updates to PEP 318 from Kevin Smith, including PEP title

pep-0000.txt

@@ -117,7 +117,7 @@ Index by Category
  S   314  Metadata for Python Software Packages v1.1   Kuchling
  S   315  Enhanced While Loop                          Carroll
  S   317  Eliminate Implicit Exception Instantiation   Taschuk
- S   318  Function Modifier Syntax                     Smith
+ S   318  Function/Method Decorator Syntax             Smith
  S   754  IEEE 754 Floating Point Special Values       Warnes
 
  Finished PEPs (done, implemented in CVS)
@@ -328,7 +328,7 @@ Numerical Index
  S   315  Enhanced While Loop                          Carroll
  SD  316  Programming by Contract for Python           Way
  S   317  Eliminate Implicit Exception Instantiation   Taschuk
- S   318  Function Modifier Syntax                     Smith
+ S   318  Function/Method Decorator Syntax             Smith
  SR  666  Reject Foolish Indentation                   Creighton
  S   754  IEEE 754 Floating Point Special Values       Warnes
 

pep-0318.txt

@@ -1,5 +1,5 @@
 PEP: 318
-Title: Function Modifier Syntax
+Title: Function/Method Decorator Syntax
 Version: $Revision$
 Last-Modified: $Date$
 Author: Kevin D. Smith <Kevin.Smith@theMorgue.org>
@@ -8,7 +8,7 @@ Type: Standards Track
 Content-Type: text/plain
 Created: 05-Jun-2003
 Python-Version: 2.4
-Post-History:
+Post-History: 09-Jun-2003
 
 
 Abstract
@@ -36,11 +36,16 @@ Motivation
     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 function that translates
+    Probably the simplest way to place the decorator that translates
     an instance method to a class/static method is illustrated in the
     code below.
 
@@ -58,7 +63,7 @@ Proposal
     '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 
-    modifiers in the form of a space delimited list as follows:
+    decorators in the form of a space delimited list as follows:
 
        def protected classmethod foo(self):
            perform method operation 
@@ -71,46 +76,103 @@ Proposal
 
     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 modifier.
+    sent to the function decorator.
 
-       def synchronized(lock) classmethod foo(self):
-           perform method operation 
+        def synchronized(lock) classmethod foo(self):
+            perform method operation 
 
-    Various syntaxes have been proposed in comp.lang.python.  The
+    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.
+
+    Other syntaxes have been proposed in comp.lang.python.  The 
     most common are demonstrated below.
 
         def foo(self) [synchronized(lock), classmethod]:
             perform method operation
 
-        def foo(self) {'pre': synchronized(lock), 'classmethod': True}:
-            """ Skip Montanaro syntax """
+        def foo(self) (synchronized(lock), classmethod):
             perform method operation
 
+        def foo(self) {'pre': synchronized(lock), 'classmethod': True}:
+            perform method operation
+
+    These three forms use syntax that just seems arbitrary and which 
+    does not help the user to comprehend the meaning of it.  In 
+    addition, since the order in which the decorators are applied 
+    may matter, the third, 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:
-            """ Gerrit Holl syntax """
             perform method operation
 
-    I have a strong preference for the last of the three.  The first
-    two use syntax that just seems arbitrary which does not help the
-    user to understand the meaning of it.  The third method is very
-    readable and could probably be interpreted easily by those not
-    familiar with Python.
+    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 compiled.  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 at compile time.
+
+
+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
-    modifiers should be implemented (proposed syntax shown below).  
+    decorators should be implemented (proposed syntax shown below).  
 
         def foo(self) as synchronized(lock), classmethod:
             perform method operation
 
-    More generally, 
-
-        def foo(self) as <tuple>:
-            perform method operation
-
     The proposed syntax is simple, powerful, easy to read, and 
     therefore preserves those qualities of the Python language.