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Date: Thu, 31 Jan 2013 11:00:58 -0500
Subject: Re: confusion with decorators
From: Jason Swails <jason.swails@gmail.com>
To: Chris Angelico <rosuav@gmail.com>
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On Thu, Jan 31, 2013 at 10:28 AM, Chris Angelico <rosuav@gmail.com> wrote:

>
> >> Well, that surely isn't going to work, because it always decorates the
> >> same function, the global "fcn".
> >
> >
> > I don't think this is right.  fcn is a passed function (at least if it
> acts
> > as a decorator) that is declared locally in the _protector_decorator
> scope.
> > Since newfcn is bound in the same scope and fcn is not defined inside
> > newfcn, I'm pretty sure that newfcn will just grab the fcn passed into
> the
> > decorator.
>
> Yet it adds a level of indirection that achieves nothing. Why not simply:
> def _protector_decorator(fcn):
>   return fcn
>
> ? I'm not understanding the purpose here.
>

Bad example.  A better (longer) one that is closer to my true use-case:


from somewhere.exceptions import MyTypeError
from somewhere.different import AuthorClass, RemoteAuthorClass
from urllib2 import HTTPError

class A(object):

   authorclass = AuthorClass

   def __init__(self, obj_list):
      """
      Instantiate a list of obj_list objects that may have an "author"
      attribute
      """
      self.things = []
      for o in obj_list:
         if not isinstance(o, self.authorclass):
            raise MyTypeError('Bad type given to constructor')
         self.things.append(o)

   def _protector(fcn):
      def newfcn(self, *args, **kwargs):
         try:
            return fcn(self, *args, **kwargs) # returns a string
         except AttributeError:
            return 'Attribute not available.'
         except IndexError:
            return 'Not that many AuthorClasses loaded'

      return newfcn

   @_protector
   def author(self, idx):
      return self.things[idx].author

   @_protector
   def description(self, idx):
      return self.things[idx].description

   @_protector
   def hobbies(self, idx):
      return self.things[idx].hobbies

class B(A):

   authorclass = RemoteAuthorClass

   def _protector(fcn):
      def newfcn(self, *args, **kwargs):
         try:
            return fcn(self, *args, **kwargs)
         except AttributeError:
            return 'Attribute not available'
         except IndexError:
            return 'Not that many RemoteAuthorClasses loaded'
         except HTTPError:
            return 'Could not connect'
      return fcn

Basically, while RemoteAuthorClass and AuthorClass are related (via
inheritance), the RemoteAuthorClass has the potential for HTTPError's now.
 I could just expand the A class decorator to catch the HTTPError, but
since that should not be possible in AuthorClass, I'd rather not risk
masking a bug.  I'm under no impressions that the above code will decorate
A-inherited functions with the B-decorator (I know it won't), but that's
the effect I'm trying to achieve...

Thanks!
Jason

-- 
Jason M. Swails
Quantum Theory Project,
University of Florida
Ph.D. Candidate
352-392-4032

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<font color=3D"#000000"><br></font><br><div class=3D"gmail_quote">On Thu, J=
an 31, 2013 at 10:28 AM, Chris Angelico <span dir=3D"ltr">&lt;<a href=3D"ma=
ilto:rosuav@gmail.com" target=3D"_blank">rosuav@gmail.com</a>&gt;</span> wr=
ote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div class=3D"im"><br>
&gt;&gt; Well, that surely isn&#39;t going to work, because it always decor=
ates the<br>
&gt;&gt; same function, the global &quot;fcn&quot;.<br>
&gt;<br>
&gt;<br>
&gt; I don&#39;t think this is right. =A0fcn is a passed function (at least=
 if it acts<br>
&gt; as a decorator) that is declared locally in the _protector_decorator s=
cope.<br>
&gt; Since newfcn is bound in the same scope and fcn is not defined inside<=
br>
&gt; newfcn, I&#39;m pretty sure that newfcn will just grab the fcn passed =
into the<br>
&gt; decorator.<br>
<br>
</div>Yet it adds a level of indirection that achieves nothing. Why not sim=
ply:<br>
def _protector_decorator(fcn):<br>
=A0 return fcn<br>
<br>
? I&#39;m not understanding the purpose here.<br></blockquote><div><br></di=
v><div>Bad example. =A0A better (longer) one that is closer to my true use-=
case:</div><div><br></div><div><div><br></div><div>from somewhere.exception=
s import MyTypeError</div>
<div>from somewhere.different import AuthorClass, RemoteAuthorClass</div><d=
iv>from urllib2 import HTTPError</div><div><br></div><div>class A(object):<=
/div><div>=A0 =A0</div><div>=A0 =A0authorclass =3D AuthorClass</div><div><b=
r></div>
<div>=A0 =A0def __init__(self, obj_list):</div><div>=A0 =A0 =A0 &quot;&quot=
;&quot;</div><div>=A0 =A0 =A0 Instantiate a list of obj_list objects that m=
ay have an &quot;author&quot;</div><div>=A0 =A0 =A0 attribute</div><div>=A0=
 =A0 =A0 &quot;&quot;&quot;</div>
<div>=A0 =A0 =A0 self.things =3D []</div><div>=A0 =A0 =A0 for o in obj_list=
:</div><div>=A0 =A0 =A0 =A0 =A0if not isinstance(o, self.authorclass):</div=
><div>=A0 =A0 =A0 =A0 =A0 =A0 raise MyTypeError(&#39;Bad type given to cons=
tructor&#39;)</div><div>=A0 =A0 =A0 =A0 =A0self.things.append(o)</div>
<div><br></div><div>=A0 =A0def _protector(fcn):</div><div>=A0 =A0 =A0 def n=
ewfcn(self, *args, **kwargs):</div><div>=A0 =A0 =A0 =A0 =A0try:</div><div>=
=A0 =A0 =A0 =A0 =A0 =A0 return fcn(self, *args, **kwargs) # returns a strin=
g</div><div>=A0 =A0 =A0 =A0 =A0except AttributeError:</div>
<div>=A0 =A0 =A0 =A0 =A0 =A0 return &#39;Attribute not available.&#39;</div=
><div>=A0 =A0 =A0 =A0 =A0except IndexError:</div><div>=A0 =A0 =A0 =A0 =A0 =
=A0 return &#39;Not that many AuthorClasses loaded&#39;</div><div><br></div=
><div>=A0 =A0 =A0 return newfcn</div>
<div><br></div><div>=A0 =A0@_protector</div><div>=A0 =A0def author(self, id=
x):</div><div>=A0 =A0 =A0 return self.things[idx].author</div><div><br></di=
v><div>=A0 =A0@_protector</div><div>=A0 =A0def description(self, idx):</div=
><div>=A0 =A0 =A0 return self.things[idx].description</div>
<div><br></div><div>=A0 =A0@_protector</div><div>=A0 =A0def hobbies(self, i=
dx):</div><div>=A0 =A0 =A0 return self.things[idx].hobbies</div><div><br></=
div><div>class B(A):</div><div>=A0 =A0</div><div>=A0 =A0authorclass =3D Rem=
oteAuthorClass</div>
<div>=A0 =A0</div><div>=A0 =A0def _protector(fcn):</div><div>=A0 =A0 =A0 de=
f newfcn(self, *args, **kwargs):</div><div>=A0 =A0 =A0 =A0 =A0try:</div><di=
v>=A0 =A0 =A0 =A0 =A0 =A0 return fcn(self, *args, **kwargs)</div><div>=A0 =
=A0 =A0 =A0 =A0except AttributeError:</div>
<div>=A0 =A0 =A0 =A0 =A0 =A0 return &#39;Attribute not available&#39;</div>=
<div>=A0 =A0 =A0 =A0 =A0except IndexError:</div><div>=A0 =A0 =A0 =A0 =A0 =
=A0 return &#39;Not that many RemoteAuthorClasses loaded&#39;</div><div>=A0=
 =A0 =A0 =A0 =A0except HTTPError:</div><div>
=A0 =A0 =A0 =A0 =A0 =A0 return &#39;Could not connect&#39;</div><div>=A0 =
=A0 =A0 return fcn</div><div><br></div><div>Basically, while RemoteAuthorCl=
ass and AuthorClass are related (via inheritance), the RemoteAuthorClass ha=
s the potential for HTTPError&#39;s now. =A0I could just expand the A class=
 decorator to catch the HTTPError, but since that should not be possible in=
 AuthorClass, I&#39;d rather not risk masking a bug. =A0I&#39;m under no im=
pressions that the above code will decorate A-inherited functions with the =
B-decorator (I know it won&#39;t), but that&#39;s the effect I&#39;m trying=
 to achieve...</div>
<div><br></div><div>Thanks!</div><div>Jason</div><div><br></div></div></div=
>-- <br>Jason M. Swails<br>Quantum Theory Project,<br>University of Florida=
<br>Ph.D. Candidate<br>352-392-4032

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