PEP: 359 Title: The "make" Statement Version: $Revision$ Last-Modified:
$Date$ Author: Steven Bethard <steven.bethard@gmail.com> Status:
Withdrawn Type: Standards Track Content-Type: text/x-rst Created:
05-Apr-2006 Python-Version: 2.6 Post-History: 05-Apr-2006, 06-Apr-2006,
13-Apr-2006

Abstract

This PEP proposes a generalization of the class-declaration syntax, the
make statement. The proposed syntax and semantics parallel the syntax
for class definition, and so:

    make <callable> <name> <tuple>:
        <block>

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.

Withdrawal Notice

This PEP was withdrawn at Guido's request[2]. Guido didn't like it, and
in particular didn't like how the property use-case puts the instance
methods of a property at a different level than other instance methods
and requires fixed names for the property functions.

Motivation

Class statements provide two nice facilities to Python:

(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
    being created to know the name it is being assigned.

Thus in a simple class statement like:

    class C(object):
        x = 1
        def foo(self):
            return 'bar'

the metaclass (type) gets called with something like:

    C = type('C', (object,), {'x':1, 'foo':<function foo at ...>})

The class statement is just syntactic sugar for the above assignment
statement, but clearly a very useful sort of syntactic sugar. It 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. 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 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[3], 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):
        ...
        def get_x(self):
            ...
        def set_x(self):
            ...
        x = property(get_x, set_x, "the x of the frobulation")

This issue has been brought up before, and Guido[4] and others[5] 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 block_property x:
            '''The x of the frobulation'''
            def fget(self):
                ...
            def fset(self):
                ...

with the following definition of block_property:

    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)

Example: interfaces

Guido[6] and others have occasionally suggested introducing interfaces
into python. Most suggestions have offered syntax along the lines of:

    interface IFoo:
        """Foo blah blah"""

        def fumble(name, count):
            """docstring"""

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:

    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:

    make <callable> <name> <tuple>:
        <block>

into the assignment:

    <name> = <callable>("<name>", <tuple>, <namespace>)

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[7].

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 Alyssa Coghlan).
However, investigations into the standard library[8] and Zope+Plone
code[9] 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:

    make dict params:
        x = 1
        y = 2

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 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)

Of course, rather than adding another magic method, the dict type could
just grow a classmethod something like dict.fromblock that could be used
like:

    make dict.fromblock params:
        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
violation, it is also possible to eliminate all as-clauses except for
the first by adding a few methods to Element.[10]

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

Remove the make keyword

It might be possible to remove the make keyword so that such statements
would begin with the callable being called, e.g.:

    namespace ns:
        badger = 42
        def spam():
            ...

    interface C(...):
        ...

However, almost all other Python statements begin with a keyword, and
removing the keyword would make it harder to look up this construct in
the documentation. Additionally, this would add some complexity in the
grammar and so far I (Steven Bethard) have not been able to implement
the feature without the keyword.

Removing __metaclass__ in Python 3000

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:

    class <name> <bases-tuple>:
        __metaclass__ = <metaclass>
        <block>

metaclasses could be supported by using the metaclass as the callable in
a make-statement:

    make <metaclass> <name> <bases-tuple>:
        <block>

Removing the __metaclass__ hook would simplify the BUILD_CLASS opcode a
bit.

Removing class statements in Python 3000

In the most extreme application of make-statements, the class statement
itself could be deprecated in favor of make type statements.

References

Copyright

This document has been placed in the public domain.



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[1] Michele Simionato's original suggestion
(https://mail.python.org/pipermail/python-dev/2005-October/057435.html)

[2] Guido requests withdrawal
(https://mail.python.org/pipermail/python-3000/2006-April/000936.html)

[3] Ian Bicking's setonce descriptor
(http://blog.ianbicking.org/easy-readonly-attributes.html)

[4] Guido ponders property syntax
(https://mail.python.org/pipermail/python-dev/2005-October/057404.html)

[5] Namespace-based property recipe
(http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/442418)

[6] Python interfaces
(http://www.artima.com/weblogs/viewpost.jsp?thread=86641)

[7] Make Statement patch
(http://ucsu.colorado.edu/~bethard/py/make_statement.patch)

[8] Instances of create in the stdlib
(https://mail.python.org/pipermail/python-list/2006-April/335159.html)

[9] Instances of create in Zope+Plone
(https://mail.python.org/pipermail/python-list/2006-April/335284.html)

[10] Eliminate as-clauses in with-statement XML
(https://mail.python.org/pipermail/python-list/2006-April/336774.html)