python-peps/pep-0318.txt

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
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, 
    or a tuple.  Using the latter form, the last example above 
    would look like:

        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

    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 
    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
    name when the 'def' statement is executed.

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.

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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