Overwriting property-> can't set attribute

[Gregor Horvath]

Hi,

why is this code failing?

class B(object):
pass

B.testattr = property(lambda s:"hallo")
b = B()
b.testattr = "test"


Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)

/tmp/python-14202ViU.py in <module>()
14 B.testattr = property(lambda s:"hallo")
15 b = B()
---16 b.testattr = "test"
17
18

<type 'exceptions.Att ributeError'>: can't set attribute

--
Greg

[Gregor Horvath]

Re: Overwriting property-&gt; can't set attribute

Gregor Horvath schrieb:

>
why is this code failing?

OK I answer myself :-)

Because there is not fset function definied in the property.
I have to del the attr before rebinding the attributename to another object.

--
Greg

[Raymond Hettinger]

Re: Overwriting property-&gt; can't set attribute

On Aug 22, 5:38 am, Gregor Horvath <g...@gregor-horvath.comwrot e:

why is this code failing?
>
class B(object):
     pass
>
B.testattr = property(lambda s:"hallo")
b = B()
b.testattr = "test"

First, property() only works when attached to classes, not instances.
So the assignment should be: B.testattr = property(...)

Second, the property() call in you example only defines a getter
function (the first argument) and not a setter function (the second
argument) it defaults to a read-only property. That is why
b.testattr='tes t' would still fail.

Raymond

[Bruno Desthuilliers]

Re: Overwriting property-&gt; can't set attribute

Raymond Hettinger a écrit :

On Aug 22, 5:38 am, Gregor Horvath <g...@gregor-horvath.comwrot e:

>why is this code failing?
>>
>class B(object):
> pass
>>
>B.testattr = property(lambda s:"hallo")
>b = B()
>b.testattr = "test"

>
First, property() only works when attached to classes, not instances.
So the assignment should be: B.testattr = property(...)

Mmm... You may want to reread the OP code more carefully !-)

[norseman]

Re: Overwriting property-&gt; can't set attribute

Gregor Horvath wrote:

Hi,
>
why is this code failing?
>
class B(object):
pass
>
B.testattr = property(lambda s:"hallo")
b = B()
b.testattr = "test"
>
>
Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
>
/tmp/python-14202ViU.py in <module>()
14 B.testattr = property(lambda s:"hallo")
15 b = B()
---16 b.testattr = "test"
17
18
>
<type 'exceptions.Att ributeError'>: can't set attribute
>
--
Greg
--

http://mail.python.org/mailman/listinfo/python-list

>

=============== =============== ======

b = B() # synonyms
When B.testattr = "hallo" so does b.testattr
So if in subsequent code:
B.testattr = property(lambda s:"test")
Then:
b.testattr yields "test"
unless b was essentially destroyed/reused in between times


this is how/why things like:
gc = damn-lot-of-typing-due-to-long-names
gc.something
works as if it was:
damn-lot-of-typing-due-to-long-names.something


Steve
norseman@hughes .net

[Bruno Desthuilliers]

Re: Overwriting property-&gt; can't set attribute

norseman a écrit :

Gregor Horvath wrote:

>Hi,
>>
>why is this code failing?
>>
>class B(object):
> pass
>>
>B.testattr = property(lambda s:"hallo")
>b = B()
>b.testattr = "test"
>>
>>
>Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
>>
>/tmp/python-14202ViU.py in <module>()
> 14 B.testattr = property(lambda s:"hallo")
> 15 b = B()
>---16 b.testattr = "test"
> 17
> 18
>>
><type 'exceptions.Att ributeError'>: can't set attribute
>>
>--
>Greg
>--
>http://mail.python.org/mailman/listinfo/python-list
>>

=============== =============== ======
>
b = B() # synonyms

Not exactly, no. You probably missed the call operator applied to B.

(snip erroneous explanation).

[Bruno Desthuilliers]

Re: Overwriting property-&gt; can't set attribute

Gregor Horvath a écrit :

Hi,
>
why is this code failing?
>
class B(object):
pass
>
B.testattr = property(lambda s:"hallo")
b = B()
b.testattr = "test"
>
>
Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
>
/tmp/python-14202ViU.py in <module>()
14 B.testattr = property(lambda s:"hallo")
15 b = B()
---16 b.testattr = "test"
17
18
>
<type 'exceptions.Att ributeError'>: can't set attribute
>

It's not failing, it's doing exactly what's expected. You made testattr
a read-only property by not providing a setter.