What is object

[Alan Bromborsky]

I am looking at the Basic class and wonder what you are doing with the
following statement:

def __new__(cls, *args, **assumptions):
obj = object.__new__(cls)

What I am trying today is give the Symbol class a new data member so
that is x is a symbol I can define
x.latex_name to go with x.name. However the __new__ command for symbol
is inscrutable to me so that if I define
a cls.latex_name in __new__ for symbol it is global to the class. I
asked about the above statement since the
__new_stage2__ function returns a basic object and there is where the
symbol name is stored.

[Ronan Lamy]

object.__new__ is the default object creator. The statement you cite
creates an empty instance (= without any instance attribute) of the
class cls and assigns it to the empty variable obj.
Generally speaking, __new__ is a special method that is called to create
instances of a class. Its return value is the instance you want to
create. A statement like x = Symbol(name, ...) is basically syntactic
sugar for x = Symbol.__new__(Symbol, name, ...). If Symbol, which is a
subclass of Basic, did not override Basic.__new__, the statement would
be equivalent to x = Basic.__new__(Symbol, name, ...) — compare the 2nd
line of __new_stage2__.

Note that __new__ is always a class method, which means that its first
argument is the class you want to instantiate (that's the reason why
it's called "cls"). So, writing cls.latex_name = <whatever> will
obviously create a *class* attribute. What you want is to create an
*instance* attribute. For that, you simply need to take your
newly-created instance (i.e. "obj", after the second line of
__new_stage2__) and add an attribute to it in the usual, straightforward
manner (i.e. obj.latex_name = <whatever>)

Hope this helps.
Ronan

[Alan Bromborsky]

Ronan Lamy wrote:
> Le vendredi 20 novembre 2009 � 16:15 -0500, Alan Bromborsky a �crit :

>
>> I am looking at the Basic class and wonder what you are doing with the
>> following statement:
>>
>> def __new__(cls, *args, **assumptions):
>> obj = object.__new__(cls)
>>
>> What I am trying today is give the Symbol class a new data member so
>> that is x is a symbol I can define
>> x.latex_name to go with x.name. However the __new__ command for symbol
>> is inscrutable to me so that if I define
>> a cls.latex_name in __new__ for symbol it is global to the class. I
>> asked about the above statement since the
>> __new_stage2__ function returns a basic object and there is where the
>> symbol name is stored.
>>
>
> object.__new__ is the default object creator. The statement you cite
> creates an empty instance (= without any instance attribute) of the
> class cls and assigns it to the empty variable obj.
> Generally speaking, __new__ is a special method that is called to create
> instances of a class. Its return value is the instance you want to
> create. A statement like x = Symbol(name, ...) is basically syntactic
> sugar for x = Symbol.__new__(Symbol, name, ...). If Symbol, which is a
> subclass of Basic, did not override Basic.__new__, the statement would

> be equivalent to x = Basic.__new__(Symbol, name, ...) � compare the 2nd

> line of __new_stage2__.
>
> Note that __new__ is always a class method, which means that its first
> argument is the class you want to instantiate (that's the reason why
> it's called "cls"). So, writing cls.latex_name = <whatever> will
> obviously create a *class* attribute. What you want is to create an
> *instance* attribute. For that, you simply need to take your
> newly-created instance (i.e. "obj", after the second line of
> __new_stage2__) and add an attribute to it in the usual, straightforward
> manner (i.e. obj.latex_name = <whatever>)
>
> Hope this helps.
> Ronan
>

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>
>
If I modify __new_stage2__ to:

def __new_stage2__(cls, name, commutative=True, **assumptions):
assert isinstance(name, str),`type(name)`
obj = Basic.__new__(cls, **assumptions)
obj.is_commutative = commutative
print '__new_stage2__(name) =',name
obj.latex_name = None
if '|' in name:
namelst = name.split('|',1)
name = namelst[0]
obj.latex_name = namelst[1]
obj.name = name
return obj

I get the error:

File "/home/brombo/sympy/sympy/core/symbol.py", line 61, in __new_stage2__
obj.latex_name = None
AttributeError: 'Symbol' object has no attribute 'latex_name'

Any further suggestions would be welcome.

[Ronan Lamy]

Le samedi 21 novembre 2009 à 17:17 -0500, Alan Bromborsky a écrit :
> Ronan Lamy wrote:

> > Le vendredi 20 novembre 2009 à 16:15 -0500, Alan Bromborsky a écrit :
> >
> >> I am looking at the Basic class and wonder what you are doing with the
> >> following statement:
> >>
> >> def __new__(cls, *args, **assumptions):
> >> obj = object.__new__(cls)
> >>
> >> What I am trying today is give the Symbol class a new data member so
> >> that is x is a symbol I can define
> >> x.latex_name to go with x.name. However the __new__ command for symbol
> >> is inscrutable to me so that if I define
> >> a cls.latex_name in __new__ for symbol it is global to the class. I
> >> asked about the above statement since the
> >> __new_stage2__ function returns a basic object and there is where the
> >> symbol name is stored.
> >>
> >
> > object.__new__ is the default object creator. The statement you cite
> > creates an empty instance (= without any instance attribute) of the
> > class cls and assigns it to the empty variable obj.
> > Generally speaking, __new__ is a special method that is called to create
> > instances of a class. Its return value is the instance you want to
> > create. A statement like x = Symbol(name, ...) is basically syntactic
> > sugar for x = Symbol.__new__(Symbol, name, ...). If Symbol, which is a
> > subclass of Basic, did not override Basic.__new__, the statement would

> > be equivalent to x = Basic.__new__(Symbol, name, ...) — compare the 2nd

That's because Symbol defines the special attribute __slots__, so it's
impossible to create instance attributes whose name isn't in
Symbol.__slots__.
You can just delete the line __slots__ = ['is_commutative', 'name']. It
will only impact performance, but nothing else, and your code should
work.

[Alan Bromborsky]

Ronan Lamy wrote:
> Le samedi 21 novembre 2009 � 17:17 -0500, Alan Bromborsky a �crit :
>
>> Ronan Lamy wrote:
>>
>>> Le vendredi 20 novembre 2009 � 16:15 -0500, Alan Bromborsky a �crit :

>>>
>>>
>>>> I am looking at the Basic class and wonder what you are doing with the
>>>> following statement:
>>>>
>>>> def __new__(cls, *args, **assumptions):
>>>> obj = object.__new__(cls)
>>>>
>>>> What I am trying today is give the Symbol class a new data member so
>>>> that is x is a symbol I can define
>>>> x.latex_name to go with x.name. However the __new__ command for symbol
>>>> is inscrutable to me so that if I define
>>>> a cls.latex_name in __new__ for symbol it is global to the class. I
>>>> asked about the above statement since the
>>>> __new_stage2__ function returns a basic object and there is where the
>>>> symbol name is stored.
>>>>
>>>>
>>> object.__new__ is the default object creator. The statement you cite
>>> creates an empty instance (= without any instance attribute) of the
>>> class cls and assigns it to the empty variable obj.
>>> Generally speaking, __new__ is a special method that is called to create
>>> instances of a class. Its return value is the instance you want to
>>> create. A statement like x = Symbol(name, ...) is basically syntactic
>>> sugar for x = Symbol.__new__(Symbol, name, ...). If Symbol, which is a
>>> subclass of Basic, did not override Basic.__new__, the statement would

>>> be equivalent to x = Basic.__new__(Symbol, name, ...) � compare the 2nd

Thank you. I just added latex_name to __slots__.