Re: Conditionally implementing __iter__ in new style classes

2005-07-08 Thread Bengt Richter 
On Thu, 07 Jul 2005 22:04:31 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

[EMAIL PROTECTED] (Bengt Richter) writes:

 On Thu, 07 Jul 2005 09:51:42 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

[EMAIL PROTECTED] (Bengt Richter) writes:

 On Wed, 06 Jul 2005 17:57:42 +0200, Thomas Heller [EMAIL PROTECTED] 
 wrote:

I'm trying to implement __iter__ on an abstract base class while I don't
know whether subclasses support that or not.

 Will a property or custom descriptor do what you want? E.g.

   class Base(object):
  ... def __getIter(self):
  ... if hasattr(self, Iterator):
  ... return self.Iterator
  ... raise AttributeError, name
  ... __iter__ = property(__getIter)
 [...]

Yep, that's exactly what I need - thanks.

 BTW, I forgot to mention that you could use property as a decorator
 in the above single-argument case:

   class Base(object):
  ... @property
  ... def __iter__(self):
  ... if hasattr(self, Iterator):
  ... return self.Iterator
  ... raise AttributeError, name

Of course.  I didn't spot this, but I cannot use this anyway for 2.3
compatibility.

  ...
   class Concrete(Base):
  ... def Iterator(self):
  ... yield 1
  ... yield 2
  ... yield 3
  ...
   iter(Base())
  Traceback (most recent call last):
File stdin, line 1, in ?
  TypeError: iteration over non-sequence
   iter(Concrete())
  generator object at 0x02EF152C
   list(iter(Concrete()))
  [1, 2, 3]

 Hope there isn't a gotcha for your use case in the way an instance attribute
 of the same name is allowed. A custom descriptor could eliminate that.

   inst = Concrete()
   list(iter(inst))
  [1, 2, 3]
   inst.__init__ = 'abc'
   list(iter(inst))
  [1, 2, 3]
   inst.__init__
  'abc'

I don't understand what you mean here.  A __iter__ instance attribute?

Yes, but it seems very unlikely to cause a problem, especially since iter(inst)
bypasses it, as you probably would want. In other words, never mind ;-)

Regards,
Bengt Richter
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Re: Conditionally implementing __iter__ in new style classes

2005-07-07 Thread Thomas Heller 
[EMAIL PROTECTED] (Bengt Richter) writes:

 On Wed, 06 Jul 2005 17:57:42 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

I'm trying to implement __iter__ on an abstract base class while I don't
know whether subclasses support that or not.

 Will a property or custom descriptor do what you want? E.g.

   class Base(object):
  ... def __getIter(self):
  ... if hasattr(self, Iterator):
  ... return self.Iterator
  ... raise AttributeError, name
  ... __iter__ = property(__getIter)
  ...
   class Concrete(Base):
  ... def Iterator(self):
  ... yield 1
  ... yield 2
  ... yield 3
  ...
   iter(Base())
  Traceback (most recent call last):
File stdin, line 1, in ?
  TypeError: iteration over non-sequence
   iter(Concrete())
  generator object at 0x02EF152C
   list(iter(Concrete()))
  [1, 2, 3]

Yep, that's exactly what I need - thanks.

Thomas
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Re: Conditionally implementing __iter__ in new style classes

2005-07-07 Thread Bengt Richter 
On Thu, 07 Jul 2005 09:51:42 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

[EMAIL PROTECTED] (Bengt Richter) writes:

 On Wed, 06 Jul 2005 17:57:42 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

I'm trying to implement __iter__ on an abstract base class while I don't
know whether subclasses support that or not.

 Will a property or custom descriptor do what you want? E.g.

   class Base(object):
  ... def __getIter(self):
  ... if hasattr(self, Iterator):
  ... return self.Iterator
  ... raise AttributeError, name
  ... __iter__ = property(__getIter)
[...]

Yep, that's exactly what I need - thanks.

BTW, I forgot to mention that you could use property as a decorator
in the above single-argument case:

  class Base(object):
 ... @property
 ... def __iter__(self):
 ... if hasattr(self, Iterator):
 ... return self.Iterator
 ... raise AttributeError, name
 ...
  class Concrete(Base):
 ... def Iterator(self):
 ... yield 1
 ... yield 2
 ... yield 3
 ...
  iter(Base())
 Traceback (most recent call last):
   File stdin, line 1, in ?
 TypeError: iteration over non-sequence
  iter(Concrete())
 generator object at 0x02EF152C
  list(iter(Concrete()))
 [1, 2, 3]

Hope there isn't a gotcha for your use case in the way an instance attribute
of the same name is allowed. A custom descriptor could eliminate that.

  inst = Concrete()
  list(iter(inst))
 [1, 2, 3]
  inst.__init__ = 'abc'
  list(iter(inst))
 [1, 2, 3]
  inst.__init__
 'abc'



Regards,
Bengt Richter
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Re: Conditionally implementing __iter__ in new style classes

2005-07-07 Thread Dieter Maurer 
Thomas Heller [EMAIL PROTECTED] writes on Wed, 06 Jul 2005 18:07:10 +0200:
 Thomas Heller [EMAIL PROTECTED] writes:
 ...
  class Base:
  def __getattr__(self, name):
  if name == __iter__ and hasattr(self, Iterator):
  return self.Iterator
  raise AttributeError, name
 
  class Concrete(Base):
  def Iterator(self):
  yield 1
 ...
  If, however, I make Base a newstyle class, this will not work any
  longer.  __getattr__ is never called for __iter__ (neither is
  __getattribute__, btw).  Probably this has to do with data descriptors
  and non-data descriptors, but I'm too tired at the moment to think
  further about this.
 
  Is there any way I could make the above code work with new style
  classes?
 
 I forgot to mention this: The Base class also implements a __getitem__
 method which should be used for iteration if the .Iterator method in the
 subclass is not available.  So it seems impossible to raise an exception
 in the __iter__ method if .Iterator is not found - __iter__ MUST return
 an iterator if present.

Then, it should return an interator (a new object) that uses
the __getitem__ method to iterate.


Dieter
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Re: Conditionally implementing __iter__ in new style classes

2005-07-07 Thread Thomas Heller 
[EMAIL PROTECTED] (Bengt Richter) writes:

 On Thu, 07 Jul 2005 09:51:42 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

[EMAIL PROTECTED] (Bengt Richter) writes:

 On Wed, 06 Jul 2005 17:57:42 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

I'm trying to implement __iter__ on an abstract base class while I don't
know whether subclasses support that or not.

 Will a property or custom descriptor do what you want? E.g.

   class Base(object):
  ... def __getIter(self):
  ... if hasattr(self, Iterator):
  ... return self.Iterator
  ... raise AttributeError, name
  ... __iter__ = property(__getIter)
 [...]

Yep, that's exactly what I need - thanks.

 BTW, I forgot to mention that you could use property as a decorator
 in the above single-argument case:

   class Base(object):
  ... @property
  ... def __iter__(self):
  ... if hasattr(self, Iterator):
  ... return self.Iterator
  ... raise AttributeError, name

Of course.  I didn't spot this, but I cannot use this anyway for 2.3
compatibility.

  ...
   class Concrete(Base):
  ... def Iterator(self):
  ... yield 1
  ... yield 2
  ... yield 3
  ...
   iter(Base())
  Traceback (most recent call last):
File stdin, line 1, in ?
  TypeError: iteration over non-sequence
   iter(Concrete())
  generator object at 0x02EF152C
   list(iter(Concrete()))
  [1, 2, 3]

 Hope there isn't a gotcha for your use case in the way an instance attribute
 of the same name is allowed. A custom descriptor could eliminate that.

   inst = Concrete()
   list(iter(inst))
  [1, 2, 3]
   inst.__init__ = 'abc'
   list(iter(inst))
  [1, 2, 3]
   inst.__init__
  'abc'

I don't understand what you mean here.  A __iter__ instance attribute?

Thomas
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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread Thomas Heller 
Thomas Heller [EMAIL PROTECTED] writes:

 I'm trying to implement __iter__ on an abstract base class while I don't
 know whether subclasses support that or not.
 Hope that makes sense, if not, this code should be clearer:

 class Base:
 def __getattr__(self, name):
 if name == __iter__ and hasattr(self, Iterator):
 return self.Iterator
 raise AttributeError, name

 class Concrete(Base):
 def Iterator(self):
 yield 1
 yield 2
 yield 3

 The idea is that if a subclass of Base defines an 'Iterator' method,
 instances are iterable.  They are not iterable otherwise.

 The above gives the expected behaviour: iter(Base()) raises a
 TypeError: iteration over non-sequence, and iter(Concrete()) returns a
 generator.

 If, however, I make Base a newstyle class, this will not work any
 longer.  __getattr__ is never called for __iter__ (neither is
 __getattribute__, btw).  Probably this has to do with data descriptors
 and non-data descriptors, but I'm too tired at the moment to think
 further about this.

 Is there any way I could make the above code work with new style
 classes?

I forgot to mention this: The Base class also implements a __getitem__
method which should be used for iteration if the .Iterator method in the
subclass is not available.  So it seems impossible to raise an exception
in the __iter__ method if .Iterator is not found - __iter__ MUST return
an iterator if present.

Thomas
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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread harold fellermann 
 I'm trying to implement __iter__ on an abstract base class while I 
 don't
 know whether subclasses support that or not.
 Hope that makes sense, if not, this code should be clearer:

 class Base:
 def __getattr__(self, name):
 if name == __iter__ and hasattr(self, Iterator):
 return self.Iterator
 raise AttributeError, name

 class Concrete(Base):
 def Iterator(self):
 yield 1
 yield 2
 yield 3

I don't know how to achieve it, but why don't you simply use

class Base:
pass

class Concrete(Base):
def __iter__(self) :
yield 1
yield 2
yield 3


What is the advantage to have a baseclass that essentially does
some method renaming __iter__ == Iterator?

- harold -

--
Always remember that you are unique;
just like everyone else.
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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread infidel 
I'm not sure I understand why you would want to.  Just don't define
__iter__ on your newstyle class and you'll get the expected behavior.

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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread infidel 
Why not define an Iterator method in your Base class that does the
iteration using __getitem__, and any subclass that wants to do
something else just defines its own Iterator method?  For that matter,
you could just use the __iter__ methods of Base and Concrete instead of
a separate method.

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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread infidel 
Something like this:

 class Base(object):
... def __getitem__(self, key):
... return key
... def __iter__(self):
... yield self[1]
... yield self['foo']
... yield self[3.0]
...
 class ConcreteIterable(Base):
... def __iter__(self):
... yield True
... yield 'Blue'
... yield 'Foo'
...
 class ConcreteNotIterable(Base):
... pass
...
 [x for x in Base()]
[1, 'foo', 3.0]
 [x for x in ConcreteIterable()]
[True, 'Blue', 'Foo']
 [x for x in ConcreteNotIterable()]
[1, 'foo', 3.0]


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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread Leif K-Brooks 
Thomas Heller wrote:
 I forgot to mention this: The Base class also implements a __getitem__
 method which should be used for iteration if the .Iterator method in the
 subclass is not available.  So it seems impossible to raise an exception
 in the __iter__ method if .Iterator is not found - __iter__ MUST return
 an iterator if present.

def Iterator(self):
for index in xrange(len(self)):
yield self[index]

def __iter__(self):
return self.Iterator()

...and then override Iterator in subclasses. But this raises the
question of why you need to use a specially-named method instead of
having subclasses override the __iter__.
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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread Peter Otten 
Thomas Heller wrote:

 I'm trying to implement __iter__ on an abstract base class while I don't
 know whether subclasses support that or not.
 Hope that makes sense, if not, this code should be clearer:
 
 class Base:
 def __getattr__(self, name):
 if name == __iter__ and hasattr(self, Iterator):
 return self.Iterator
 raise AttributeError, name
 
 Is there any way I could make the above code work with new style
 classes?

Obligatory metaclass approach:

class Base:
class __metaclass__(type):
def __new__(mcl, name, bases, classdict):
try:
classdict[__iter__] = classdict[Iterator]
except KeyError:
pass
return type.__new__(mcl, name, bases, classdict)

class Alpha(Base):
def Iterator(self): yield 42

class Beta(Base):
def __getitem__(self, index):
return [1, 2, 3, ganz viele][index]


for item in Alpha(): print item
for item in Beta(): print item,
print

Peter

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Re: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread Bengt Richter 
On Wed, 06 Jul 2005 17:57:42 +0200, Thomas Heller [EMAIL PROTECTED] wrote:

I'm trying to implement __iter__ on an abstract base class while I don't
know whether subclasses support that or not.
Hope that makes sense, if not, this code should be clearer:

class Base:
def __getattr__(self, name):
if name == __iter__ and hasattr(self, Iterator):
return self.Iterator
raise AttributeError, name

class Concrete(Base):
def Iterator(self):
yield 1
yield 2
yield 3

The idea is that if a subclass of Base defines an 'Iterator' method,
instances are iterable.  They are not iterable otherwise.

The above gives the expected behaviour: iter(Base()) raises a
TypeError: iteration over non-sequence, and iter(Concrete()) returns a
generator.

If, however, I make Base a newstyle class, this will not work any
longer.  __getattr__ is never called for __iter__ (neither is
__getattribute__, btw).  Probably this has to do with data descriptors
and non-data descriptors, but I'm too tired at the moment to think
further about this.

Is there any way I could make the above code work with new style
classes?
Will a property or custom descriptor do what you want? E.g.

  class Base(object):
 ... def __getIter(self):
 ... if hasattr(self, Iterator):
 ... return self.Iterator
 ... raise AttributeError, name
 ... __iter__ = property(__getIter)
 ...
  class Concrete(Base):
 ... def Iterator(self):
 ... yield 1
 ... yield 2
 ... yield 3
 ...
  iter(Base())
 Traceback (most recent call last):
   File stdin, line 1, in ?
 TypeError: iteration over non-sequence
  iter(Concrete())
 generator object at 0x02EF152C
  list(iter(Concrete()))
 [1, 2, 3]

Regards,
Bengt Richter
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RE: Conditionally implementing __iter__ in new style classes

2005-07-06 Thread Delaney, Timothy (Tim) 
Thomas Heller wrote:

 I forgot to mention this: The Base class also implements a __getitem__
 method which should be used for iteration if the .Iterator method in
 the subclass is not available.  So it seems impossible to raise an
 exception in the __iter__ method if .Iterator is not found - __iter__
 MUST return an iterator if present.

Again, why not just let subclasses implement __iter__? Calling iter() on
an instance of the base class (or any subclass that does not implement
__iter__) will return an iterator that uses __getitem__. Calling iter()
on a subclass instance that does implement __iter__ will return the
iterator from the subclass __iter__.

Tim Delaney
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