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pypy / pypy / doc / objspace-proxies.rst

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What PyPy can do for your objects

Thanks to the Object Space architecture, any feature that is based on proxying, extending, changing or otherwise controlling the behavior of all objects in a running program is easy to implement on top of PyPy.

Here is what we have implemented so far, in historical order:

  • Thunk Object Space: lazily computed objects, computing only when an operation is performed on them; lazy functions, computing their result only if and when needed; and a way to globally replace an object with another.
  • Dump Object Space: dumps all operations performed on all the objects into a large log file. For debugging your applications.
  • Transparent Proxies Extension: adds new proxy objects to the Standard Object Space that enable applications to control operations on application and builtin objects, e.g lists, dictionaries, tracebacks.

Which object space to use can be chosen with the :config:`objspace.name` option.

The Thunk Object Space

This small object space, meant as a nice example, wraps another object space (e.g. the standard one) and adds two capabilities: lazily computed objects, computed only when an operation is performed on them, and "become", a more obscure feature which allows to completely and globally replaces an object with another.

Example usage of lazily computed objects:

$ py.py -o thunk
>>>> from __pypy__ import thunk
>>>> def f():
....    print 'computing...'
....    return 6*7
....
>>>> x = thunk(f)
>>>> x
computing...
42
>>>> x
42
>>>> y = thunk(f)
>>>> type(y)
computing...
<type 'int'>

Example of how one object can be instantly and globally replaced with another:

$ py.py -o thunk
>>>> from __pypy__ import become
>>>> x = object()
>>>> lst = [1, 2, x, 4]
>>>> become(x, 3)
>>>> lst
[1, 2, 3, 4]

There is also a decorator for functions whose result can be computed lazily (the function appears to return a result, but it is not really invoked before the result is used, if at all):

$ py.py -o thunk
>>>> from __pypy__ import lazy
>>>> @lazy
.... def f(x):
....    print 'computing...'
....    return x * 100
....
>>>> lst = [f(i) for i in range(10)]
>>>> del lst[1:9]
>>>> lst
computing...
computing...
[0, 900]

Implementation

The implementation is short (see `pypy/objspace/thunk.py`_). For the purpose of become(), it adds an internal field w_thunkalias to each object, which is either None (in the common case) or a reference to the object that this object was replaced with. When any space operation is invoked, the chain of w_thunkalias references is followed and the underlying object space really operates on the new objects instead of the old ones.

For the laziness part, the function thunk() returns an instance of a new internal class W_Thunk which stores the user-supplied callable and arguments. When a space operation follows the w_thunkalias chains of objects, it special-cases W_Thunk: it invokes the stored callable if necessary to compute the real value and then stores it in the w_thunkalias field of the W_Thunk. This has the effect of replacing the latter with the real value.

Interface

In a PyPy running with (or translated with) the Thunk Object Space, the __pypy__ module exposes the following interface:

  • thunk(f, *args, **kwargs): returns something that behaves like the result of the call f(*args, **kwargs) but the call is done lazily.
  • is_thunk(obj): return True if obj is a thunk that is not computed yet.
  • become(obj1, obj2): globally replace obj1 with obj2.
  • lazy(callable): should be used as a function decorator - the decorated function behaves lazily: all calls to it return a thunk object.

The Dump Object Space

When PyPy is run with (or translated with) the Dump Object Space, all operations between objects are dumped to a file called pypy-space-dump. This should give a powerful way to debug applications, but so far the dump can only be inspected in a text editor; better browsing tools are needed before it becomes really useful.

Try:

$ py.py -o dump
>>>> 2+3
5
>>>> (exit py.py here)
$ more pypy-space-dump

On my machine the add between 2 and 3 starts at line 3152 (!) and returns at line 3164. All the rest is start-up, printing, and shutdown.

Transparent Proxies

PyPy's Transparent Proxies allow routing of operations on objects to a callable. Application level code can customize objects without interfering with the type system - type(proxied_list) is list holds true when 'proxied_list' is a proxied built-in list - while giving you full control on all operations that are performed on the proxied_list.

See [D12.1] for more context, motivation and usage of transparent proxies.

Example of the core mechanism

The following example proxies a list and will return 42 on any add operation to the list:

$ py.py --objspace-std-withtproxy
>>>> from __pypy__ import tproxy
>>>> def f(operation, *args, **kwargs):
>>>>    if operation == '__add__':
>>>>         return 42
>>>>    raise AttributeError
>>>>
>>>> i = tproxy(list, f)
>>>> type(i)
list
>>>> i + 3
42

Example of recording all operations on builtins

Suppose we want to have a list which stores all operations performed on it for later analysis. We can use the small `lib_pypy/tputil.py`_ module to help with transparently proxying builtin instances:

from tputil import make_proxy

history = []
def recorder(operation):
    history.append(operation)
    return operation.delegate()

>>>> l = make_proxy(recorder, obj=[])
>>>> type(l)
list
>>>> l.append(3)
>>>> len(l)
1
>>>> len(history)
2

make_proxy(recorder, obj=[]) creates a transparent list proxy where we can delegate operations to in the recorder function. Calling type(l) does not lead to any operation being executed at all.

Note that append shows up as __getattribute__ and that type(lst) does not show up at all - the type is the only aspect of the instance which the controller cannot change.

Transparent Proxy PyPy builtins and support

If you are using the --objspace-std-withtproxy option the __pypy__ module provides the following builtins:

  • tproxy(type, controller): returns a proxy object representing the given type and forwarding all operations on this type to the controller. On each such operation controller(opname, *args, **kwargs) is invoked.
  • get_tproxy_controller(obj): returns the responsible controller for a given object. For non-proxied objects None is returned.

tputil helper module

The `lib_pypy/tputil.py`_ module provides:

  • make_proxy(controller, type, obj): function which creates a transparent proxy controlled by the given 'controller' callable. The proxy will appear as a completely regular instance of the given type but all operations on it are send to the specified controller - which receives a ProxyOperation instance on each such operation. A non-specified type will default to type(obj) if obj was specified.

    ProxyOperation instances have the following attributes:

    proxyobj: the transparent proxy object of this operation.

    opname: the operation name of this operation

    args: positional arguments for this operation

    kwargs: keyword arguments for this operation

    obj: (if provided to make_proxy): a concrete object

    If you have specified a concrete object instance obj to your make_proxy invocation, you may call proxyoperation.delegate() to delegate the operation to this object instance.

Further points of interest

A lot of tasks could be performed using transparent proxies, including, but not limited to:

  • Remote versions of objects, on which we can directly perform operations (think about transparent distribution)
  • Access to persistent storage such as a database (imagine an SQL object mapper which looks like a real object)
  • Access to external data structures, such as other languages, as normal objects (of course some operations could raise exceptions, but since they are purely done on application level, that is not real problem)

Implementation Notes

PyPy's standard object space allows to internally have multiple implementations of a type and change the implementation at run time while application level code consistently sees the exact same type and object. Multiple performance optimizations using this features are already implemented: see the document about alternative object implementations. Transparent Proxies use the architecture to provide control back to application level code.

Transparent proxies are implemented on top of the standard object space, in `pypy/objspace/std/proxy_helpers.py`_, `pypy/objspace/std/proxyobject.py`_ and `pypy/objspace/std/transparent.py`_. To use them you will need to pass a --objspace-std-withtproxy option to py.py or translate.py. This registers implementations named W_TransparentXxx - which usually correspond to an appropriate W_XxxObject - and includes some interpreter hacks for objects that are too close to the interpreter to be implemented in the std objspace. The types of objects that can be proxied this way are user created classes & functions, lists, dicts, exceptions, tracebacks and frames.

[D12.1]High-Level Backends and Interpreter Feature Prototypes, PyPy EU-Report, 2007, http://codespeak.net/pypy/extradoc/eu-report/D12.1_H-L-Backends_and_Feature_Prototypes-2007-03-22.pdf