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Looking for assignement operator

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
Oct 17 '06 #1
17 1459
On 10/17/06, Alexander Eisenhuth <ne******@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
si***@brunningo nline.net
http://www.brunningonline.net/simon/blog/
Oct 17 '06 #2

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

Oct 17 '06 #3
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.
Oct 17 '06 #4
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.

Oct 17 '06 #5


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

Oct 17 '06 #6
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 'o****@xiludom. gro'.split('@')])"
Oct 17 '06 #7
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 'o****@xiludom. gro'.split('@')])"
Oct 17 '06 #8
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
Oct 17 '06 #9
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

Oct 17 '06 #10

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