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[Python-ideas] PEP 30xx: Access to Module/Class/Function CurrentlyBeing Defined (this)
Jim Jewettjimjjewett at gmail.com
Mon Apr 23 05:05:14 CEST 2007
(Please note that several groups were Cc'd. For now, please limitfollowups to python-3000. This would *probably* be backported to 2.6,but that wouldn't be decided until the implementation strategy wassettled.)PEP: 30XXTitle: Access to Module/Class/Function Currently Being Defined (this)Version: $Revision$Last-Modified: $Date$Author: Jim J. Jewett <jimjjewett at gmail.com>Status: DraftType: Standards TrackContent-Type: text/plainCreated: 22-Apr-2007Python-Version: 3.0Post-History: 22-Apr-2007Abstract It is common to need a reference to the current module, class, or function, but there is currently no entirely correct way to do this. This PEP proposes adding the keywords __module__, __class__, and __function__.Rationale Many modules export various functions, classes, and other objects, but will perform additional activities (such as running unit tests) when run as a script. The current idiom is to test whether the module's name has been set to magic value. if __name__ == "__main__": ... More complicated introspection requires a module to (attempt to) import itself. If importing the expected name actually produces a different module, there is no good workaround. Proposal: Add a __module__ keyword which refers to the module currently being defined (executed). (But see open issues.) if __module__ is sys.main: ... # assuming PEP 3020, Cannon Class methods are passed the current instance; from this they can determine self.__class__ (or cls, for classmethods). Unfortunately, this reference is to the object's actual class, which may be a subclass of the defining class. The current workaround is to repeat the name of the class, and assume that the name will not be rebound. class C(B): def meth(self): super(C, self).meth() # Hope C is never rebound. class D(C): def meth(self): super(C, self).meth() # ?!? issubclass(D,C), so it "works" Proposal: Add a __class__ keyword which refers to the class currently being defined (executed). (But see open issues.) class C(B): def meth(self): super(__class__, self).meth() Note that super calls may be further simplified by PEP 30XX, Jewett. The __class__ (or __this_class__) attribute came up in attempts to simplify the explanation and/or implementation of that PEP, but was separated out as an independent decision. Note that __class__ (or __this_class__) is not quite the same as the __thisclass__ property on bound super objects. The existing super.__thisclass__ property refers to the class from which the Method Resolution Order search begins. In the above class D, it would refer to (the current reference of name) C. Functions (including methods) often want access to themselves, usually for a private storage location. While there are several workarounds, all have their drawbacks. def counter(_total=[0]): # _total shouldn't really appear in the _total[0] += 1 # signature at all; the list wrapping and return _total[0] # [0] unwrapping obscure the code @annotate(total=0) def counter(): counter.total += 1 # Assume name counter is never rebound return counter.total class _wrap(object): # class exists only to provide storage __total=0 def f(self): self.__total += 1 return self.__total accum=_wrap().f # set module attribute to a bound method Proposal: Add a __function__ keyword which refers to the function (or method) currently being defined (executed). (But see open issues.) @annotate(total=0) def counter(): __function__.total += 1 # Always refers to this function obj return __function__.totalBackwards Compatibility While a user could be using these names already, __anything__ names are explicitly reserved to the interpreter. It is therefore acceptable to introduce special meaning to these names within a single feature release.Implementation Ideally, these names would be keywords treated specially by the bytecode compiler. Guido has suggested [1] using a cell variable filled in by the metaclass. Michele Simionato has provided a prototype using bytecode hacks [2].Open Issues - Are __module__, __class__, and __function__ the right names? In particular, should the names include the word "this", either as __this_module__, __this_class__, and __this_function__, (format discussed on the python-3000 and python-ideas lists) or as __thismodule__, __thisclass__, and __thisfunction__ (inspired by, but conflicting with, current usage of super.__thisclass__). - Are all three keywords needed, or should this enhancement be limited to a subset of the objects? Should methods be treated separately from other functions?References [1] Fixing super anyone? Guido van Rossumhttp://mail.python.org/pipermail/python-3000/2007-April/006671.html [2] Descriptor/Decorator challenge, Michele Simionatohttp://groups.google.com/group/comp.lang.python/browse_frm/thread/a6010c7494871bb1/62a2da68961caeb6?lnk=gst&q=simionato+challenge&rnum=1&hl=en#62a2da68961caeb6Copyright This document has been placed in the public domain. Local Variables:mode: indented-textindent-tabs-mode: nilsentence-end-double-space: tfill-column: 70coding: utf-8End:
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