Looking for assignement operator

[Alexander Eisenhuth]

Hello,

is there a assignement operator, that i can overwrite?

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

a = MyInt(10)

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


Thanks
Alexander

 

[Simon Brunning]

Hello,

is there a assignement operator, that i can overwrite?

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

 

[Rob Wolfe]

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]

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(isinstance(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]

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(isinstance(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

 

[Jerry]

Hello,

is there a assignement operator, that i can overwrite?

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

_val = property(self.getval, self.setval)

 

[Bruno Desthuilliers]

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.



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]

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(isinstance(val, int))
self._val = val

_val = property(self.getval, 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(_getval, _setval)


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

 

[Alexander Eisenhuth]

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]

class MyClassescriptors 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.

 

[Bruno Desthuilliers]

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(_getval, _setval)

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(isinstance(val, int))
self._val = val

_val = property(self.getval, 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.getval, self.setval)
NameError: name 'self' is not defined


HTH

 

[Jerry]

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?

 

[Bruno Desthuilliers]

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 ?

 

[Tommi]

Could the "traits" package be of help?

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

 

[Bruno Desthuilliers]

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

Could the "traits" package be of help?

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

How could it help ?

 

[Robert Kern]

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


How could it help ?

It doesn't. (I am an Enthought developer.)

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

 

[Tommi]

(please don't top-post - corrected)
(sorry)


How could it help ?

To me they just looked a bit alike:

--- op's example ---
a = MyInt(10)
# Here i need to overwrite the assignement operator
a = 12

--- traits' example ---
moe = Child()
# NOTIFICATION in action
moe.age = 10

 

[Bruno Desthuilliers]

Bruno Desthuilliers wrote:
(about Traits)


To me they just looked a bit alike:

--- op's example ---
a = MyInt(10)
# Here i need to overwrite the assignement operator
a = 12

--- traits' example ---
moe = Child()
# NOTIFICATION in action
moe.age = 10

You do understand the difference between rebinding a name and modifying
a mutable object, do you ?

FWIW, you definitively don't need Traits here - properties are enough.

class Something(object):
@apply
def age():
def fget(self):
return self._age
def fset(self, val):
self._age = MyInt(val)
return property(**locals())

s = Something()
s.age = 42
type(s.age)

But this is not what the OP was asking for...