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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Getting lazy with decorators
Date: Tue, 26 Jun 2012 08:57:39 +0200
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Xref: csiph.com comp.lang.python:24461

Josh English wrote:

> On Sunday, June 24, 2012 1:07:45 AM UTC-7, Peter Otten wrote:
>> 
>> You cannot access a class instance because even the class itself doesn't
>> exist yet. You could get hold of the class namespace with
>> sys._getframe(),
>> 
>> def add_help(f):
>>     exec """\
>> def help_%s(self):
>>     f = getattr(self, %r)
>>     self.show_help(f)
>> """ % (f.__name__[3:], f.__name__) in sys._getframe(1).f_locals
>>     return f
>> 
>> but here's a simpler approach:
>> 
>> import cmd
>> 
>> def add_help(f):
>>     def help(self):
>>         self.show_help(f)
>>     f.help = help
>>     return f
>> 
>> 
>> class BaseCmd(cmd.Cmd):
>>     def __init__(self, *args, **kwargs):
>>         cmd.Cmd.__init__(self, *args, **kwargs)
>> 
>>     def show_help(self, func):
>>         print "\n".join((line.strip() for line in
>> func.__doc__.splitlines()))
>> 
>>     def __getattr__(self, name):
>>         if name.startswith("help_"):
>>             helpfunc = getattr(self, "do_" + name[5:]).help
>>             setattr(self.__class__, name, helpfunc)
>>             return getattr(self, name)
>>         raise AttributeError
>> 
>>     @add_help
>>     def do_done(self, line):
>>         """done
>>         Quits this and goes to higher level or quits the application.
>>         I mean, what else do you expect?
>>         """
>>         return True
>> 
>> if __name__=='__main__':
>>     c = BaseCmd()
>>     c.cmdloop()
> 
> 
> Okay. If I understand this, you are adding a help attribute to the class
> method. The help attribute is itself a function.
> 
> There is nothing in the documentation (that I have found) that points to
> this solution. 

That's because I "invented" it.

@deco 
def func(...): ...

is equivalent to

def func(...): ...
func = deco(func)

so only one assignment is ever made in the enclosing namespace. If you have 
more targets you have to put them somewhere else. Making the help_xxx() 
function an attribute of do_xxx() seemed the obvious choice.

> Even after reading the do_help method in the cmd.Cmd
> source, I don't see this as working.

do_help(name) 

looks up a "help_" + name method and invokes it if that lookup succeeds.

> Yet it works.

In the example above do_help() looks for a help_done attribute which doesn't 
exist. As a fallback  the __getattr__() method is invoked with the 
"help_done" argument, finds that the name starts with "help_" and proceeds 
with

>>             helpfunc = getattr(self, "do_" + name[5:]).help

i. e. it looks for getattr(self, "do_help") which does exist and then stores 
its help attribute (if that doesn't exist an AttributeError is implicitly 
raised) in helpfunc. helpfunc is then added as "help_done" to the class

>>             setattr(self.__class__, name, helpfunc)

and looked up again in the instance:

>>             return getattr(self, name)

This time it exists and from the class attribute help_done an instance 
method is created, returned from __getattr__() and invoked by do_help().

PS: Stefan is probably right that you should just override do_help()