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confusion with decorators

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  confusion with decorators Jason Swails <jason.swails@gmail.com> - 2013-01-30 19:34 -0500
    Re: confusion with decorators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-01-31 05:46 +0000
      Re: confusion with decorators Jason Swails <jason.swails@gmail.com> - 2013-01-31 08:25 -0500
        Re: confusion with decorators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-01 10:16 +1100
          Re: confusion with decorators Jason Swails <jason.swails@gmail.com> - 2013-01-31 22:13 -0500
      Re: confusion with decorators Chris Angelico <rosuav@gmail.com> - 2013-02-01 02:28 +1100
      Re: confusion with decorators Jason Swails <jason.swails@gmail.com> - 2013-01-31 11:00 -0500
      Re: confusion with decorators Jason Swails <jason.swails@gmail.com> - 2013-01-31 12:53 -0500
      Re: confusion with decorators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-01 08:31 +1100
    Re: confusion with decorators 88888 Dihedral <dihedral88888@googlemail.com> - 2013-02-01 03:26 -0800
    Re: confusion with decorators 88888 Dihedral <dihedral88888@googlemail.com> - 2013-02-01 03:26 -0800

#37974 — confusion with decorators

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Hello,

I was having some trouble understanding decorators and inheritance and all
that.  This is what I was trying to do:

# untested
class A(object):
   def _protector_decorator(fcn):
      def newfcn(self, *args, **kwargs):
         return fcn(self, *args, **kwargs)
      return newfcn

   @_protector_decorator
   def my_method(self, *args, **kwargs):
      """ do something here """

class B(A):
   def _protector_decorator(fcn):
      def newfcn(self, *args, **kwargs):
         raise MyException('I do not want B to be able to access the
protected functions')
      return newfcn

The goal of all that was to be able to change the behavior of my_method
inside class B simply by redefining the decorator. Basically, what I want
is B.my_method() to be decorated by B._protector_decorator, but in the code
I'm running it's decorated by A._protector_decorator.

I presume this is because once the decorator is applied to my_method in
class A, A.my_method is immediately bound to the new, 'decorated' function,
which is subsequently inherited (and not decorated, obviously), by B.

Am I correct here?  My workaround was to simply copy the method from class
A to class B, after which B._protector_decorator decorated the methods in
B.  While this doesn't make the use of decorators completely pointless (the
decorators actually do something in each class, it's just different), it
does add a bunch of code duplication which I was at one point hopeful to
avoid.

I'm still stumbling around with decorators a little, but this exercise has
made them a lot clearer to me.

Thanks!
Jason

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#37983

On Wed, 30 Jan 2013 19:34:03 -0500, Jason Swails wrote:

> Hello,
> 
> I was having some trouble understanding decorators and inheritance and 
> all that.  This is what I was trying to do:
> 
> # untested
> class A(object):
>    def _protector_decorator(fcn):
>       def newfcn(self, *args, **kwargs):
>          return fcn(self, *args, **kwargs)
>       return newfcn

Well, that surely isn't going to work, because it always decorates the 
same function, the global "fcn". 

You probably want to add an extra parameter to the newfcn definition:

def newfcn(self, fcn, *args, **kwargs):


Also, I trust you realise that this is a pointless decorator that doesn't 
do anything useful? It just adds an extra layer of indirection, without 
adding any functionality.


>   @_protector_decorator
>   def my_method(self, *args, **kwargs):
>      """ do something here """
>
> class B(A):
>   def _protector_decorator(fcn):
>       def newfcn(self, *args, **kwargs):
>          raise MyException('I do not want B to be able to access the
>                             protected functions')
>       return newfcn


That's not going to work, because B's _protector_decorator never gets 
called. True, it overrides A's _protector_decorator, but too late. A has 
already used it to decorate the methods, and B does not override those 
methods, so A's version are inherited.

But even if it could work, it relies on class B protecting class A from 
B. All B needs do to overcome the protection is ... *not* define the 
magic decorator.


> The goal of all that was to be able to change the behavior of my_method
> inside class B simply by redefining the decorator. Basically, what I
> want is B.my_method() to be decorated by B._protector_decorator, but in
> the code I'm running it's decorated by A._protector_decorator.

Yes. Remember that you don't have a B.my_method, so B merely inherits 
A.my_method.


> I presume this is because once the decorator is applied to my_method in
> class A, A.my_method is immediately bound to the new, 'decorated'
> function, which is subsequently inherited (and not decorated,
> obviously), by B.

Correct.

> Am I correct here?  My workaround was to simply copy the method from
> class A to class B, after which B._protector_decorator decorated the
> methods in B.

That's not a work-around, that's an anti-pattern.

Why is B inheriting from A if you don't want it to be able to use A's 
methods? That's completely crazy, if you don't mind me saying so. If you 
don't want B to access A's methods, simply don't inherit from A.

I really don't understand what you are trying to accomplish here. 
Possibly Java.

http://dirtsimple.org/2004/12/python-is-not-java.html
http://dirtsimple.org/2004/12/java-is-not-python-either.html


But you can accomplish something close to what you are after like this:


import functools

def decorate(func):
    @functools.wraps(func)
    def inner(self, *args, **kwargs):
        protector = getattr(self, '_protect', None)
        if protector is not None:
            protector()
        return func(self, *args, **kwargs)
    return inner


class A(object):
    @decorate
    def mymethod(self):
        """Do something useful."""


class B(A):
    def _protect(self):
        raise RuntimeError("I'm sorry Dave, I'm afraid I cannot do that.")



Try studying that to see how it works, and then try studying it to 
realise how pointless it is, since it too relies on class B protecting 
class A from B.


-- 
Steven

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#37992

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On Thu, Jan 31, 2013 at 12:46 AM, Steven D'Aprano <
steve+comp.lang.python@pearwood.info> wrote:

> On Wed, 30 Jan 2013 19:34:03 -0500, Jason Swails wrote:
>
> > Hello,
> >
> > I was having some trouble understanding decorators and inheritance and
> > all that.  This is what I was trying to do:
> >
> > # untested
> > class A(object):
> >    def _protector_decorator(fcn):
> >       def newfcn(self, *args, **kwargs):
> >          return fcn(self, *args, **kwargs)
> >       return newfcn
>
> 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.

The following code illustrates what I'm trying to say (I think):

test.py:
#!/usr/bin/env python

a = 3

print 'Global namespace:', a

def myfunc(a):
   def nested_func():
      print 'nested_func a is:', a, 'id(a) =', id(a)

   print 'passed a is:', a, 'id(a) = ', id(a)
   nested_func()

myfunc(10)

$ python test.py
Global namespace: 3
passed a is: 10 id(a) =  6416096
nested_func a is: 10 id(a) = 6416096

Likewise, newfcn will use the function bound to the passed argument to the
decorator.  This syntax appears to work in my 'real' program.


> You probably want to add an extra parameter to the newfcn definition:
>
> def newfcn(self, fcn, *args, **kwargs):
>

I don't think I want to do that, since fcn  will simply become the first
argument that I pass to the decorated myfunc(), and if it's not callable
I'll get a traceback.

Also, I trust you realise that this is a pointless decorator that doesn't
> do anything useful? It just adds an extra layer of indirection, without
> adding any functionality.
>

Naturally.  I tried to contrive the simplest example to demonstrate what I
wanted.  In retrospect I should've picked something functional instead.

> Am I correct here?  My workaround was to simply copy the method from
> > class A to class B, after which B._protector_decorator decorated the
> > methods in B.
>
> That's not a work-around, that's an anti-pattern.
>
> Why is B inheriting from A if you don't want it to be able to use A's
> methods? That's completely crazy, if you don't mind me saying so. If you
> don't want B to access A's methods, simply don't inherit from A.
>
> I really don't understand what you are trying to accomplish here.
>

Again, my example code is over-simplified.  A brief description of my class
is a list of 'patch' (diff) files with various attributes.  If I want
information from any of those files, I instantiate a Patch instance (and
cache it for later use if desired) and return any of the information I want
from that patch (like when it was created, who created it, what files will
be altered in the patch, etc.).

But a lot of these patches are stored online, so I wanted a new class (a
RemotePatchList) to handle lists of patches in an online repository.  I can
do many of the things with an online patch that I can with one stored
locally, but not everything, hence my desire to squash the methods I don't
want to support.

I'd imagine a much more sensible approach is to generate a base class that
implements all methods common to both and simply raises an exception in
those methods that aren't.  I agree it doesn't make much sense to inherit
from an object that has MORE functionality than you want.

However, my desire to use decorators was not to disable methods in one
class vs. another.  The _protector_decorator (a name borrowed from my
actual code), is designed to wrap a function call inside a try/except, to
account for specific exceptions I might raise inside.  One of my classes
deals with local file objects, and the other deals with remote file objects
via urllib.  Naturally, the latter has other exceptions that can be raised,
like HTTPError and the like.  So my desire was to override the decorator to
handle more types of exceptions, but leave the underlying methods intact
without duplicating them.

I can do this without decorators easily enough, but I thought the decorator
syntax was a bit more elegant and I saw an opportunity to learn more about
them.

Possibly Java.
>

I took a Java class in high school once ~10 years ago... haven't used it
since. :)  Truth be told, outside of Python, the languages I can work in
are Fortran (and to a much lesser extent), C and C++.

import functools
>

I need to support Python 2.4, and the docs suggest this is 2.5+.  Too bad,
too, since functools appears pretty useful.

Thanks for the help!
Jason

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#38016

Jason Swails wrote:

> On Thu, Jan 31, 2013 at 12:46 AM, Steven D'Aprano <
> steve+comp.lang.python@pearwood.info> 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.  

It certainly isn't. Sorry for the noise.


[...]
> Again, my example code is over-simplified.  A brief description of my
> class
> is a list of 'patch' (diff) files with various attributes.  If I want
> information from any of those files, I instantiate a Patch instance (and
> cache it for later use if desired) and return any of the information I
> want from that patch (like when it was created, who created it, what files
> will be altered in the patch, etc.).
> 
> But a lot of these patches are stored online, so I wanted a new class (a
> RemotePatchList) to handle lists of patches in an online repository.  I
> can do many of the things with an online patch that I can with one stored
> locally, but not everything, hence my desire to squash the methods I don't
> want to support.


Normally, subclasses should extend functionality, not take it away. A
fundamental principle of OO design is that anywhere you could sensibly
allow an instance, should also be able to use a subclass.

So if you have a Patch class, and a RemotePatch subclass, then everything
that a Patch can do, a RemotePatch can do too, because RemotePatch
instances *are also* instances of Patch.

But the rule doesn't go in reverse: you can't necessarily use a Patch
instance where you were using a RemotePatch. Subclasses are allowed to do
*more*, but they shouldn't do *less*.

On the other hand, if you have a Patch class, and a RemotePatchList class,
inheritance does not seem to be the right relationship here. A
RemotePatchList does not seem to be a kind of Patch, but a kind of list.


> I'd imagine a much more sensible approach is to generate a base class that
> implements all methods common to both and simply raises an exception in
> those methods that aren't.  I agree it doesn't make much sense to inherit
> from an object that has MORE functionality than you want.

If a method is not common to both, it doesn't belong in the base class. The
base should only include common methods.

In fact, I'm usually rather suspicious of base classes that don't ever get
used except as a base for subclassing. I'm not saying it's wrong, but it
could be excessive abstraction. Abstraction is good, but you can have too
much of a good thing. If the base class is not used, consider a flatter
hierarchy:

    class Patch:  ...
    class RemotePatch(Patch):  ...


rather than:

    class PatchBase:  ...
    class Patch(PatchBase):  ...
    class RemotePatch(Patch):  ...

although this is okay:

    class PatchBase:  ...
    class Patch(PatchBase):  ...
    class RemotePatch(PatchBase):  ...


> However, my desire to use decorators was not to disable methods in one
> class vs. another.  The _protector_decorator (a name borrowed from my
> actual code), is designed to wrap a function call inside a try/except, to
> account for specific exceptions I might raise inside.

Ah, your example looked like you were trying to implement some sort of
access control, where some methods were flagged as "protected" to prevent
subclasses from using them. Hence my quip about Java. What you describe
here makes more sense.


> One of my classes 
> deals with local file objects, and the other deals with remote file
> objects
> via urllib.  Naturally, the latter has other exceptions that can be
> raised,
> like HTTPError and the like.  So my desire was to override the decorator
> to handle more types of exceptions, but leave the underlying methods
> intact without duplicating them.

    >>> decorated(3)
    4

One way to do that is to keep a list of exceptions to catch:


class Patch:
    catch_these = [SpamException, HamException]
    def method(self, arg):
        try:
            do_this()
        except self.catch_these:
            do_that()

The subclass can then extend or replace that list:

class RemotePatch(Patch):
    catch_these = Patch.catch_these + [EggsException, CheeseException]




>> import functools
> 
> I need to support Python 2.4, and the docs suggest this is 2.5+.  Too bad,
> too, since functools appears pretty useful.

functools.wraps is pretty simple. You can use this as an equivalent:

# `functools.wraps` was added in Python 2.5.
def wraps(func_to_wrap):
    """Return a decorator that wraps its argument.

    This is a reimplementation of functools.wraps() which copies the name,
    module, docstring and attributes of the base function to the decorated
    function. wraps() is available in the standard library from Python 2.5.

    >>> def undecorated(x):
    ...     '''This is a doc string.'''
    ...     return x+1
    ...
    >>> undecorated.__module__ = 'parrot'
    >>> undecorated.attr = 'something'
    >>> @wraps(undecorated)
    ... def decorated(x):
    ...     return undecorated(x)
    ...
    >>> decorated(3)
    4
    >>> decorated.__doc__
    'This is a doc string.'
    >>> decorated.attr
    'something'
    >>> decorated.__module__
    'parrot'
    >>> decorated.__name__
    'undecorated'

    """
    def decorator(func):
        def f(*args, **kwargs):
            return func(*args, **kwargs)
        f.__doc__ = func_to_wrap.__doc__
        try:
            f.__name__ = func_to_wrap.__name__
        except Exception:
            # Older versions of Python (2.3 and older perhaps?)
            # don't allow assigning to function __name__.
            pass
        f.__module__ = func_to_wrap.__module__
        if hasattr(func_to_wrap, '__dict__'):
            f.__dict__.update(func_to_wrap.__dict__)
        return f
    return decorator


The doctest passes for Python 2.4.



-- 
Steven

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#38022

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On Thu, Jan 31, 2013 at 6:16 PM, Steven D'Aprano <
steve+comp.lang.python@pearwood.info> wrote:

>
> Normally, subclasses should extend functionality, not take it away. A
> fundamental principle of OO design is that anywhere you could sensibly
> allow an instance, should also be able to use a subclass.
>
> So if you have a Patch class, and a RemotePatch subclass, then everything
> that a Patch can do, a RemotePatch can do too, because RemotePatch
> instances *are also* instances of Patch.
>
> But the rule doesn't go in reverse: you can't necessarily use a Patch
> instance where you were using a RemotePatch. Subclasses are allowed to do
> *more*, but they shouldn't do *less*.
>
> On the other hand, if you have a Patch class, and a RemotePatchList class,
> inheritance does not seem to be the right relationship here. A
> RemotePatchList does not seem to be a kind of Patch, but a kind of list.
>
>
> > I'd imagine a much more sensible approach is to generate a base class
> that
> > implements all methods common to both and simply raises an exception in
> > those methods that aren't.  I agree it doesn't make much sense to inherit
> > from an object that has MORE functionality than you want.
>
> If a method is not common to both, it doesn't belong in the base class. The
> base should only include common methods.
>

Yes, I agree here.  The only reason I was considering NOT doing this was
because I wanted to control the exception that gets raised rather than let
through a simple NameError.  The reason, in case you care, is that I like
creating my own custom excepthook() which optionally suppresses tracebacks
of the base exception class of my program (which can be overridden by a
--debug option of some sort).

That way I don't worry about returning error codes and the like and my
exceptions double as error messages which don't scare users away.  Of
course, if I didn't raise the exception myself, then I definitely want to
know what line that error occurred on so I can fix it (since that typically
means it's a bug or error I did not handle gracefully).

I suppose I could get the same effect by dumping everything into a main()
function somewhere and wrapping that in a try/except where I catch my base
class, but I like the flexibility


> In fact, I'm usually rather suspicious of base classes that don't ever get
> used except as a base for subclassing. I'm not saying it's wrong, but it
> could be excessive abstraction. Abstraction is good, but you can have too
> much of a good thing. If the base class is not used, consider a flatter
> hierarchy:
>
>     class Patch:  ...
>     class RemotePatch(Patch):  ...
>
>
> rather than:
>
>     class PatchBase:  ...
>     class Patch(PatchBase):  ...
>     class RemotePatch(Patch):  ...
>
> although this is okay:
>
>     class PatchBase:  ...
>     class Patch(PatchBase):  ...
>     class RemotePatch(PatchBase):  ...
>

This last one is what I've settled on.  Patch and RemotePatch have common
functionality.  But RemotePatch can be downloaded and Patch can be parsed
through (in my app, if you're going to spend the time to parse through the
whole RemotePatch, it just gets downloaded and instantiated as a Patch).
 So this last form of inheritance made the most sense to me.


>
>
> > However, my desire to use decorators was not to disable methods in one
> > class vs. another.  The _protector_decorator (a name borrowed from my
> > actual code), is designed to wrap a function call inside a try/except, to
> > account for specific exceptions I might raise inside.
>
> Ah, your example looked like you were trying to implement some sort of
> access control, where some methods were flagged as "protected" to prevent
> subclasses from using them. Hence my quip about Java. What you describe
> here makes more sense.
>
>
> > One of my classes
> > deals with local file objects, and the other deals with remote file
> > objects
> > via urllib.  Naturally, the latter has other exceptions that can be
> > raised,
> > like HTTPError and the like.  So my desire was to override the decorator
> > to handle more types of exceptions, but leave the underlying methods
> > intact without duplicating them.
>
>     >>> decorated(3)
>     4
>
> One way to do that is to keep a list of exceptions to catch:
>
>
> class Patch:
>     catch_these = [SpamException, HamException]
>     def method(self, arg):
>         try:
>             do_this()
>         except self.catch_these:
>             do_that()
>
> The subclass can then extend or replace that list:
>
> class RemotePatch(Patch):
>     catch_these = Patch.catch_these + [EggsException, CheeseException]
>

Ha! I use this technique all the time to avoid code duplication (it's used
several times in the program I'm writing).  It didn't even occur to me in
this context... Thanks for pointing this out!

As always, the time you put into responses and helping is appreciated.

All the best,
Jason

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#37997

On Fri, Feb 1, 2013 at 12:25 AM, Jason Swails <jason.swails@gmail.com> wrote:
> On Thu, Jan 31, 2013 at 12:46 AM, Steven D'Aprano
> <steve+comp.lang.python@pearwood.info> wrote:
>>
>> On Wed, 30 Jan 2013 19:34:03 -0500, Jason Swails wrote:
>>
>> > Hello,
>> >
>> > I was having some trouble understanding decorators and inheritance and
>> > all that.  This is what I was trying to do:
>> >
>> > # untested
>> > class A(object):
>> >    def _protector_decorator(fcn):
>> >       def newfcn(self, *args, **kwargs):
>> >          return fcn(self, *args, **kwargs)
>> >       return newfcn
>>
>> 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.

ChrisA

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#37998

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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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#38004

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On Thu, Jan 31, 2013 at 11:00 AM, Jason Swails <jason.swails@gmail.com>wrote:

>
>
> 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...
>

The approach I'm switching to here is to make the decorators wrappers
instead that are passed the functions that need to be called.  Basically,
wrap at run-time rather than 'compile time' (i.e., when the Python code is
'compiled' into class definitions).  That way each child of the main class
can simply change the wrapping behavior by implementing the wrapping
functions instead of duplicating all of the code.  And since this part of
the code is not performance-intensive, I don't care about the overhead of
extra function calls.

It seems to me to be the more appropriate course of action here, since
decorators don't seem to naturally lend themselves to what I'm trying to do.

--Jason

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#38012

Steven D'Aprano wrote:

>> def _protector_decorator(fcn):
>>     def newfcn(self, *args, **kwargs):
>>         return fcn(self, *args, **kwargs)
>>     return newfcn
> 
> Well, that surely isn't going to work, because it always decorates the
> same function, the global "fcn".

Good grief, I can't believe I failed to see that fcn was declared as a 
parameter to _protector_decorator.


> You probably want to add an extra parameter to the newfcn definition:
> 
> def newfcn(self, fcn, *args, **kwargs):

And that's also rubbish. The right place for the fcn parameter is the
decorator function itself, exactly where it already is.

Whatever crack I was smoking yesterday, it must have been pretty awful
stuff.



-- 
Steven

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#38036

Jason Swails於 2013年1月31日星期四UTC+8上午8時34分03秒寫道：
> Hello,
> 
> 
> I was having some trouble understanding decorators and inheritance and all that.  This is what I was trying to do:
> 
> 
> 
> # untested
> class A(object):
>    def _protector_decorator(fcn):
> 
>       def newfcn(self, *args, **kwargs):
>          return fcn(self, *args, **kwargs)
>       return newfcn
> 
> 
> 
>    @_protector_decorator
>    def my_method(self, *args, **kwargs):
>       """ do something here """
> 
> 
> 
> class B(A):
>    def _protector_decorator(fcn):
>       def newfcn(self, *args, **kwargs):
> 
>          raise MyException('I do not want B to be able to access the protected functions')
>       return newfcn
> 
> 
> 
> The goal of all that was to be able to change the behavior of my_method inside class B simply by redefining the decorator. Basically, what I want is B.my_method() to be decorated by B._protector_decorator, but in the code I'm running it's decorated by A._protector_decorator.
> 
> 
> 
> I presume this is because once the decorator is applied to my_method in class A, A.my_method is immediately bound to the new, 'decorated' function, which is subsequently inherited (and not decorated, obviously), by B.
> 
> 
> 
> Am I correct here?  My workaround was to simply copy the method from class A to class B, after which B._protector_decorator decorated the methods in B.  While this doesn't make the use of decorators completely pointless (the decorators actually do something in each class, it's just different), it does add a bunch of code duplication which I was at one point hopeful to avoid.
> 
> 
> 
> I'm still stumbling around with decorators a little, but this exercise has made them a lot clearer to me.
> 
> 
> Thanks!
> Jason

It sounds that you need a decorator mapper to 
perform the functionality of your designs.

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#38037

Jason Swails於 2013年1月31日星期四UTC+8上午8時34分03秒寫道：
> Hello,
> 
> 
> I was having some trouble understanding decorators and inheritance and all that.  This is what I was trying to do:
> 
> 
> 
> # untested
> class A(object):
>    def _protector_decorator(fcn):
> 
>       def newfcn(self, *args, **kwargs):
>          return fcn(self, *args, **kwargs)
>       return newfcn
> 
> 
> 
>    @_protector_decorator
>    def my_method(self, *args, **kwargs):
>       """ do something here """
> 
> 
> 
> class B(A):
>    def _protector_decorator(fcn):
>       def newfcn(self, *args, **kwargs):
> 
>          raise MyException('I do not want B to be able to access the protected functions')
>       return newfcn
> 
> 
> 
> The goal of all that was to be able to change the behavior of my_method inside class B simply by redefining the decorator. Basically, what I want is B.my_method() to be decorated by B._protector_decorator, but in the code I'm running it's decorated by A._protector_decorator.
> 
> 
> 
> I presume this is because once the decorator is applied to my_method in class A, A.my_method is immediately bound to the new, 'decorated' function, which is subsequently inherited (and not decorated, obviously), by B.
> 
> 
> 
> Am I correct here?  My workaround was to simply copy the method from class A to class B, after which B._protector_decorator decorated the methods in B.  While this doesn't make the use of decorators completely pointless (the decorators actually do something in each class, it's just different), it does add a bunch of code duplication which I was at one point hopeful to avoid.
> 
> 
> 
> I'm still stumbling around with decorators a little, but this exercise has made them a lot clearer to me.
> 
> 
> Thanks!
> Jason

It sounds that you need a decorator mapper to 
perform the functionality of your designs.

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