PEP: 318 Title: Decorators for Functions, Methods and Classes Version: $Revision$ Last-Modified: $Date$ Author: Kevin D. Smith , Jim Jewett , Skip Montanaro Status: Draft 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 Abstract ======== The current method for declaring class and static methods 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 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:: 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:: def foo(cls): pass foo = synchronized(lock)(foo) foo = classmethod(foo) with an alternative that places the decoration in the function's declaration:: def foo(cls) using [synchronized(lock), classmethod]: 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. 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, attributing the bracketed syntax to an earlier proposal on ``comp.lang.python`` by `Gareth McCaughan`_. .. _syntactic support for decorators: http://www.python.org/doc/essays/ppt/python10/py10keynote.pdf .. _10th python conference: http://www.python.org/workshops/2002-02/ .. _michael hudson raised the topic: http://mail.python.org/pipermail/python-dev/2002-February/020005.html .. _he later said: http://mail.python.org/pipermail/python-dev/2002-February/020017.html .. _gareth mccaughan: 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. Design Goals ============ The new syntax should * work for arbitrary wrappers, including user-defined callables and the existing builtins ``classmethod()`` and ``staticmethod`` * work with multiple wrappers per definition * make it obvious what is happening; at the very least it should be obvious that new users can safely ignore it when writing their own code * not make future extensions more difficult * be easy to type; programs that use it are expected to use it very 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 * not needlessly complicate secondary support tools such as language-sensitive editors and other "`toy parser tools out there`_" .. _toy parser tools out there: http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=mailman.1010809396.32158.python-list%40python.org Proposed Syntax =============== The currently proposed syntax for function decorators is:: def func(arg1, arg2, ...) [dec1, dec2, ...]: pass The decorators are near the declaration of the function's API but are clearly secondary. The square brackets make it possible to fairly easily break long lists of decorators across multiple lines. Class decorators are defined in an analogous fashion:: class MyClass(base1, base2) [dec1, dec2, ...]: pass Alternate Proposals =================== Several other syntaxes have been proposed:: def func(arg1, arg2, ...) as dec1, dec2, ...: pass The absence of brackets makes it cumbersome to break long lists of decorators across multiple lines, and the keyword "as" doesn't have the same meaning as its use in the ``import`` statement. Plenty of `alternatives to "as"`_ have been proposed. :-) .. _alternatives to "as": http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=mailman.236.1079968472.742.python-list%40python.org&rnum=2&prev=/groups%3Fq%3Dpython%2Bpep%2B318%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3Dmailman.236.1079968472.742.python-list%2540python.org%26rnum%3D2 :: def [dec1, dec2, ...] func(arg1, arg2, ...): pass This form has the disadvantage that the decorators visually assume higher priority than the function name and argument list. :: def func [dec1, dec2, ...] (arg1, arg2, ...): pass Quixote's `Python Template Language`_ uses this form, but only supports a single decorator chosen from a restricted set. For short lists it works okay, but for long list it separates the argument list from the function name. .. _Python Template Language: http://www.mems-exchange.org/software/quixote/doc/PTL.html :: using: dec1 dec2 ... def foo(arg1, arg2, ...): pass The function definition is not nested within the using: block making it impossible to tell which objects following the block will be decorated. Nesting the function definition within the using: block suggests nesting of namespaces that doesn't exist. The name ``foo`` would actually exist at the same scope as the using: block. Finally, it would require the introduction of a new keyword. Guido proposed and implementated a patch to support interpretation of a `list of decorators`_ as a prefix to function definitions :: [dec1, dec2, ...] def foo(arg1, arg2, ...): pass This appears to be his current favorite, but negative sentiment runs high, mostly because that syntax, though useless except for side effects of the list, is already legal and thus creates a special case. .. _list of decorators: http://python.org/sf/926860 Why [...]? ---------- For syntax options which use a list-like syntax to specify the decorators a few alternatives have been proposed: ``[|...|]``, ``*[...]*``, and ``<...>``. None have gained traction. The alternatives which involve square brackets only serve to make it obvious that the decorator construct is not a list. They do nothing to make parsing any easier. The '<...>' alternative presents parsing problems because '<' and '>' already parse as un-paired. They present a further parsing ambiguity because a right angle bracket might be a greater than symbol instead of a closer for the decorators. Current Implementation ====================== Michael Hudson posted a `patch`_ at Starship, which implements the proposed syntax changes for both functions and classes and left-first application of decorators:: def func(arg1, arg2, ...) [dec1, dec2]: pass is equivalent to:: def func(arg1, arg2, ...): pass func = dec2(dec1(func)) though without the intermediate creation of a variable named ``func``. .. _patch: http://starship.python.net/crew/mwh/hacks/meth-syntax-sugar-3.diff 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. 1. Define a function to be executed at exit. Note that the function isn't actually "wrapped" in the usual sense. :: def onexit(f): import atexit atexit.register(f) return f def func() [onexit]: ... 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``.) :: def singleton(cls): instances = {} def getinstance(): if cls not in instances: instances[cls] = cls() return instances[cls] return getinstance class MyClass [singleton]: ... 3. Add attributes to a function. (Based on an example posted by Anders Munch on ``python-dev``.) :: def attrs(**kwds): def decorate(f): for k in kwds: setattr(f, k, kwds[k]) return f return decorate def mymethod(f) [attrs(versionadded="2.2", author="Guido van Rossum")]: ... 4. Enforce function argument and return types. (Note that this is not exactly correct, as the returned new_f doesn't have "func" as its func_name attribute.) :: def accepts(*types): def check_accepts(f): assert len(types) == f.func_code.co_argcount def new_f(*args, **kwds): for (a, t) in zip(args, types): assert isinstance(a, t), \ "arg %r does not match %s" % (a,t) return f(*args, **kwds) return new_f return check_accepts def returns(rtype): def check_returns(f): def new_f(*args, **kwds): result = f(*args, **kwds) assert isinstance(result, rtype), \ "return value %r does not match %s" % (result,rtype) return result return new_f return check_returns def func(arg1, arg2) [accepts(int, (int,float)), returns((int,float))]: return arg1 * arg2 5. Declare that a class implements a particular (set of) interface(s). This is from a posting by Bob Ippolito on ``python-dev`` based on experience with `PyProtocols`_. .. _PyProtocols: http://peak.telecommunity.com/PyProtocols.html :: def provides(*interfaces): """ An actual, working, implementation of provides for the current implementation of PyProtocols. Not particularly important for the PEP text. """ def provides(typ): declareImplementation(typ, instancesProvide=interfaces) return typ return provides class IBar(Interface): """Declare something about IBar here""" class Foo(object) [provides(IBar)]: """Implement something here...""" Of course, all these examples are possible today, though without the syntactic support. Open Issues =========== 1. It's not yet certain that class decorators will be incorporated into the language at this 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``. 2. Decorators which wrap a function and return a different function should be able to easily change the func_name attribute without constructing it with new.function(). Perhaps the func_name attribute should be writable. .. _strong arguments: http://mail.python.org/pipermail/python-dev/2004-March/thread.html Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 End: