.__container,
                                         self.__contained)

    def __findattr__(self, name, *args):
        # Some things are our own
        if name.startswith('_WrapperImplicit__'):
            if args: return setattr(self, name, *args)
            else:    return getattr(self, name)
        # setattr stores the name on the contained object directly
        if args:
            return setattr(self.__contained, name, args[0])
        # Other special names
        if name == 'aq_parent':
            return self.__container
        elif name == 'aq_self':
            return self.__contained
        elif name == 'aq_base':
            base = self.__contained
            try:
                while 1:
                    base = base.aq_self
            except AttributeError:
                return base
        # no acquisition for _ names
        if name.startswith('_'):
            return getattr(self.__contained, name)
        # Everything else gets wrapped
        missing = []
        which = self.__contained
        obj = getattr(which, name, missing)
        if obj is missing:
            which = self.__container
            obj = getattr(which, name, missing)
            if obj is missing:
                raise AttributeError, name
        of = getattr(obj, '__of__', missing)
        if of is not missing:
            return of(self)
        elif type(obj) == types.MethodType:
            return MethodWrapper(self, obj)
        return obj


class WrapperExplicit:
    def __init__(self, contained, container):
        self.__contained = contained
        self.__container = container

    def __repr__(self):
        return '<Wrapper: [%s | %s]>' % (self.__container,
                                         self.__contained)

    def __findattr__(self, name, *args):
        # Some things are our own
        if name.startswith('_WrapperExplicit__'):
            if args: return setattr(self, name, *args)
            else:    return getattr(self, name)
        # setattr stores the name on the contained object directly
        if args:
            return setattr(self.__contained, name, args[0])
        # Other special names
        if name == 'aq_parent':
            return self.__container
        elif name == 'aq_self':
            return self.__contained
        elif name == 'aq_base':
            base = self.__contained
            try:
                while 1:
                    base = base.aq_self
            except AttributeError:
                return base
        elif name == 'aq_acquire':
            return self.aq_acquire
        # explicit acquisition only
        obj = getattr(self.__contained, name)
        if type(obj) == types.MethodType:
            return MethodWrapper(self, obj)
        return obj

    def aq_acquire(self, name):
        # Everything else gets wrapped
        missing = []
        which = self.__contained
        obj = getattr(which, name, missing)
        if obj is missing:
            which = self.__container
            obj = getattr(which, name, missing)
            if obj is missing:
                raise AttributeError, name
        of = getattr(obj, '__of__', missing)
        if of is not missing:
            return of(self)
        elif type(obj) == types.MethodType:
            return MethodWrapper(self, obj)
        return obj


class Implicit:
    def __of__(self, container):
        return WrapperImplicit(self, container)

    def __findattr__(self, name, *args):
        # ignore setattrs
        if args:
            return setattr(self, name, args[0])
        obj = getattr(self, name)
        missing = []
        of = getattr(obj, '__of__', missing)
        if of is not missing:
            return of(self)
        return obj


class Explicit(Implicit):
    def __of__(self, container):
        return WrapperExplicit(self, container)


# tests
class C(Implicit):
    color = 'red'

class A(Implicit):
    def report(self):
        return self.color

# simple implicit acquisition
c = C()
a = A()
c.a = a
assert c.a.report() == 'red'

d = C()
d.color = 'green'
d.a = a
assert d.a.report() == 'green'

try:
    a.report()
except AttributeError:
    pass
else:
    assert 0, 'AttributeError expected'


# special names
assert c.a.aq_parent is c
assert c.a.aq_self is a

c.a.d = d
assert c.a.d.aq_base is d
assert c.a is not a


# no acquisition on _ names
class E(Implicit):
    _color = 'purple'

class F(Implicit):
    def report(self):
        return self._color

e = E()
f = F()
e.f = f
try:
    e.f.report()
except AttributeError:
    pass
else:
    assert 0, 'AttributeError expected'


# explicit
class G(Explicit):
    color = 'pink'

class H(Explicit):
    def report(self):
        return self.aq_acquire('color')

    def barf(self):
        return self.color

g = G()
h = H()
g.h = h
assert g.h.report() == 'pink'

i = G()
i.color = 'cyan'
i.h = h
assert i.h.report() == 'cyan'

try:
    g.i.barf()
except AttributeError:
    pass
else:
    assert 0, 'AttributeError expected'

C++-like access control can also be accomplished, although less cleanly because of the difficulty of figuring out what method is being called from the runtime call stack:

import sys
import types

PUBLIC = 0
PROTECTED = 1
PRIVATE = 2

try:
    getframe = sys._getframe
except ImportError:
    def getframe(n):
        try: raise Exception
        except Exception:
            frame = sys.exc_info()[2].tb_frame
        while n > 0:
            frame = frame.f_back
            if frame is None:
                raise ValueError, 'call stack is not deep enough'
        return frame


class AccessViolation(Exception):
    pass


class Access:
    def __findattr__(self, name, *args):
        methcache = self.__dict__.setdefault('__cache__', {})
        missing = []
        obj = getattr(self, name, missing)
        # if obj is missing we better be doing a setattr for
        # the first time
        if obj is not missing and type(obj) == types.MethodType:
            # Digusting hack because there's no way to
            # dynamically figure out what the method being
            # called is from the stack frame.
            methcache[obj.im_func.func_code] = obj.im_class
        #
        # What's the access permissions for this name?
        access, klass = getattr(self, '__access__', {}).get(
            name, (PUBLIC, 0))
        if access is not PUBLIC:
            # Now try to see which method is calling us
            frame = getframe(0).f_back
            if frame is None:
                raise AccessViolation
            # Get the class of the method that's accessing
            # this attribute, by using the code object cache
            if frame.f_code.co_name == '__init__':
                # There aren't entries in the cache for ctors,
                # because the calling mechanism doesn't go
                # through __findattr__().  Are there other
                # methods that might have the same behavior?
                # Since we can't know who's __init__ we're in,
                # for now we'll assume that only protected and
                # public attrs can be accessed.
                if access is PRIVATE:
                    raise AccessViolation
            else:
                methclass = self.__cache__.get(frame.f_code)
                if not methclass:
                    raise AccessViolation
                if access is PRIVATE and methclass is not klass:
                    raise AccessViolation
                if access is PROTECTED and not issubclass(methclass,
                                                          klass):
                    raise AccessViolation
        # If we got here, it must be okay to access the attribute
        if args:
            return setattr(self, name, *args)
        return obj

# tests
class A(Access):
    def __init__(self, foo=0, name='A'):
        self._foo = foo
        # can't set private names in __init__
        self.__initprivate(name)

    def __initprivate(self, name):
        self._name = name

    def getfoo(self):
        return self._foo

    def setfoo(self, newfoo):
        self._foo = newfoo

    def getname(self):
        return self._name

A.__access__ = {'_foo'      : (PROTECTED, A),
                '_name'     : (PRIVATE, A),
                '__dict__'  : (PRIVATE, A),
                '__access__': (PRIVATE, A),
                }

class B(A):
    def setfoo(self, newfoo):
        self._foo = newfoo + 3

    def setname(self, name):
        self._name = name

b = B(1)
b.getfoo()

a = A(1)
assert a.getfoo() == 1
a.setfoo(2)
assert a.getfoo() == 2

try:
    a._foo
except AccessViolation:
    pass
else:
    assert 0, 'AccessViolation expected'

try:
    a._foo = 3
except AccessViolation:
    pass
else:
    assert 0, 'AccessViolation expected'

try:
    a.__dict__['_foo']
except AccessViolation:
    pass
else:
    assert 0, 'AccessViolation expected'


b = B()
assert b.getfoo() == 0
b.setfoo(2)
assert b.getfoo() == 5
try:
    b.setname('B')
except AccessViolation:
    pass
else:
    assert 0, 'AccessViolation expected'

assert b.getname() == 'A'

Here's an implementation of the attribute hook described in PEP 213 (except that hooking on attribute deletion isn't supported by the current reference implementation).

class Pep213:
    def __findattr__(self, name, *args):
        hookname = '__attr_%s__' % name
        if args:
            op = 'set'
        else:
            op = 'get'
        # XXX: op = 'del' currently not supported
        missing = []
        meth = getattr(self, hookname, missing)
        if meth is missing:
            if op == 'set':
                return setattr(self, name, *args)
            else:
                return getattr(self, name)
        else:
            return meth(op, *args)


def computation(i):
    print 'doing computation:', i
    return i + 3


def rev_computation(i):
    print 'doing rev_computation:', i
    return i - 3


class X(Pep213):
    def __init__(self, foo=0):
        self.__foo = foo

    def __attr_foo__(self, op, val=None):
        if op == 'get':
            return computation(self.__foo)
        elif op == 'set':
            self.__foo = rev_computation(val)
        # XXX: 'del' not yet supported

x = X()
fooval = x.foo
print fooval
x.foo = fooval + 5
print x.foo
# del x.foo

Reference Implementation

The reference implementation, as a patch to the Python core, can be found at this URL:

http://sourceforge.net/patch/?func=detailpatch&patch_id=102613&group_id=5470

Rejection

There are serious problems with the recursion-protection feature. As described here it's not thread-safe, and a thread-safe solution has other problems. In general, it's not clear how helpful the recursion-protection feature is; it makes it hard to write code that needs to be callable inside __findattr__ as well as outside it. But without the recursion-protection, it's hard to implement __findattr__ at all (since __findattr__ would invoke itself recursively for every attribute it tries to access). There seems to be no good solution here.

It's also dubious how useful it is to support __findattr__ both for getting and for setting attributes -- __setattr__ gets called in all cases alrady.

The examples can all be implemented using __getattr__ if care is taken not to store instance variables under their own names.

Source: https://github.com/python/peps/blob/master/pep-0231.txt