Declarative properties

[Artur Siekielski]

Hi.

I would like to have declarative properties in Python, ie. something
like slots definitions in defclass in Common Lisp. It seems that even
Java will have it, using a library ( https://bean-properties.dev.java.net/
).

I know about 'property' function in Python, but it's normal usage
isn't declarative, because I have to code imperatively getters and
setters:

class Person(object):
def __init__(self, name):
self._name = name
def _get_name(self) :
return self._name
def _set_name(self, new_name):
self._name = new_name
name = property(_get_n ame, _set_name)

I would like to have something like that:

class Person(object):
name = property('_name ')

I assume that this causes "generation " of instance field '_name' and
default getters and setters. And if I would like to customize (g|
s)etters, I would write them by hand, or, better, use more declarative
approach (like @NotNull annotations in Java version).

I could probably code a solution to my problem with help of wonderful
introspection functionalities . But I'm looking for a standard and/or
proven way of doing this. Maybe some PEP is being prepared for this?

Regards,
Artur

[Marc 'BlackJack' Rintsch]

On Thu, 11 Oct 2007 11:48:18 +0000, Artur Siekielski wrote:

class Person(object):
def __init__(self, name):
self._name = name
def _get_name(self) :
return self._name
def _set_name(self, new_name):
self._name = new_name
name = property(_get_n ame, _set_name)

This is more easily spelled:

class Person(object):
def __init__(self, name):
self.name = name

I would like to have something like that:

class Person(object):
name = property('_name ')

I assume that this causes "generation " of instance field '_name' and
default getters and setters.

But why? Default getters and setters are unnecessary and if you need
something other than the default you need to write it anyway more
explicitly.

Ciao,
Marc 'BlackJack' Rintsch

[Bruno Desthuilliers]

Artur Siekielski a écrit :

Hi.

I would like to have declarative properties in Python, ie. something
like slots definitions in defclass in Common Lisp. It seems that even
Java will have it, using a library ( https://bean-properties.dev.java.net/
).

I know about 'property' function

Actually, it's a class.

in Python, but it's normal usage
isn't declarative, because I have to code imperatively getters and
setters:

Indeed. What would be the use of a property (AKA computed attribute) if
you don't need to do something specific ?

class Person(object):
def __init__(self, name):
self._name = name
def _get_name(self) :
return self._name
def _set_name(self, new_name):
self._name = new_name
name = property(_get_n ame, _set_name)

While it's often seen as demonstration code for a property, it's
actually totally useless. If all you want to do is to get and set an
attribute (I mean, without any computation anywhere), then just use an
attribute - you will always be able to turn it into a property if and
whene the need arises.

I would like to have something like that:

class Person(object):
name = property('_name ')

I assume that this causes "generation " of instance field '_name' and
default getters and setters.

So far, it's a waste of time. That's exactly what you'd get with a plain
attribute, with far less complexity and overhead.

And if I would like to customize (g|
s)etters, I would write them by hand,

Which bring us back to how properties actually work !-)

or, better, use more declarative
approach (like @NotNull annotations in Java version).

I could probably code a solution to my problem with help of wonderful
introspection functionalities . But I'm looking for a standard and/or
proven way of doing this. Maybe some PEP is being prepared for this?

I guess you want to have a look at the descriptor protocol - it's what
makes properties work, and it allow you to write your own custom 'smart
attributes'. If you want automatic validation/conversion, you could
write custom descriptors working with the FormEncode package (I started
such a thing for Elixir, but had no time to finish it so far). It would
then looks like:

class Person(object):
name = StringField(emp ty=False)

// etc

HTH

[Dan Stromberg]

On Thu, 11 Oct 2007 13:46:12 +0000, Marc 'BlackJack' Rintsch wrote:

On Thu, 11 Oct 2007 13:04:53 +0000, Artur Siekielski wrote:

>On Oct 11, 2:27 pm, Marc 'BlackJack' Rintsch <[e-mail] wrote:

>>But why? Default getters and setters are unnecessary and if you need
something other than the default you need to write it anyway more
explicitly.

I see some problems with your approach:

1. If I use instance field 'name' which is accessed directly by other
classes,
and later I decide to implement nonstandard getter, I must refactor
'Person' class
and in some places change 'name' to '_name' (assuming this is now the
field's name).
The problem is that I cannot automatically change 'name' to '_name'
everywhere, because
in some places I want public property value (eg. validated and
formatted), and in other
places raw property value.

So what? Otherwise you carry *always* the baggage of a public property
and a private attribute whether you need this or not. At least for me it
would be unnecessary in most cases.

That "baggage" of carrying around "unneeded" methods is something the
computer carries for you - IE, no big deal in 99.99% of all cases.

The "baggage" of possibly fixing (AKA "generalizi ng") how your attributes
are accessed is something you lug around while your deadline looms.

Here's some code that defines such methods for you:

#!/usr/bin/env python

def gimme_set_get(f oo, attribute):
lst = [ \
'def set_%s(self, value):' % attribute, \
' self._%s = value' % attribute, \
'def get_%s(self):' % attribute, \
' return self._%s' % attribute, \
'foo.set_%s = set_%s' % (attribute, attribute), \
'foo.get_%s = get_%s' % (attribute, attribute) \
]
s = '\n'.join(lst)
code = compile(s, '<string>', 'exec')
eval(code)

class foo:
def __init__(self, value):
self.public_val ue = value
gimme_set_get(f oo, 'via_accessor_m ethod_only')

f = foo(1)
f.set_via_acces sor_method_only (1/9.0)
print f.get_via_acces sor_method_only ()

print dir(f)

[Marc 'BlackJack' Rintsch]

On Thu, 11 Oct 2007 09:58:48 -0700, Dan Stromberg wrote:

On Thu, 11 Oct 2007 13:46:12 +0000, Marc 'BlackJack' Rintsch wrote:

>On Thu, 11 Oct 2007 13:04:53 +0000, Artur Siekielski wrote:

>>1. If I use instance field 'name' which is accessed directly by other
classes,
and later I decide to implement nonstandard getter, I must refactor
'Person' class
and in some places change 'name' to '_name' (assuming this is now the
field's name).
The problem is that I cannot automatically change 'name' to '_name'
everywhere, because
in some places I want public property value (eg. validated and
formatted), and in other
places raw property value.

So what? Otherwise you carry *always* the baggage of a public property
and a private attribute whether you need this or not. At least for me it
would be unnecessary in most cases.

That "baggage" of carrying around "unneeded" methods is something the
computer carries for you - IE, no big deal in 99.99% of all cases.

It shows twice as much attributes if I inspect the objects and I don't know
which are merely useless default getters and setters. And it is more and
more complex code. Code can be cut down a bit by some metaclass magic but
this brings in another complexity.

The "baggage" of possibly fixing (AKA "generalizi ng") how your attributes
are accessed is something you lug around while your deadline looms.

Sorry I don't get it. If I want to customize the access to a "normal"
attribute I simply turn it into a property.

Here's some code that defines such methods for you:

#!/usr/bin/env python

def gimme_set_get(f oo, attribute):
lst = [ \
'def set_%s(self, value):' % attribute, \
' self._%s = value' % attribute, \
'def get_%s(self):' % attribute, \
' return self._%s' % attribute, \
'foo.set_%s = set_%s' % (attribute, attribute), \
'foo.get_%s = get_%s' % (attribute, attribute) \
]
s = '\n'.join(lst)
code = compile(s, '<string>', 'exec')
eval(code)

class foo:
def __init__(self, value):
self.public_val ue = value
gimme_set_get(f oo, 'via_accessor_m ethod_only')

f = foo(1)
f.set_via_acces sor_method_only (1/9.0)
print f.get_via_acces sor_method_only ()

print dir(f)

And the benefit of this evil ``eval`` dance is exactly what!?

Ciao,
Marc 'BlackJack' Rintsch

[Paul Hankin]

On Oct 11, 12:48 pm, Artur Siekielski <artur.siekiel. [e-mail]>
wrote:

Hi.

I would like to have declarative properties in Python, ie. something
like slots definitions in defclass in Common Lisp. It seems that even
Java will have it, using a library (https://bean-properties.dev.java.net/
).

I know about 'property' function in Python, but it's normal usage
isn't declarative, because I have to code imperatively getters and
setters:

class Person(object):
def __init__(self, name):
self._name = name
def _get_name(self) :
return self._name
def _set_name(self, new_name):
self._name = new_name
name = property(_get_n ame, _set_name)

I would like to have something like that:

class Person(object):
name = property('_name ')

Here's something that does what I think you want. I think your use
case is quite unusual in that you expect the public accessors to
change (keeping private use the same), and that you know you're going
to have a costly rewrite of your class when that happens.

Attributes that are declared with 'magic_attribut e' access the private
attribute '_<name>', unless there's a '_get_<name>' or '_set_<name>'
method in which case those are called instead.

def magic_attribute (_name):
def set(self, value):
try:
getattr(self, '_set' + _name)(value)
except AttributeError:
setattr(self, _name, value)
def get(self):
try:
return getattr(self, '_get' + _name)()
except AttributeError:
return getattr(self, _name)
return property(get, set)

# An example use...
class C(object):
x = magic_attribute ('_x')
y = magic_attribute ('_y')

# Override the default set method for x
def _set_x(self, value):
self._x = value + ' world'

# Override the default get method for y
def _get_y(self):
return self._y.upper()

# Test it works...
c = C()
c.x = c.y = 'hello'
print c.x
print c.y

# hello world
# HELLO

--
Paul Hankin

[Paul Hankin]

On Oct 11, 7:42 pm, Marc 'BlackJack' Rintsch <[e-mail] wrote:

Sorry I don't get it. If I want to customize the access to a "normal"
attribute I simply turn it into a property.

I *think* I understand Artur's problem: he wants to be able to add
(for example) clean-up and validation code to public accesses of these
attributes, but he doesn't want this code run internally to the class.
Or another way of saying the same thing: he wants two views of a
variable; one internal to the class, another to the public.

If I understand correctly, his plan is to use 'X._name' internally to
code in his class, but the public uses 'X.name'. Initially, one is an
alias for the other, but later he writes getters and setters to
customise access through the public version.

[I could be totally wrong of course]
--
Paul Hankin

[George Sakkis]

On Oct 11, 7:48 am, Artur Siekielski <artur.siekiel. [e-mail]>
wrote:

I know about 'property' function in Python, but it's normal usage
isn't declarative, because I have to code imperatively getters and
setters:

class Person(object):
def __init__(self, name):
self._name = name
def _get_name(self) :
return self._name
def _set_name(self, new_name):
self._name = new_name
name = property(_get_n ame, _set_name)

I would like to have something like that:

class Person(object):
name = property('_name ')

By now you must have been convinced that default getters/setters is
not a very useful idea in Python but this does not mean you can't do
it; it's actually straightforward :

def make_property(a ttr):
return property(lambda self: getattr(self,at tr),
lambda self, value: setattr(self, attr, value))

class Person(object):
name = make_property(' _name')
def __init__(self, name):
self.name = name

You could take it even further by removing the need to repeat the
attribute's name twice. Currently this can done only through
metaclasses but in the future a class decorator would be even better:

def PropertyMaker(* names, **kwds):
format = kwds.get('forma t', '_%s')
def meta(cls,bases, attrdict):
for name in names:
attrdict[name] = make_property(f ormat % name)
return type(cls,bases, attrdict)
return meta

class Person(object):
__metaclass__ = PropertyMaker(' name', format='__%s__' )

def __init__(self, name):
self.name = name
print self.__name__
George

[George Sakkis]

On Oct 11, 7:04 pm, George Sakkis <george.sak...@ gmail.comwrote:

You could take it even further by removing the need to repeat the
attribute's name twice. Currently this can done only through
metaclasses but in the future a class decorator would be even
better:

Replying to myself here, but actually metaclasses is not the only way;
another solution involves a descriptor class:
class Property(object ):

# cache the mapping of types to 'private' attribute names
_type2attrname = {}

def __init__(self, format='_%s'):
self._format = format

def __get__(self, obj, type=None):
try: name = self._type2attr name[type(obj)]
except KeyError:
self._type2attr name[type(obj)] = name =
self._get_propn ame(obj)
return getattr(obj, name)

def __set__(self, obj, value):
try: name = self._type2attr name[type(obj)]
except KeyError:
self._type2attr name[type(obj)] = name =
self._get_propn ame(obj)
setattr(obj, name, value)

def _get_propname(s elf, obj):
for cls in type(obj).mro() :
for name,value in cls.__dict__.it eritems():
if value is self:
return self._format % name
assert False # unreachable
#---- example ------------------------------------

class Person(object):
name = Property()

def __init__(self, name):
self.name = name

p = Person('John')
q = Person('Mike')
print p.name, q.name
print p.__dict__
George

[Artur Siekielski]

George Sakkis wrote:

By now you must have been convinced that default getters/setters is
not a very useful idea in Python but this does not mean you can't do
it;

It's a perfect summary of my thoughts after reading this thread. I
will use public attributes (with access customizable with properties)
and remember that in Python I can do everything :).
Thanks everybody.