Looking for assignement operator

[Alexander Eisenhuth]

Hello,

is there a assignement operator, that i can overwrite?

class MyInt:
def __init__(self, val):
assert(isinstan ce(val, int))
self._val = val

a = MyInt(10)

# Here i need to overwrite the assignement operator
a = 12


Thanks
Alexander

[Simon Brunning]

Re: Looking for assignement operator

On 10/17/06, Alexander Eisenhuth <newsuser@staco m-software.dewrot e:

Hello,
>
is there a assignement operator, that i can overwrite?

Soirry, no, assignment is a statement, not an operator, and can't be overridden.

--
Cheers,
Simon B
simon@brunningo nline.net

http://www.brunningonline.net/simon/blog/

[Rob Wolfe]

Re: Looking for assignement operator


Alexander Eisenhuth wrote:

Hello,
>
is there a assignement operator, that i can overwrite?

You can't overwrite assignment operator, but you can
overwrite methods of numeric objects:

http://docs.python.org/ref/numeric-types.html

HTH,
Rob

[Laurent Pointal]

Re: Looking for assignement operator

Alexander Eisenhuth a écrit :

Hello,
>
is there a assignement operator, that i can overwrite?

Adding to Simon Brunning reply (assignment is a statement).

class MyInt:
def __init__(self, val):
assert(isinstan ce(val, int))
self._val = val
>
a = MyInt(10)
>
# Here i need to overwrite the assignement operator
a = 12

Here you bind the 12 (Python int value) to name 'a', then 'a' has the
int type, not your MyInt (which value has been lost).

You may define a 'set' method, and write:
a = MyInt(10)
a.set(12)

And, if a is a member of another class, you may define an accessor for
that 'a' member in that class, which automatically call your set method
when giving an int value.

b.a = MyInt(10)
b.a = 12 ---b.a.set(12)

A+

Laurent.

[Steven D'Aprano]

Re: Looking for assignement operator

On Tue, 17 Oct 2006 15:50:47 +0200, Alexander Eisenhuth wrote:

Hello,
>
is there a assignement operator, that i can overwrite?

No.

We were just discussing the reasons why Python will not and can not have
an assignment operator just a few days ago. Check the archives for more
details.

class MyInt:
def __init__(self, val):
assert(isinstan ce(val, int))

isinstance() considered harmful:

http://www.canonical.org/~kragen/isinstance/

self._val = val

Seems kind of pointless. What does MyInt do that ordinary ints don't?
Apart from slow your program down and require extra programming effort.

a = MyInt(10)
>
# Here i need to overwrite the assignement operator a = 12

Can't happen. "a" is just a name, not an object with methods that can be
called. "a" can be bound to anything, not just MyInt instances. Objects
like MyInt(10) can have no name, one name or many names:

mylist = [0, MyInt(10), 20, 30] # MyInt instance has no name
x = MyInt(10) # MyInt instance has one name
x = y = z = MyInt(10) # MyInt instance has many names



--
Steven.

[Jerry]

Re: Looking for assignement operator



On Oct 17, 8:50 am, Alexander Eisenhuth <newsu...@staco m-software.de>
wrote:

Hello,
>
is there a assignement operator, that i can overwrite?
>
class MyInt:
def __init__(self, val):
assert(isinstan ce(val, int))
self._val = val
>
a = MyInt(10)
>
# Here i need to overwrite the assignement operator
a = 12
>
Thanks
Alexander

I believe the property function is what you are looking for. e.g.

class MyClass:
def __init__(self, val):
self.setval(val )

def getval(self):
return self._val

def setval(self, val):
assert(isinstan ce(val, int))
self._val = val

_val = property(self.g etval, self.setval)

--
Jerry

[Bruno Desthuilliers]

Re: Looking for assignement operator

Steven D'Aprano wrote:

On Tue, 17 Oct 2006 15:50:47 +0200, Alexander Eisenhuth wrote:
>

>Hello,
>>
>is there a assignement operator, that i can overwrite?

>
No.
>
We were just discussing the reasons why Python will not and can not have
an assignment operator just a few days ago. Check the archives for more
details.
>
>

>class MyInt:
> def __init__(self, val):
> assert(isinstan ce(val, int))

>
isinstance() considered harmful:

Trying to convert val to an int would probably be better indeed:

class MyInt(object):
def __init__(self, val):
self.val = int(val)

My 2 cents
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Bruno Desthuilliers]

Re: Looking for assignement operator

Jerry wrote:
(snip)

I believe the property function is what you are looking for.

It is not.

e.g.
>
class MyClass:

Descriptors don't work fine with old-style classes. Should be:

class MyClass(object) :

def __init__(self, val):
self.setval(val )
>
def getval(self):
return self._val
>
def setval(self, val):
assert(isinstan ce(val, int))
self._val = val
>
_val = property(self.g etval, self.setval)

NameError : self is not defined.
Should be :
_val = property(getval , setval)

but then - since setval() now calls _vals.__set__() , which itself calls
setval(), you have a nice infinite recursion (well, almost infinite -
hopefully, Python takes care of it).

May I kindly suggest that you learn more about properties and test your
code before posting ?-)

Anyway, even with the following correct code, this won't solve the OP's
question:
class MyClass(object) :
def __init__(self, val):
self.val = val

def _getval(self):
return self._val

def _setval(self, val):
self._val = int(val)

val = property(_getva l, _setval)


m = MyClass(42)
m
=<__main__.MyCl ass object at 0x2ae5eaa00410>
m.val
=42
m = 42
m
=42
type(m)
=<type 'int'>


--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Alexander Eisenhuth]

Re: Looking for assignement operator

Wow, thanks a lot for your quick answers.

That assignement is no operator, but a statemant is a pity, but indeed I came
foward with overwritten methods for numeric types

Regards
Alexander

[Jerry]

Re: Looking for assignement operator

class MyClass:Descrip tors don't work fine with old-style classes.
Interesting, I have used this construct before in Python 2.4.3 and not
run into the recursion problem you talk about. Also, it has worked
fine for me. Perhaps you can post a link to your source so that I
could study it and understand what circumstances my solution works and
what the recommended construct actually is.

May I kindly suggest that you learn more about properties and test your
code before posting ?-)

I did test this on Python 2.4.3 in Mac OS X 10.4 and it worked fine.

--
Jerry

[Bruno Desthuilliers]

Re: Looking for assignement operator

Jerry wrote:

>class MyClass:Descrip tors don't work fine with old-style classes.

Interesting, I have used this construct before in Python 2.4.3 and not
run into the recursion problem you talk about.

The recursion problem doesn't occur with you original code (for the good
reason that there's a name error way before). It doesn't even occur when
the cause of the name error is corrected, since the first (explicit)
call to setval() in the __init__ rebind self._val to the value passed -
so the property is in fact *never* used.

Also, it has worked
fine for me.

For a very peculiar definition of "works fine" !-)

Perhaps you can post a link to your source

class MyClass:
def __init__(self, val):
self.setval(val )
print "in __init__, after setval(): self._val is %s" \
% type(self._val)

def getval(self):
print "in getval - you won't see me unless you explicitely call
getval"
return self._val

def setval(self, val):
print "in setval"
self._val = val
print "you wont see me no more unless you explicitely call setval"

_val = property(getval , setval)

so that I
could study it and understand what circumstances my solution works

It doesn't work in any circumstances.

and
what the recommended construct actually is.

class MyWorkingClass( object):
def __init__(self, val):
self.val = val

def _setval(self, val):
print "_setval to %s" % val
self._val = val

def _getval(self):
print "_getval"
return self._val

val = property(_getva l, _setval)

>May I kindly suggest that you learn more about properties and test your
>code before posting ?-)

I did test this on Python 2.4.3 in Mac OS X 10.4 and it worked fine.

Here's the exact code you posted:

class MyClass:
def __init__(self, val):
self.setval(val )

def getval(self):
return self._val

def setval(self, val):
assert(isinstan ce(val, int))
self._val = val

_val = property(self.g etval, self.setval)

And here's the result:

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/usr/tmp/python-30955cPK.py", line 1, in ?
class MyClass:
File "/usr/tmp/python-30955cPK.py", line 15, in MyClass
_val = property(self.g etval, self.setval)
NameError: name 'self' is not defined


HTH
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Jerry]

Re: Looking for assignement operator

Okay, very well, then I put a couple of extra 'self' identifiers in
there when I hand-copied the code over. That would be my mistake for
letting my fingers do the walking and forgetting my brain. Is there
anything else wrong with my code?

--
Jerry

[Bruno Desthuilliers]

Re: Looking for assignement operator

Jerry wrote:

Okay, very well, then I put a couple of extra 'self' identifiers in
there when I hand-copied the code over.

You should try copy/paste - it's both safer and less work !-)

That would be my mistake for
letting my fingers do the walking and forgetting my brain. Is there
anything else wrong with my code?

You mean something I didn't cover in my 2 previous posts ?

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

[Tommi]

Re: Looking for assignement operator

Could the "traits" package be of help?

http://code.enthought.com/traits/

Alexander Eisenhuth wrote:

Hello,
>
is there a assignement operator, that i can overwrite?
>
class MyInt:
def __init__(self, val):
assert(isinstan ce(val, int))
self._val = val
>
a = MyInt(10)
>
# Here i need to overwrite the assignement operator
a = 12
>
>
Thanks
Alexander

[Bruno Desthuilliers]

Re: Looking for assignement operator

Tommi wrote:
(please don't top-post - corrected)

>
>
Alexander Eisenhuth wrote:

>Hello,
>>
>is there a assignement operator, that i can overwrite?
>>
>class MyInt:
> def __init__(self, val):
> assert(isinstan ce(val, int))
> self._val = val
>>
>a = MyInt(10)
>>
># Here i need to overwrite the assignement operator
>a = 12
>>

Could the "traits" package be of help?
>

http://code.enthought.com/traits/

>

How could it help ?

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"