Type emulation issues with new style classes

[Chris]

Is there any way to make the class Z behave the same way as class Y?

Chris

class Y:
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __getattr__(sel f, name):
if name == '__int__':
return lambda: self.value

class Z(object):
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __getattr__(sel f, name):
if name == '__int__':
return lambda: self.value

y, z = Y(), Z()
print int(y) 42 print int(z)

TypeError: int() argument must be a string or a number

[Carmine Noviello]

class Z(object):
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __int__(self):
return self.value

Umm, maybe I haven't understood what you need.

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Take a look to http://www.pycrash.org
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[Chris]

I need to have Python call the the emulation methods for numbers, lists,
etc, when those methods are provided though __getattr__ rather than defined
in the instance or class __dict__. I've been up and down PEP 252 trying to
determine how Python determines the presence or absence of emulation methods
for new style classes, but I'm not having any luck.

"Carmine Noviello" wrote:

class Z(object):
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __int__(self):
return self.value
Umm, maybe I haven't understood what you need.

I need to know how I can do what I previously demonstrated with an old style
class, that is hook into Python's introspection mechanism to dynamically
provide an __int__ method (or optionally __str__, __lt__, __le__, __eq__
,__ne__, __gt__, __ge__, __len__, __getitem__, __add__, __sub__, __mul__,
__floordiv__, __mod__, __divmod__, __pow__, __lshift__, ... ad nausuem).
Old style classes will let me do this, but I cannot determine how to do this
with new style classes.

Chris

[Aahz]

In article <C0************ ****@nwrdny03.g nilink.net>,
Chris <fe************ *@spamgourmet.c om> wrote:

class Z(object):
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __getattr__(sel f, name):
if name == '__int__':
return lambda: self.value

IIRC, __getattribute_ _ may do what you want, but I don't have time to
check.
--
Aahz (aa**@pythoncra ft.com) <*> http://www.pythoncraft.com/

"Do not taunt happy fun for loops. Do not change lists you are looping over."
--Remco Gerlich, comp.lang.pytho n

[Chris]

Tried it earlier, same result.

Chris

"Aahz" wrote:

IIRC, __getattribute_ _ may do what you want, but I don't have time to
check.
--
Aahz (aa**@pythoncra ft.com) <*> http://www.pythoncraft.com/
"Do not taunt happy fun for loops. Do not change lists you are looping over." --Remco Gerlich, comp.lang.pytho n

[Michael Hudson]

"Chris" <fe************ *@spamgourmet.c om> writes:

Is there any way to make the class Z behave the same way as class Y?

You need a custom metaclass for Z, and implement __getattr__ there.

HTH.

Cheers,
mwh

--
Lisp does badly because we refuse to lie. When people ask us if
we can solve insoluble problems we say that we can't, and because
they expect us to lie to them, they find some other language
where the truth is less respected. -- Tim Bradshaw, comp.lang.lisp

[Chris]

I tried a couple variation of that, and __getattr__, when defined in a
metaclass is never called when accessing an attribute on an instance of a
class derived from that metaclass.

Here is some testing I did:

class Zmeta(type):
def __getattribute_ _(*args):
raise Exception('call ed __getattribute_ _ with %s' % str(args))
def __getattr__(*ar gs):
raise Exception('call ed __getattr__ with %s' % str(args))

Z = Zmeta('Z', (), {'value': 42})
z = Z()

int(z) TypeError: int() argument must be a string or a number z.__int__ AttributeError: 'Z' object has no attribute '__int__' Z.__int__

Exception: called __getattribute_ _ with (<class '__main__.Z'>, '__int__')

It appears (and is confirmed in: Metaclass Programming in Python Pt. 2
http://www-106.ibm.com/developerwork...a2/?ca=dnt-434) that
metaclass attributes are available to instances (classes) but not instances
of instances.

Chris

"Michael Hudson" wrote:

Is there any way to make the class Z behave the same way as class Y?

You need a custom metaclass for Z, and implement __getattr__ there.

HTH.

Cheers,
mwh

--
Lisp does badly because we refuse to lie. When people ask us if
we can solve insoluble problems we say that we can't, and because
they expect us to lie to them, they find some other language
where the truth is less respected. -- Tim Bradshaw, comp.lang.lisp

[Rainer Deyke]

Chris wrote:

class Z(object):
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __getattr__(sel f, name):
if name == '__int__':
return lambda: self.value

def __int__(self):
return self.__getattr_ _('__int__')()
--
Rainer Deyke - ra*****@eldwood .com - http://eldwood.com

[Greg Chapman]

On Sat, 28 Feb 2004 11:33:54 GMT, "Chris" <fe************ *@spamgourmet.c om>
wrote:

class Z(object):
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __getattr__(sel f, name):
if name == '__int__':
return lambda: self.value

The following allows your test case to work, but it may have various subtle
problems. AddDynOper emulates __getattr__ semantics: the descriptor is not
installed if the type already has an attribute of the given name:

class DynOperDescr(ob ject):
def __init__(self, name):
self.name = name
def __get__(self, instance, typ):
if instance is None:
return self
return instance.__geta ttr__(self.name )

def AddDynOper(decl cls, name):
if not hasattr(declcls , name):
setattr(declcls , name, DynOperDescr(na me))

class Z(object):
value = 42
def __getattr__(sel f, name):
if name == "__int__":
return lambda : self.value
raise AttributeError( name)
AddDynOper(Z, "__int__")

---
Greg Chapman

[Chris]

I'm looking for a way to do this without explicity definining every
emulation method, i.e. (__str__, __lt__, __le__, __eq__, __ne__, __gt__,
__ge__, __len__, __getitem__, __add__, __sub__, __mul__, __floordiv__,
__mod__, __divmod__, __pow__, __lshift__, ... ad nausuem). Plus, there's a
few this isn't going to work for several of the the arithmetic operators,
such as when I want to have __add__ undefined so Python will attpemt
__radd__. For example:

class Y:
value = 42
def __getattr__(sel f, name):
if name == '__coerce__':
raise AttributeError
if name == '__add__':
raise AttributeError
if name == '__radd__':
return lambda other: self.value + other.value

y1, y2 = Y(), Y()
print int(y1) #This works 42 print y1 + y2 #This works too 84

class Z(object):
value = 42
def __getattr__(sel f, name):
if name == '__add__':
raise AttributeError
if name == '__radd__':
return lambda other: Z.value + Z.value
if name == '__int__':
return lambda: self.value
def __int__(self):
return self.__getattr_ _('__int__')()
def __add__(self, other):
return self.__getattr_ _('__add__')()
def __radd__(self, other):
return self.__getattr_ _('__radd__')()

z1, z2 = Z(), Z()
# This following will work now, after explicitly
# adding __int__ to the namespace of Z, which I'm
# trying to avoid.
print int(z1) 42 print y1 + y2 # This bombs

TypeError: 'NoneType' object is not callable

"Rainer Deyke" wrote: Chris wrote:

class Z(object):
value = 42
def __hasattr__(sel f, name):
if name == '__int__':
return True
def __getattr__(sel f, name):
if name == '__int__':
return lambda: self.value

def __int__(self):
return self.__getattr_ _('__int__')()
--
Rainer Deyke - ra*****@eldwood .com - http://eldwood.com