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ding dong the witch is dead. or rather, the decorator discussion is. 

updating the pep.

(I'm not sure if the "Community Concensus" section should be trimmed
down radically now - it's a lot of words for a rejected form, and the
case for the form is still available on the web and in the mailing
list archives... opinions, anyone?)
This commit is contained in:
Anthony Baxter 2004-09-01 15:02:22 +00:00
parent 1373ea5150
commit b13a4cc305
1 changed files with 213 additions and 148 deletions
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pep-0318.txt
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@ -8,49 +8,49 @@ Type: Standards Track
Content-Type: text/x-rst
Created: 05-Jun-2003
Python-Version: 2.4
Post-History: 09-Jun-2003, 10-Jun-2003, 27-Feb-2004, 23-Mar-2004, 30-Aug-2004
Post-History: 09-Jun-2003, 10-Jun-2003, 27-Feb-2004, 23-Mar-2004, 30-Aug-2004,
2-Sep-2004
WarningWarningWarning
=====================
The final decision on the syntax for 2.4a3 is not yet made. This will
be done before 2.4a3, and this document will be updated to match.
Note also that this document does not attempt to cover the huge number
of potential alternative syntaxes, nor is it an attempt to
exhaustively list all the positives and negatives of each form.
This document is meant to describe the decorator syntax and the
process that resulted in the decisions that were made. It does not
attempt to cover the huge number of potential alternative syntaxes,
nor is it an attempt to exhaustively list all the positives and
negatives of each form.
Abstract
========
The current method for transforming functions and methods (for
instance, declaring them as a class or static method) is awkward and
can lead to code that is difficult to understand. Ideally, these
transformations should be made at the same point in the code where the
declaration itself is made. This PEP introduces new syntax for
transformations of a function or method declaration.
The current method for transforming functions and methods (for instance,
declaring them as a class or static method) is awkward and can lead to
code that is difficult to understand. Ideally, these transformations
should be made at the same point in the code where the declaration
itself is made. This PEP introduces new syntax for transformations of a
function or method declaration.
Motivation
==========
The current method of applying a transformation to a function or
method places the actual translation after the function body. For
large functions this separates a key component of the function's
behavior from the definition of the rest of the function's external
interface. For example::
The current method of applying a transformation to a function or method
places the actual translation after the function body. For large
functions this separates a key component of the function's behavior from
the definition of the rest of the function's external interface. For
example::
def foo(self):
perform method operation
foo = classmethod(foo)
This becomes less readable with longer methods. It also seems less
than pythonic to name the function three times for what is
conceptually a single declaration. A solution to this problem is to
move the transformation of the method closer to the method's own
declaration. While the new syntax is not yet final, the intent is to
replace::
than pythonic to name the function three times for what is conceptually
a single declaration. A solution to this problem is to move the
transformation of the method closer to the method's own declaration.
While the new syntax is not yet final, the intent is to replace::
def foo(cls):
pass
@ -65,41 +65,39 @@ declaration::
def foo(cls):
pass
Modifying classes in this fashion is also possible, though the
benefits are not as immediately apparent. Almost certainly, anything
which could be done with class decorators could be done using
metaclasses, but using metaclasses is sufficiently obscure that there
is some attraction to having an easier way to make simple
modifications to classes. For Python 2.4, only function/method
decorators are being added.
Modifying classes in this fashion is also possible, though the benefits
are not as immediately apparent. Almost certainly, anything which could
be done with class decorators could be done using metaclasses, but
using metaclasses is sufficiently obscure that there is some attraction
to having an easier way to make simple modifications to classes. For
Python 2.4, only function/method decorators are being added.
Why Is This So Hard?
--------------------
Two decorators (``classmethod()`` and ``staticmethod()``) have
been available in Python since version 2.2. It's been assumed since
Two decorators (``classmethod()`` and ``staticmethod()``) have been
available in Python since version 2.2. It's been assumed since
approximately that time that some syntactic support for them would
eventually be added to the language. Given this assumption, one might
wonder why it's been so difficult to arrive at a consensus.
Discussions have raged off-and-on at times in both comp.lang.python
and the python-dev mailing list about how best to implement function
decorators. There is no one clear reason why this should be so, but a
few problems seem to be most problematic.
wonder why it's been so difficult to arrive at a consensus. Discussions
have raged off-and-on at times in both comp.lang.python and the
python-dev mailing list about how best to implement function decorators.
There is no one clear reason why this should be so, but a few problems
seem to be most problematic.
* Disagreement about where the "declaration of intent" belongs.
Almost everyone agrees that decorating/transforming a function at
the end of its definition is suboptimal. Beyond that there seems to
be no clear consensus where to place this information.
Almost everyone agrees that decorating/transforming a function at the
end of its definition is suboptimal. Beyond that there seems to be no
clear consensus where to place this information.
* Syntactic constraints. Python is a syntactically simple language
with fairly strong constraints on what can and can't be done without
"messing things up" (both visually and with regards to the language
parser). There's no obvious way to structure this information so
that people new to the concept will think, "Oh yeah, I know what
you're doing." The best that seems possible is to keep new users
from creating a wildly incorrect mental model of what the syntax
means.
you're doing." The best that seems possible is to keep new users from
creating a wildly incorrect mental model of what the syntax means.
* Overall unfamiliarity with the concept. For people who have a
passing acquaintance with algebra (or even basic arithmetic) or have
@ -109,7 +107,7 @@ few problems seem to be most problematic.
strong preexisting meme that captures the concept.
* Syntax discussions in general appear to cause more contention than
almost anything else. Readers are pointed to the ternary operator
almost anything else. Readers are pointed to the ternary operator
discussions that were associated with PEP 308 for another example of
this.
@ -117,12 +115,12 @@ few problems seem to be most problematic.
Background
==========
There is general agreement that syntactic support is desirable to the
current state of affairs. Guido mentioned `syntactic support for
decorators`_ in his DevDay keynote presentation at the `10th Python
Conference`_, though `he later said`_ it was only one of several
extensions he proposed there "semi-jokingly". `Michael Hudson raised
the topic`_ on ``python-dev`` shortly after the conference,
There is general agreement that syntactic support is desirable to
the current state of affairs. Guido mentioned `syntactic support
for decorators`_ in his DevDay keynote presentation at the `10th
Python Conference`_, though `he later said`_ it was only one of
several extensions he proposed there "semi-jokingly". `Michael Hudson
raised the topic`_ on ``python-dev`` shortly after the conference,
attributing the initial bracketed syntax to an earlier proposal on
``comp.lang.python`` by `Gareth McCaughan`_.
@ -138,18 +136,21 @@ attributing the initial bracketed syntax to an earlier proposal on
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=slrna40k88.2h9o.Gareth.McCaughan%40g.local
Class decorations seem like an obvious next step because class
definition and function definition are syntactically similar.
definition and function definition are syntactically similar,
however Guido remains unconvinced, and class decorators will almost
certainly not be in Python 2.4.
The discussion continued on and off on python-dev from February 2002
through July 2004. Hundreds and hundreds of posts were made, with
people proposing many possible syntax variations. Guido took a list
of proposals to `EuroPython 2004`_, where a discussion took place.
Subsequent to this, he decided that for 2.4a2 we'd have the
`Java-style`_ @decorator syntax. Barry Warsaw named this the
'pie-decorator' syntax, in honor of the Pie-thon Parrot shootout which
was announced about the same time as the decorator syntax, and because
the @ looks a little like a pie. Guido `outlined his case`_ on
Python-dev, including `this piece`_ on the various rejected forms.
The discussion continued on and off on python-dev from February
2002 through July 2004. Hundreds and hundreds of posts were made,
with people proposing many possible syntax variations. Guido took
a list of proposals to `EuroPython 2004`_, where a discussion took
place. Subsequent to this, he decided that we'd have the `Java-style`_
@decorator syntax, and this appeared for the first time in 2.4a2.
Barry Warsaw named this the 'pie-decorator' syntax, in honor of the
Pie-thon Parrot shootout which was occured around the same time as
the decorator syntax, and because the @ looks a little like a pie.
Guido `outlined his case`_ on Python-dev, including `this piece`_
on some of the (many) rejected forms.
.. _EuroPython 2004:
http://www.python.org/doc/essays/ppt/euro2004/euro2004.pdf
@ -199,8 +200,8 @@ The new syntax should
frequently
* not make it more difficult to scan through code quickly. It should
still be easy to search for all definitions, a particular
definition, or the arguments that a function accepts
still be easy to search for all definitions, a particular definition,
or the arguments that a function accepts
* not needlessly complicate secondary support tools such as
language-sensitive editors and other "`toy parser tools out
@ -208,8 +209,8 @@ The new syntax should
* allow future compilers to optimize for decorators. With the hope of
a JIT compiler for Python coming into existence at some point this
tends to require the syntax for decorators to come before the
function definition
tends to require the syntax for decorators to come before the function
definition
* move from the end of the function, where it's currently hidden, to
the front where it is more `in your face`_
@ -246,8 +247,8 @@ This is equivalent to::
func = dec2(dec1(func))
without the intermediate assignment to the variable ``func``. The
decorators are near the function declaration. The @ sign makes it
clear that something new is going on here.
decorators are near the function declaration. The @ sign makes it clear
that something new is going on here.
The decorator statement is limited in what it can accept -- arbitrary
expressions will not work. Guido preferred this because of a `gut
@ -278,8 +279,8 @@ Decorator Location
The first syntax point is the location of the decorators. For the
following examples, we use the @syntax used in 2.4a2.
Decorators before the def statement are the first alternative,
and the syntax used in 2.4a2::
Decorators before the def statement are the first alternative, and the
syntax used in 2.4a2::
@classmethod
def foo(arg1,arg2):
@ -290,36 +291,36 @@ and the syntax used in 2.4a2::
def bar(low,high):
pass
There have been a number of objections raised to this location --
the primary one is that it's the first real Python case where a
line of code has a result on a following line. The syntax that
will be in 2.4a3 will also require one decorator per line (in a2,
multiple decorators can be specified on the same line).
There have been a number of objections raised to this location -- the
primary one is that it's the first real Python case where a line of code
has a result on a following line. The syntax available for in 2.4a3
requires one decorator per line (in a2, multiple decorators could be
specified on the same line).
People also complained that the syntax got unworldly quickly when
multiple decorators were used. The point was made, though, that the
chances of a large number of decorators being used on a single function
were small and thus this was not a large worry.
Some of the advantages of this form are that the decorators live
outside the method body -- they are obviously executed at the time
the function is defined.
Some of the advantages of this form are that the decorators live outside
the method body -- they are obviously executed at the time the function
is defined.
Another advantage is that being prefix to the function definition fit the
idea of knowing about a change to the semantics of the code before the
code itself, thus knowing how to interpret the code's semantics
Another advantage is that being prefix to the function definition fit
the idea of knowing about a change to the semantics of the code before
the code itself, thus knowing how to interpret the code's semantics
properly without having to go back and change your initial perceptions
if the syntax did not come before the function definition.
Guido decided `he preferred`_ having the decorators on the line before
the 'def', because it was felt that a long argument list would mean
that the decorators would be 'hidden'
the 'def', because it was felt that a long argument list would mean that
the decorators would be 'hidden'
.. _he preferred:
http://mail.python.org/pipermail/python-dev/2004-March/043756.html
The second form is the decorators between the def and the function
name, or the function name and the argument list::
The second form is the decorators between the def and the function name,
or the function name and the argument list::
def @classmethod foo(arg1,arg2):
pass
@ -336,12 +337,12 @@ name, or the function name and the argument list::
There are a couple of objections to this form. The first is that it
breaks easily 'greppability' of the source -- you can no longer search
for 'def foo(' and find the definition of the function. The second,
more serious, objection is that in the case of multiple decorators,
the syntax would be extremely unwieldy.
more serious, objection is that in the case of multiple decorators, the
syntax would be extremely unwieldy.
The next form, which has had a number of strong proponents, is to
have the decorators between the argument list and the trailing ``:``
in the 'def' line::
The next form, which has had a number of strong proponents, is to have
the decorators between the argument list and the trailing ``:`` in the
'def' line::
def foo(arg1,arg2) @classmethod:
pass
@ -349,8 +350,8 @@ in the 'def' line::
def bar(low,high) @accepts(int,int),@returns(float):
pass
Guido `summarized the arguments`_ against this form (many of which
also apply to the previous form) as:
Guido `summarized the arguments`_ against this form (many of which also
apply to the previous form) as:
- it hides crucial information (e.g. that it is a static method)
after the signature, where it is easily missed
@ -364,9 +365,8 @@ also apply to the previous form) as:
.. _summarized the arguments:
http://mail.python.org/pipermail/python-dev/2004-August/047112.html
The next form is that the decorator syntax go inside the method
body at the start, in the same place that docstrings currently
live:
The next form is that the decorator syntax go inside the method body at
the start, in the same place that docstrings currently live:
def foo(arg1,arg2):
@classmethod
@ -377,16 +377,16 @@ live:
@returns(float)
pass
The primary objection to this form is that it requires "peeking
inside" the method body to determine the decorators. In addition,
even though the code is inside the method body, it is not executed
when the method is run. Guido felt that docstrings were not a good
counter-example, and that it was quite possible that a 'docstring'
decorator could help move the docstring to outside the function body.
The primary objection to this form is that it requires "peeking inside"
the method body to determine the decorators. In addition, even though
the code is inside the method body, it is not executed when the method
is run. Guido felt that docstrings were not a good counter-example, and
that it was quite possible that a 'docstring' decorator could help move
the docstring to outside the function body.
The final form is a new block that encloses the method's code. For
this example, we'll use a 'decorate' keyword, as it makes no sense
with the @syntax. ::
The final form is a new block that encloses the method's code. For this
example, we'll use a 'decorate' keyword, as it makes no sense with the
@syntax. ::
decorate:
classmethod
@ -400,8 +400,8 @@ with the @syntax. ::
pass
This form would result in inconsistent indentation for decorated and
undecorated methods. In addition, a decorated method's body would
start three indent levels in.
undecorated methods. In addition, a decorated method's body would start
three indent levels in.
Syntax forms
@ -420,7 +420,7 @@ Syntax forms
The major objections against this syntax are that the @ symbol is
not currently used in Python (and is used in both IPython and Leo),
and that the @ symbol is not meaningful. Another objection is that
and that the @ symbol is not meaningful. Another objection is that
this "wastes" a currently unused character (from a limited set) on
something that is not perceived as a major use.
@ -437,8 +437,8 @@ Syntax forms
This is a variant on the @decorator syntax -- it has the advantage
that it does not break IPython and Leo. Its major disadvantage
compared to the @syntax is that the | symbol looks like both a
capital I and a lowercase l.
compared to the @syntax is that the | symbol looks like both a capital
I and a lowercase l.
* list syntax::
@ -474,28 +474,29 @@ Syntax forms
* ``decorate()``
The ``decorate()`` proposal was that no new syntax be implemented --
instead a magic function that used introspection to manipulate the
following function. Both Jp Calderone and Philip Eby produced
The ``decorate()`` proposal was that no new syntax be implemented
-- instead a magic function that used introspection to manipulate
the following function. Both Jp Calderone and Philip Eby produced
implementations of functions that did this. Guido was pretty firmly
against this -- with no new syntax, the magicness of a function like
this is extremely high:
Using functions with "action-at-a-distance" through
sys.settraceback may be okay for an obscure feature that can't be
had any other way yet doesn't merit changes to the language, but
that's not the situation for decorators. The widely held view
here is that decorators need to be added as a syntactic feature to
avoid the problems with the postfix notation used in 2.2 and 2.3.
Decorators are slated to be an important new language feature and
their design needs to be forward-looking, not constrained by what
can be implemented in 2.3.
Using functions with "action-at-a-distance" through sys.settraceback
may be okay for an obscure feature that can't be had any other
way yet doesn't merit changes to the language, but that's not
the situation for decorators. The widely held view here is that
decorators need to be added as a syntactic feature to avoid the
problems with the postfix notation used in 2.2 and 2.3. Decorators
are slated to be an important new language feature and their
design needs to be forward-looking, not constrained by what can be
implemented in 2.3.
* _`new keyword (and block)`
This idea was the consensus alternate from comp.lang.python. Robert
Brewer wrote up a detailed `J2 proposal`_ document outlining the
arguments in favor of this. The issues with this form are:
This idea was the consensus alternate from comp.lang.python (more
on this in `Community Consensus`_ below.) Robert Brewer wrote up a
detailed `J2 proposal`_ document outlining the arguments in favor of
this form. The initial issues with this form are:
- It requires a new keyword, and therefore a ``from __future__
import decorators`` statement.
@ -508,12 +509,39 @@ Syntax forms
code block, but isn't. Attempts to use statements in this block
will cause a syntax error, which may confuse users.
See `Community Consensus`_ below.
A few days later, Guido `rejected the proposal`_ on two main grounds,
firstly:
... the syntactic form of an indented block strongly
suggests that its contents should be a sequence of statements, but
in fact it is not -- only expressions are allowed, and there is an
implicit "collecting" of these expressions going on until they can
be applied to the subsequent function definition. ...
and secondly:
... the keyword starting the line that heads a block
draws a lot of attention to it. This is true for "if", "while",
"for", "try", "def" and "class". But the "using" keyword (or any
other keyword in its place) doesn't deserve that attention; the
emphasis should be on the decorator or decorators inside the suite,
since those are the important modifiers to the function definition
that follows. ...
Readers are invited to read `the full response`_.
.. _J2 proposal:
http://www.aminus.org/rbre/python/pydec.html
There are plenty of other variants and proposals on `the wiki page`_.
.. _rejected the proposal:
http://mail.python.org/pipermail/python-dev/2004-September/048518.html
.. _the full response:
http://mail.python.org/pipermail/python-dev/2004-September/048518.html
* Other forms
There are plenty of other variants and proposals on `the wiki page`_.
.. _the wiki page:
http://www.python.org/moin/PythonDecorators
@ -522,18 +550,18 @@ There are plenty of other variants and proposals on `the wiki page`_.
Why @?
------
There is some history in Java using @ initially as a marker in
`Javadoc comments`_ and later in Java 1.5 for `annotations`_, which
are similar to Python decorators. The fact that @ was previously
unused as a token in Python also means it's clear there is no
possibility of such code being parsed by an earlier version of Python,
leading to possibly subtle semantic bugs. It also means that ambiguity
of what is a decorator and what isn't is removed. of That said, @ is
still a fairly arbitrary choice. Some have suggested using | instead.
There is some history in Java using @ initially as a marker in `Javadoc
comments`_ and later in Java 1.5 for `annotations`_, which are similar
to Python decorators. The fact that @ was previously unused as a token
in Python also means it's clear there is no possibility of such code
being parsed by an earlier version of Python, leading to possibly subtle
semantic bugs. It also means that ambiguity of what is a decorator
and what isn't is removed. of That said, @ is still a fairly arbitrary
choice. Some have suggested using | instead.
For syntax options which use a list-like syntax (no matter where it
appears) to specify the decorators a few alternatives were proposed:
``[|...|]``, ``*[...]*``, and ``<...>``.
``[|...|]``, ``*[...]*``, and ``<...>``.
.. _Javadoc comments:
http://java.sun.com/j2se/javadoc/writingdoccomments/
@ -545,8 +573,8 @@ Current Implementation, History
===============================
Guido asked for a volunteer to implement his preferred syntax, and Mark
Russell stepped up and posted a `patch`_ to SF. The syntax accepted
for 2.4a2 is::
Russell stepped up and posted a `patch`_ to SF. This new syntax was
available in 2.4a2. ::
@dec2
@dec1
@ -561,6 +589,10 @@ This is equivalent to::
though without the intermediate creation of a variable named ``func``.
The version implemented in 2.4a2 allowed multiple ``@decorator`` clauses
on a single line. In 2.4a3, this was tightened up to only allowing one
decorator per line.
A `previous patch`_ from Michael Hudson which implements the
list-after-def syntax is also still kicking around.
@ -572,19 +604,44 @@ stated that he'd re-examine a community proposal, if the community
could come up with a community consensus, a decent proposal, and an
implementation. After an amazing number of posts, collecting a vast
number of alternatives in the `Python wiki`_, a community consensus
emerged (below). As at time of writing, we're waiting for Guido's
decision.
emerged (below). Guido `subsequently rejected`_ this alternate form,
but added:
In Python 2.4a3 (to be released this Thursday), everything remains
as currently in CVS. For 2.4b1, I will consider a change of @ to
some other single character, even though I think that @ has the
advantage of being the same character used by a similar feature
in Java. It's been argued that it's not quite the same, since @
in Java is used for attributes that don't change semantics. But
Python's dynamic nature makes that its syntactic elements never mean
quite the same thing as similar constructs in other languages, and
there is definitely significant overlap. Regarding the impact on
3rd party tools: IPython's author doesn't think there's going to be
much impact; Leo's author has said that Leo will survive (although
it will cause him and his users some transitional pain). I actually
expect that picking a character that's already used elsewhere in
Python's syntax might be harder for external tools to adapt to,
since parsing will have to be more subtle in that case. But I'm
frankly undecided, so there's some wiggle room here. I don't want
to consider further syntactic alternatives at this point: the buck
has to stop at some point, everyone has had their say, and the show
must go on.
.. _Python wiki:
http://www.python.org/moin/PythonDecorators
.. _subsequently rejected:
http://mail.python.org/pipermail/python-dev/2004-September/048518.html
Community Consensus
-------------------
The consensus that emerged was for the proposed J2 syntax: the new
keyword ``using`` prefixing a block of decorators before the ``def``
statement. For example::
[editor's note: should this section be removed now?]
The consensus that emerged on comp.lang.python was the proposed J2
syntax (the "J2" was how it was referenced on the PythonDecorators wiki
page): the new keyword ``using`` prefixing a block of decorators before
the ``def`` statement. For example::
using:
classmethod
@ -613,15 +670,21 @@ Sparks produced `a patch`_.
.. _a patch:
http://www.python.org/sf/1013835
As noted previously, Guido rejected this form, outlining his problems
with it in `a message`_ to python-dev and comp.lang.python.
.. _a message:
http://mail.python.org/pipermail/python-dev/2004-September/048518.html
Examples
========
Much of the discussion on ``comp.lang.python`` and the ``python-dev``
mailing list focuses on the use of decorators as a cleaner way to use
the ``staticmethod()`` and ``classmethod()`` builtins. This
capability is much more powerful than that. This section presents
some examples of use.
the ``staticmethod()`` and ``classmethod()`` builtins. This capability
is much more powerful than that. This section presents some examples of
use.
1. Define a function to be executed at exit. Note that the function
isn't actually "wrapped" in the usual sense. ::
@ -639,9 +702,8 @@ some examples of use.
is for example purposes only.
2. Define a class with a singleton instance. Note that once the class
disappears enterprising programmers would have to be more creative
to create more instances. (From Shane Hathaway on ``python-dev``.)
::
disappears enterprising programmers would have to be more creative to
create more instances. (From Shane Hathaway on ``python-dev``.) ::
def singleton(cls):
instances = {}
@ -734,14 +796,17 @@ Open Issues
===========
1. It's not yet certain that class decorators will be incorporated
into the language at this point. Guido expressed skepticism about
into the language at a future point. Guido expressed skepticism about
the concept, but various people have made some `strong arguments`_
(search for ``PEP 318 -- posting draft``) on their behalf in
``python-dev``.
``python-dev``. It's exceedingly unlikely that class decorators
will be in Python 2.4.
.. _strong arguments:
http://mail.python.org/pipermail/python-dev/2004-March/thread.html
2. The choice of the ``@`` character will be re-examined before
Python 2.4b1.
Copyright
=========