PEP: 213 Title: Attribute Access Handlers Author: Paul Prescod
<paul@prescod.net> Status: Deferred Type: Standards Track Content-Type:
text/x-rst Created: 21-Jul-2000 Python-Version: 2.1 Post-History:

Introduction

It is possible (and even relatively common) in Python code and in
extension modules to "trap" when an instance's client code attempts to
set an attribute and execute code instead. In other words, it is
possible to allow users to use attribute assignment/ retrieval/deletion
syntax even though the underlying implementation is doing some
computation rather than directly modifying a binding.

This PEP describes a feature that makes it easier, more efficient and
safer to implement these handlers for Python instances.

Justification

Scenario 1

You have a deployed class that works on an attribute named "stdout".
After a while, you think it would be better to check that stdout is
really an object with a "write" method at the moment of assignment.
Rather than change to a setstdout method (which would be incompatible
with deployed code) you would rather trap the assignment and check the
object's type.

Scenario 2

You want to be as compatible as possible with an object model that has a
concept of attribute assignment. It could be the W3C Document Object
Model or a particular COM interface (e.g. the PowerPoint interface). In
that case you may well want attributes in the model to show up as
attributes in the Python interface, even though the underlying
implementation may not use attributes at all.

Scenario 3

A user wants to make an attribute read-only.

In short, this feature allows programmers to separate the interface of
their module from the underlying implementation for whatever purpose.
Again, this is not a new feature but merely a new syntax for an existing
convention.

Current Solution

To make some attributes read-only:

    class foo:
        def __setattr__( self, name, val ):
            if name=="readonlyattr":
                raise TypeError
            elif name=="readonlyattr2":
                raise TypeError
        ...
        else:
            self.__dict__["name"]=val

This has the following problems:

1.  The creator of the method must be intimately aware of whether
    somewhere else in the class hierarchy __setattr__ has also been
    trapped for any particular purpose. If so, she must specifically
    call that method rather than assigning to the dictionary. There are
    many different reasons to overload __setattr__ so there is a decent
    potential for clashes. For instance object database implementations
    often overload setattr for an entirely unrelated purpose.
2.  The string-based switch statement forces all attribute handlers to
    be specified in one place in the code. They may then dispatch to
    task-specific methods (for modularity) but this could cause
    performance problems.
3.  Logic for the setting, getting and deleting must live in
    __getattr__, __setattr__ and __delattr__. Once again, this can be
    mitigated through an extra level of method call but this is
    inefficient.

Proposed Syntax

Special methods should declare themselves with declarations of the
following form:

    class x:
        def __attr_XXX__(self, op, val ):
            if op=="get":
                return someComputedValue(self.internal)
            elif op=="set":
                self.internal=someComputedValue(val)
            elif op=="del":
                del self.internal

Client code looks like this:

    fooval=x.foo
    x.foo=fooval+5
    del x.foo

Semantics

Attribute references of all three kinds should call the method. The op
parameter can be "get"/"set"/"del". Of course this string will be
interned so the actual checks for the string will be very fast.

It is disallowed to actually have an attribute named XXX in the same
instance as a method named __attr_XXX__.

An implementation of __attr_XXX__ takes precedence over an
implementation of __getattr__ based on the principle that __getattr__ is
supposed to be invoked only after finding an appropriate attribute has
failed.

An implementation of __attr_XXX__ takes precedence over an
implementation of __setattr__ in order to be consistent. The opposite
choice seems fairly feasible also, however. The same goes for __del_y__.

Proposed Implementation

There is a new object type called an attribute access handler. Objects
of this type have the following attributes:

    name (e.g. XXX, not __attr__XXX__)
    method (pointer to a method object)

In PyClass_New, methods of the appropriate form will be detected and
converted into objects (just like unbound method objects). These are
stored in the class __dict__ under the name XXX. The original method is
stored as an unbound method under its original name.

If there are any attribute access handlers in an instance at all, a flag
is set. Let's call it "I_have_computed_attributes" for now. Derived
classes inherit the flag from base classes. Instances inherit the flag
from classes.

A get proceeds as usual until just before the object is returned. In
addition to the current check whether the returned object is a method it
would also check whether a returned object is an access handler. If so,
it would invoke the getter method and return the value. To remove an
attribute access handler you could directly fiddle with the dictionary.

A set proceeds by checking the "I_have_computed_attributes" flag. If it
is not set, everything proceeds as it does today. If it is set then we
must do a dictionary get on the requested object name. If it returns an
attribute access handler then we call the setter function with the
value. If it returns any other object then we discard the result and
continue as we do today. Note that having an attribute access handler
will mildly affect attribute "setting" performance for all sets on a
particular instance, but no more so than today, using __setattr__. Gets
are more efficient than they are today with __getattr__.

The I_have_computed_attributes flag is intended to eliminate the
performance degradation of an extra "get" per "set" for objects not
using this feature. Checking this flag should have minuscule performance
implications for all objects.

The implementation of delete is analogous to the implementation of set.

Caveats

1.  You might note that I have not proposed any logic to keep the
    I_have_computed_attributes flag up to date as attributes are added
    and removed from the instance's dictionary. This is consistent with
    current Python. If you add a __setattr__ method to an object after
    it is in use, that method will not behave as it would if it were
    available at "compile" time. The dynamism is arguably not worth the
    extra implementation effort. This snippet demonstrates the current
    behavior:

        >>> def prn(*args):print args
        >>> class a:

        ...    __setattr__=prn
        >>> a().foo=5
        (<__main__.a instance at 882890>, 'foo', 5)

        >>> class b: pass
        >>> bi=b()
        >>> bi.__setattr__=prn
        >>> b.foo=5

2.  Assignment to __dict__["XXX"] can overwrite the attribute access
    handler for __attr_XXX__. Typically the access handlers will store
    information away in private __XXX variables

3.  An attribute access handler that attempts to call setattr or getattr
    on the object itself can cause an infinite loop (as with
    __getattr__) Once again, the solution is to use a special (typically
    private) variable such as __XXX.

Note

The descriptor mechanism described in PEP 252 is powerful enough to
support this more directly. A 'getset' constructor may be added to the
language making this possible:

    class C:
        def get_x(self):
            return self.__x
        def set_x(self, v):
            self.__x = v
        x = getset(get_x, set_x)

Additional syntactic sugar might be added, or a naming convention could
be recognized.