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updates from Steven Bethard

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pep-0359.txt
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@ -8,7 +8,7 @@ Type: Standards Track
Content-Type: text/x-rst
Created: 05-Apr-2006
Python-Version: 2.6
Post-History: 05-Apr-2006, 06-Apr-2006
Post-History: 05-Apr-2006, 06-Apr-2006, 13-Apr-2006
Abstract
@ -26,6 +26,19 @@ is translated into the assignment::
<name> = <callable>("<name>", <tuple>, <namespace>)
where ``<namespace>`` is the dict created by executing ``<block>``.
This is mostly syntactic sugar for::
class <name> <tuple>:
__metaclass__ = <callable>
<block>
and is intended to help more clearly express the intent of the
statement when something other than a class is being created. Of
course, other syntax for such a statement is possible, but it is hoped
that by keeping a strong parallel to the class statement, an
understanding of how classes and metaclasses work will translate into
an understanding of how the make-statement works as well.
The PEP is based on a suggestion [1]_ from Michele Simionato on the
python-dev list.
@ -35,11 +48,10 @@ Motivation
Class statements provide two nice facilities to Python:
(1) They are the standard Python means of creating a namespace. All
statements within a class body are executed, and the resulting
local name bindings are passed as a dict to the metaclass.
(1) They execute a block of statements and provide the resulting
bindings as a dict to the metaclass.
(2) They encourage DRY (don't repeat yourself) by allowing the class
(2) They encourage DRY (don't repeat yourself) by allowing the class
being created to know the name it is being assigned.
Thus in a simple class statement like::
@ -59,13 +71,153 @@ avoids not only the repetition of ``C``, but also simplifies the
creation of the dict by allowing it to be expressed as a series of
statements.
Historically, type instances (a.k.a. class objects) have been the only
objects blessed with this sort of syntactic support. But other sorts
of objects could benefit from such support. For example, property
objects take three function arguments, but because the property type
cannot be passed a namespace, these functions, though relevant only to
the property, must be declared before it and then passed as arguments
to the property call, e.g.::
Historically, type instances (a.k.a. class objects) have been the
only objects blessed with this sort of syntactic support. The make
statement aims to extend this support to other sorts of objects where
such syntax would also be useful.
Example: simple namespaces
--------------------------
Let's say I have some attributes in a module that I access like::
mod.thematic_roletype
mod.opinion_roletype
mod.text_format
mod.html_format
and since "Namespaces are one honking great idea", I'd like to be able
to access these attributes instead as::
mod.roletypes.thematic
mod.roletypes.opinion
mod.format.text
mod.format.html
I currently have two main options:
(1) Turn the module into a package, turn ``roletypes`` and ``format``
into submodules, and move the attributes to the submodules.
(2) Create ``roletypes`` and ``format`` classes, and move the
attributes to the classes.
The former is a fair chunk of refactoring work, and produces two tiny
modules without much content. The latter keeps the attributes local
to the module, but creates classes when there is no intention of ever
creating instances of those classes.
In situations like this, it would be nice to simply be able to declare
a "namespace" to hold the few attributes. With the new make
statement, I could introduce my new namespaces with something like::
make namespace roletypes:
thematic = ...
opinion = ...
make namespace format:
text = ...
html = ...
and keep my attributes local to the module without making classes that
are never intended to be instantiated. One definition of namespace
that would make this work is::
class namespace(object):
def __init__(self, name, args, kwargs):
self.__dict__.update(kwargs)
Given this definition, at the end of the make-statements above,
``roletypes`` and ``format`` would be namespace instances.
Example: GUI objects
--------------------
In GUI toolkits, objects like frames and panels are often associated
with attributes and functions. With the make-statement, code that
looks something like::
root = Tkinter.Tk()
frame = Tkinter.Frame(root)
frame.pack()
def say_hi():
print "hi there, everyone!"
hi_there = Tkinter.Button(frame, text="Hello", command=say_hi)
hi_there.pack(side=Tkinter.LEFT)
root.mainloop()
could be rewritten to group the the Button's function with its
declaration::
root = Tkinter.Tk()
frame = Tkinter.Frame(root)
frame.pack()
make Tkinter.Button hi_there(frame):
text = "Hello"
def command():
print "hi there, everyone!"
hi_there.pack(side=Tkinter.LEFT)
root.mainloop()
Example: custom descriptors
---------------------------
Since descriptors are used to customize access to an attribute, it's
often useful to know the name of that attribute. Current Python
doesn't give an easy way to find this name and so a lot of custom
descriptors, like Ian Bicking's setonce descriptor [2]_, have to hack
around this somehow. With the make-statement, you could create a
``setonce`` attribute like::
class A(object):
...
make setonce x:
"A's x attribute"
...
where the ``setonce`` descriptor would be defined like::
class setonce(object):
def __init__(self, name, args, kwargs):
self._name = '_setonce_attr_%s' % name
self.__doc__ = kwargs.pop('__doc__', None)
def __get__(self, obj, type=None):
if obj is None:
return self
return getattr(obj, self._name)
def __set__(self, obj, value):
try:
getattr(obj, self._name)
except AttributeError:
setattr(obj, self._name, value)
else:
raise AttributeError("Attribute already set")
def set(self, obj, value):
setattr(obj, self._name, value)
def __delete__(self, obj):
delattr(obj, self._name)
Note that unlike the original implementation, the private attribute
name is stable since it uses the name of the descriptor, and therefore
instances of class A are pickleable.
Example: property namespaces
----------------------------
Python's property type takes three function arguments and a docstring
argument which, though relevant only to the property, must be declared
before it and then passed as arguments to the property call, e.g.::
class C(object):
...
@ -73,60 +225,73 @@ to the property call, e.g.::
...
def set_x(self):
...
x = property(get_x, set_x, ...)
x = property(get_x, set_x, "the x of the frobulation")
There have been a few recipes [2]_ trying to work around this
behavior, but with the new make statement (and an appropriate
definition of property), the getter and setter functions can be
defined in the property's namespace like::
This issue has been brought up before, and Guido [3]_ and others [4]_
have briefly mused over alternate property syntaxes to make declaring
properties easier. With the make-statement, the following syntax
could be supported::
class C(object):
...
make property x:
def get(self):
make block_property x:
'''The x of the frobulation'''
def fget(self):
...
def set(self):
def fset(self):
...
The definition of such a property callable could be as simple as::
with the following definition of ``block_property``::
def property(name, args, namespace):
fget = namespace.get('get')
fset = namespace.get('set')
fdel = namespace.get('delete')
doc = namespace.get('__doc__')
return __builtin__.property(fget, fset, fdel, doc)
def block_property(name, args, block_dict):
fget = block_dict.pop('fget', None)
fset = block_dict.pop('fset', None)
fdel = block_dict.pop('fdel', None)
doc = block_dict.pop('__doc__', None)
assert not block_dict
return property(fget, fset, fdel, doc)
Of course, properties are only one of the many possible uses of the
make statement. The make statement is useful in essentially any
situation where a name is associated with a namespace. So, for
example, namespaces could be created as simply as::
make namespace ns:
"""This creates a namespace named ns with a badger attribute
and a spam function"""
Example: interfaces
-------------------
badger = 42
Guido [5]_ and others have occasionally suggested introducing
interfaces into python. Most suggestions have offered syntax along
the lines of::
def spam():
...
interface IFoo:
"""Foo blah blah"""
And if Python acquires interfaces, given an appropriately defined
``interface`` callable, the make statement can support interface
creation through the syntax::
def fumble(name, count):
"""docstring"""
make interface C(...):
...
but since there is currently no way in Python to declare an interface
in this manner, most implementations of Python interfaces use class
objects instead, e.g. Zope's::
This would mean that interface systems like that of Zope would no
longer have to abuse the class syntax to create proper interface
instances.
class IFoo(Interface):
"""Foo blah blah"""
def fumble(name, count):
"""docstring"""
With the new make-statement, these interfaces could instead be
declared as::
make Interface IFoo:
"""Foo blah blah"""
def fumble(name, count):
"""docstring"""
which makes the intent (that this is an interface, not a class) much
clearer.
Specification
=============
Python will translate a make statement::
Python will translate a make-statement::
make <callable> <name> <tuple>:
<block>
@ -139,25 +304,39 @@ where ``<namespace>`` is the dict created by executing ``<block>``.
The ``<tuple>`` expression is optional; if not present, an empty tuple
will be assumed.
A patch is available implementing these semantics [3]_.
A patch is available implementing these semantics [6]_.
The make statement introduces a new keyword, ``make``. Thus in Python
2.6, the make statement will have to be enabled using ``from
The make-statement introduces a new keyword, ``make``. Thus in Python
2.6, the make-statement will have to be enabled using ``from
__future__ import make_statement``.
Open Issues
===========
Keyword
-------
Does the ``make`` keyword break too much code? Originally, the make
statement used the keyword ``create`` (a suggestion due to Nick
Coghlan). However, investigations into the standard library [4]_ and
Zope+Plone code [5]_ revealed that ``create`` would break a lot more
Coghlan). However, investigations into the standard library [7]_ and
Zope+Plone code [8]_ revealed that ``create`` would break a lot more
code, so ``make`` was adopted as the keyword instead. However, there
are still a few instances where ``make`` would break code. Is there a
better keyword for the statement?
**********
Some possible keywords and their counts in the standard library (plus
some installed packages):
* make - 2 (both in tests)
* create - 19 (including existing function in imaplib)
* build - 83 (including existing class in distutils.command.build)
* construct - 0
* produce - 0
The make-statement as an alternate constructor
----------------------------------------------
Currently, there are not many functions which have the signature
``(name, args, kwargs)``. That means that something like::
@ -169,10 +348,10 @@ Currently, there are not many functions which have the signature
is currently impossible because the dict constructor has a different
signature. Does this sort of thing need to be supported? One
suggestion, by Carl Banks, would be to add a ``__make__`` magic method
that would be called before ``__call__``. For types, the ``__make__``
method would be identical to ``__call__`` (and thus unnecessary), but
dicts could support the make statement by defining a ``__make__``
method on the dict type that looks something like::
that if found would be called instead of ``__call__``. For types,
the ``__make__`` method would be identical to ``__call__`` and thus
unnecessary, but dicts could support the make-statement by defining a
``__make__`` method on the dict type that looks something like::
def __make__(cls, name, args, kwargs):
return cls(**kwargs)
@ -185,6 +364,112 @@ could be used like::
x = 1
y = 2
So the question is, will many types want to use the make-statement as
an alternate constructor? And if so, does that alternate constructor
need to have the same name as the original constructor?
Customizing the dict in which the block is executed
---------------------------------------------------
Should users of the make-statement be able to determine in which dict
object the code is executed? This would allow the make-statement to
be used in situations where a normal dict object would not suffice,
e.g. if order and repeated names must be allowed. Allowing this sort
of customization could allow XML to be written without repeating
element names, and with nesting of make-statements corresponding to
nesting of XML elements::
make Element html:
make Element body:
text('before first h1')
make Element h1:
attrib(style='first')
text('first h1')
tail('after first h1')
make Element h1:
attrib(style='second')
text('second h1')
tail('after second h1')
If the make-statement tried to get the dict in which to execute its
block by calling the callable's ``__make_dict__`` method, the
following code would allow the make-statement to be used as above::
class Element(object):
class __make_dict__(dict):
def __init__(self, *args, **kwargs):
self._super = super(Element.__make_dict__, self)
self._super.__init__(*args, **kwargs)
self.elements = []
self.text = None
self.tail = None
self.attrib = {}
def __getitem__(self, name):
try:
return self._super.__getitem__(name)
except KeyError:
if name in ['attrib', 'text', 'tail']:
return getattr(self, 'set_%s' % name)
else:
return globals()[name]
def __setitem__(self, name, value):
self._super.__setitem__(name, value)
self.elements.append(value)
def set_attrib(self, **kwargs):
self.attrib = kwargs
def set_text(self, text):
self.text = text
def set_tail(self, text):
self.tail = text
def __new__(cls, name, args, edict):
get_element = etree.ElementTree.Element
result = get_element(name, attrib=edict.attrib)
result.text = edict.text
result.tail = edict.tail
for element in edict.elements:
result.append(element)
return result
Note, however, that the code to support this is somewhat fragile --
it has to magically populate the namespace with ``attrib``, ``text``
and ``tail``, and it assumes that every name binding inside the make
statement body is creating an Element. As it stands, this code would
break with the introduction of a simple for-loop to any one of the
make-statement bodies, because the for-loop would bind a name to a
non-Element object. This could be worked around by adding some sort
of isinstance check or attribute examination, but this still results
in a somewhat fragile solution.
It has also been pointed out that the with-statement can provide
equivalent nesting with a much more explicit syntax::
with Element('html') as html:
with Element('body') as body:
body.text = 'before first h1'
with Element('h1', style='first') as h1:
h1.text = 'first h1'
h1.tail = 'after first h1'
with Element('h1', style='second') as h1:
h1.text = 'second h1'
h1.tail = 'after second h1'
And if the repetition of the element names here is too much of a DRY
violoation, it is also possible to eliminate all as-clauses except for
the first by adding a few methods to Element. [9]_
So are there real use-cases for executing the block in a dict of a
different type? And if so, should the make-statement be extended to
support them?
Optional Extensions
===================
@ -213,7 +498,7 @@ implement the feature without the keyword.
Removing __metaclass__ in Python 3000
-------------------------------------
As a side-effect of its generality, the make statement mostly
As a side-effect of its generality, the make-statement mostly
eliminates the need for the ``__metaclass__`` attribute in class
objects. Thus in Python 3000, instead of::
@ -222,7 +507,7 @@ objects. Thus in Python 3000, instead of::
<block>
metaclasses could be supported by using the metaclass as the callable
in a make statement::
in a make-statement::
make <metaclass> <name> <bases-tuple>:
<block>
@ -234,7 +519,7 @@ opcode a bit.
Removing class statements in Python 3000
----------------------------------------
In the most extreme application of make statements, the class
In the most extreme application of make-statements, the class
statement itself could be deprecated in favor of ``make type``
statements.
@ -245,18 +530,30 @@ References
.. [1] Michele Simionato's original suggestion
(http://mail.python.org/pipermail/python-dev/2005-October/057435.html)
.. [2] Namespace-based property recipe
.. [2] Ian Bicking's setonce descriptor
(http://blog.ianbicking.org/easy-readonly-attributes.html)
.. [3] Guido ponders property syntax
(http://mail.python.org/pipermail/python-dev/2005-October/057404.html)
.. [4] Namespace-based property recipe
(http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/442418)
.. [3] Make Statement patch
.. [5] Python interfaces
(http://www.artima.com/weblogs/viewpost.jsp?thread=86641)
.. [6] Make Statement patch
(http://ucsu.colorado.edu/~bethard/py/make_statement.patch)
.. [4] Instances of create in the stdlib
.. [7] Instances of create in the stdlib
(http://mail.python.org/pipermail/python-list/2006-April/335159.html)
.. [5] Instances of create in Zope+Plone
.. [8] Instances of create in Zope+Plone
(http://mail.python.org/pipermail/python-list/2006-April/335284.html)
.. [9] Eliminate as-clauses in with-statement XML
(http://mail.python.org/pipermail/python-list/2006-April/336774.html)
Copyright
=========

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pep-3002.txt
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@ -6,8 +6,8 @@ Author: Steven Bethard <steven.bethard@gmail.com>
Status: Draft
Type: Process
Content-Type: text/x-rst
Created: 03-Mar-2006
Post-History: 03-Mar-2006
Created: 27-Mar-2006
Post-History: 27-Mar-2006, 13-Apr-2006
Abstract