python-peps/pep-0318.txt

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PEP: 318
Title: Function/Method Decorator Syntax
Version: $Revision$
Last-Modified: $Date$
Author: Kevin D. Smith <Kevin.Smith@theMorgue.org>
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 05-Jun-2003
Python-Version: 2.4
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Post-History: 09-Jun-2003, 10-Jun-2003, 27-Feb-2004
Abstract
The current method for declaring class and static methods
is awkward and can lead to code that is difficult to understand.
This PEP introduces possible new syntax which will place the
translation of instance methods to class/static methods at
the same point in the code as the method's declaration.
Motivation
The current method of translating an instance method into a
class/static method places the actual translation at a different
point in the code than the declaration of the method. The
code below demonstrates this.
def foo(self):
perform method operation
foo = classmethod(foo)
When the method is very short, it is easy to look ahead and see
that this is a class method. However, if the method is more than
15 lines or so, the translation into a class method is not
obvious. A solution to this problem is to move the translation
of the method to the same point as the method's declaration.
The proposed syntax, shown in the example below, is discussed
in the following sections.
def foo(self) as synchronized(lock), classmethod:
perform method operation
Proposal
Probably the simplest way to place the decorator that translates
an instance method to a class/static method is illustrated in the
code below.
def classmethod foo(self):
perform method operation
The code in this example will simply perform the following.
def foo(self):
perform method operation
foo = classmethod(foo)
This syntax does not introduce any new keywords and is completely
backwards compatible with any existing code. The word between the
'def' and the actual name of the method is simply a reference to
a callable object that returns a new function reference.
This syntax could also be extended to allow multiple function
decorators in the form of a space delimited list as follows:
def protected classmethod foo(self):
perform method operation
which would be equivalent to the current form:
def foo(self):
perform method operation
foo = protected(classmethod(foo))
While this syntax is simple and easy to read, it does become
cluttered and more obscure if you wish to allow arguments to be
sent to the function decorator.
def synchronized(lock) classmethod foo(self):
perform method operation
Instead of placing the decorators in front of the function name,
a better place might be after it, as shown below. The word 'as' is
added simply as a separator to assist in readability.
def foo(self) as synchronized(lock), classmethod:
perform method operation
This syntax is quite clear and could probably be interpreted
by those not familiar with Python. The proposed syntax can be
generalized as follows:
'def' NAME '(' PARAMETERS ')' ['as' DECORATORS] ':'
where DECORATORS is a comma-separated list of expressions,
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or a tuple. Using the latter form, the last example above
would look like:
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def foo(self) as (synchronized(lock), classmethod):
perform method operation
This form make is possible for the list of decorators to
span multiple lines without using the line continuation operator.
Alternate Syntaxes
Other syntaxes have been proposed in comp.lang.python and
python-dev. Unfortunately, no one syntax has come out as a clear
winner in the lengthy discussions. The most common suggestions
are demonstrated below. The proposed syntax is also included
for easy comparison.
Proposed Syntax
def foo(self) as synchronized(lock), classmethod:
perform method operation
def foo(self) as (synchronized(lock), classmethod):
perform method operation
Prefix Forms
def [synchronized(lock), classmethod] foo(self):
perform method operation
def synchronized(lock), classmethod foo(self):
perform method operation
# Same as above, but only identifiers are allowed
sync = synchronized(lock)
def sync, classmethod foo(self):
perform method operation
# Java-like
sync = synchronized(lock)
def @sync @classmethod foo(self):
perform method operation
Postfix Forms
def foo(self) [synchronized(lock), classmethod]:
perform method operation
def foo(self) (synchronized(lock), classmethod):
perform method operation
def foo(self) {'pre': synchronized(lock), 'classmethod': True}:
perform method operation
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I'm not as fond of the forms that use '[ ]' since code like
'foo()[a]' looks as if you are getting the item 'a' from 'foo()'.
Although, this isn't as much of an issue when using '[ ]' in
a prefix form. The Java-like syntax adds new syntax that is
very arbitrary and is almost Perl-ish. In addition, since the
order in which the decorators are applied may matter, the last,
dictionary-style, syntax must be eliminated.
Implementation Issues
In the following example there are two function decorators:
synchronized(lock) and classmethod.
def foo(self) as synchronized(lock), classmethod:
perform method operation
Since these all appear within the operation of the 'def'
itself, it makes sense that synchronized, lock, and
classmethod must exist at the time that the definition
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is executed. In addition, each of these arguments will be
evaluated before being applied to the compiled function.
This means that arguments like synchronized(lock) must
return a descriptor that will be applied to foo. Therefore,
the code above translates to:
def foo(self):
perform method operation
foo = classmethod(<returned-descriptor>(foo))
In the example above, <returned-descriptor> refers to the
descriptor returned by evaluating synchronized(lock).
It could easily be argued that the descriptors should be applied
in reverse order to make the application of the descriptor look
more like the resultant code. I tend to prefer this form.
def foo(self):
perform method operation
foo = <returned-descriptor>(classmethod(foo))
In either case, the modified function is bound to the function
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name when the 'def' statement is executed.
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Open Issues
It is not clear at the moment if it is even possible to have
multiple decorators for a function. If decorators are required
to take a function/method and return a descriptor, it might
not even be possible to wrap multiple decorators. This should
be explored since the best syntax for multiple decorators
may not be the same as the best syntax for a single decorator.
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Current Implementations
I am not personally familiar enough with Python's source to
implement the proposed syntax; however, Michael Hudson has
implemented the "square-bracketed" syntax (see patch at
http://starship.python.net/crew/mwh/hacks/meth-syntax-sugar.diff).
It should be fairly simple for the Python development team
to translate this patch to the proposed syntax.
Possible Extensions
The proposed syntax is general enough that it could be used
on class definitions as well as shown below.
class foo(object) as classmodifier:
class definition here
However, there are no obvious parallels for use with other
descriptors such as property().
Conclusion
The current method of translating an instance method to a class
or static method is awkward. A new syntax for applying function
decorators should be implemented (proposed syntax shown below).
def foo(self) as synchronized(lock), classmethod:
perform method operation
The proposed syntax is simple, powerful, easy to read, and
therefore preserves those qualities of the Python language.
Copyright
This document has been placed in the public domain.
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