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PEP:359
Title:The "make" Statement
Version:55014
Last-Modified:2007-04-27 17:36:48 -0700 (Fri, 27 Apr 2007)
Author:Steven Bethard <steven.bethard at 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Status:Withdrawn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This PEP was withdrawn at Guido's request [2].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 Nick 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 violoation, 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.