[Python-Dev] [UPDATED] PEP 318 - Function/Method Decorator Syntax

[Kevin Smith]

I have updated PEP 318 to include the various other syntaxes suggested 
in the past few days at python-dev.  I have also included an Open 
Issues section which introduces a question about exactly what a 
decorator is required to do in order to function properly.

PEP: 318
Title: Function/Method Decorator Syntax

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.


Kevin Smith
Kevin.Smith at themorgue.org