data attributes override method attributes?
 

Dear All,

In the Python Tutorial, Section 9.4, it is said that

"Data attributes override method attributes with the same name."

But in my testing as follows:

#Begin
class A:
i = 10
def i():
print 'i'

A.i
<unbound method A.i>
#End

I think A.i should be the number 10 but it is the method. There must be something I misunderstand. Would you please tell me why?

Thanks,

Jayden

Re: data attributes override method attributes?
 

On Sep 25, 11:41*pm, Jayden <jayden.s...@gmail.com> wrote:
> Dear All,
>
> In the Python Tutorial, Section 9.4, it is said that
>
> "Data attributes override method attributes with the same name."
>
> But in my testing as follows:
>
> #Begin
> class A:
> * * * * i = 10
> * * * * def i():
> * * * * * * * * print 'i'
>
> A.i
> * *<unbound method A.i>
> #End
>
> I think A.i should be the number 10 but it is the method. There must be something I misunderstand. Would you please tell me why?

What the tutorial is referring to is this, I think:

class A(object):
def i(self):
print 'i'

>>> a = A()
>>> a.i = 10
>>> a.i
10

That is, you can create a data attribute on an object even if a method
attribute of the same name exists.

Re: data attributes override method attributes?
 

Jayden wrote:

> In the Python Tutorial, Section 9.4, it is said that
>
> "Data attributes override method attributes with the same name."

The tutorial is wrong here. That should be

"Instance attributes override class attributes with the same name."

As methods are usually defined in the class and data attributes are usually
set in the instance it will look like data override method attributes.
What the author had in mind:

>>> class A:
.... def i(self): print "method"
....
>>>
>>> a = A()
>>> a.i()
method
>>> a.i = 42 # this could also happen in a method with self.i = 42
>>> a.i()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable
>>> a.i
42

> But in my testing as follows:
>
> #Begin
> class A:
> i = 10
> def i():
> print 'i'
>
> A.i
> <unbound method A.i>
> #End

but

class A:
def i(self): print "i"
i = 42

print A().i # 42

If two objects are assigned to the same name the last assignment always
wins.

> I think A.i should be the number 10 but it is the method. There must be
something I misunderstand. Would you please tell me why?

No, you're right. Please file a bug report at http://bugs.python.org

Re: data attributes override method attributes?
 

On Sep 26, 12:08*am, Peter Otten <__pete...@web.de> wrote:
> Jayden wrote:
> > In the Python Tutorial, Section 9.4, it is said that
>
> > "Data attributes override method attributes with the same name."
>
> The tutorial is wrong here. That should be
>
> "Instance attributes override class attributes with the same name."
>
> As methods are usually defined in the class and data attributes are usually
> set in the instance it will look like data override method attributes.

But you can assign attributes on the class, which has the same impact,
so the tutorial is correct.

> No, you're right. Please file a bug report athttp://bugs.python.org

Didn't you just demonstrate the behaviour you're now saying is a bug?

Re: data attributes override method attributes?
 

On Tue, 25 Sep 2012 06:41:43 -0700, Jayden wrote:

> Dear All,
>
> In the Python Tutorial, Section 9.4, it is said that
>
> "Data attributes override method attributes with the same name."
>
> But in my testing as follows:
>
> #Begin
> class A:
> i = 10
> def i():
> print 'i'
>
> A.i
> <unbound method A.i>
> #End
>
> I think A.i should be the number 10 but it is the method. There must be
> something I misunderstand. Would you please tell me why?

When you create the class, two things happen: first you define a class-
level attribute i, then you define a method i. Since you can only have a
single object with the same name in the same place, the method replaces
the attribute.

In this case, classes and methods are irrelevant. It is exactly the same
behaviour as this:

i = 10
i = 20
# i now equals 20, not 10


except that instead of 20, you use a function object:

i = 10
def i():
return "something"
# i is now a function object, not 10


What the manual refers to is the use of attributes on an instance:

py> class Test(object):
.... def f(self):
.... return "something"
....
py> t = Test()
py> t.f = 20
py> t.f
20

In this case, there is an attribute called "f" (a method) which lives in
the class and is shared by all instances, and another attribute called
"f" which lives in the instance t, is not shared, and has the value 20.
This instance attribute masks the method with the same name. We can see
that it is only hidden, not gone, by creating a new instance:

py> u = Test()
py> u.f
<bound method Test.f of <__main__.Test object at 0x871390c>>



--
Steven

Re: data attributes override method attributes?
 

alex23 wrote:

> On Sep 26, 12:08 am, Peter Otten <__pete...@web.de> wrote:
>> Jayden wrote:
>> > In the Python Tutorial, Section 9.4, it is said that
>>
>> > "Data attributes override method attributes with the same name."
>>
>> The tutorial is wrong here. That should be
>>
>> "Instance attributes override class attributes with the same name."
>>
>> As methods are usually defined in the class and data attributes are
>> usually set in the instance it will look like data override method
>> attributes.
>
> But you can assign attributes on the class, which has the same impact,
> so the tutorial is correct.
>
>> No, you're right. Please file a bug report athttp://bugs.python.org
>
> Didn't you just demonstrate the behaviour you're now saying is a bug?
>

To me

"Data attributes override method attributes with the same name"

implies that data attributes take precedence over method attributes, not
that they replace them only when there is an assignment of data after the
method definition.

With your interpretation (if I understand you correctly)

"Method attributes override data attributes with the same name"

is equally correct, and therefore I think it is misleading to focus on the
type of the attributes at all.

I would even consider replacing the whole paragraph

"""
Data attributes override method attributes with the same name; to avoid
accidental name conflicts, which may cause hard-to-find bugs in large
programs, it is wise to use some kind of convention that minimizes the
chance of conflicts. Possible conventions include capitalizing method names,
prefixing data attribute names with a small unique string (perhaps just an
underscore), or using verbs for methods and nouns for data attributes.
"""
http://docs.python.org/dev/py3k/tutorial/classes.html

with something like

"Data attributes and method attributes share the same namespace. To avoid
name conflicts consider using verbs for methods and nouns for data
attributes"

Re: data attributes override method attributes?
 

Am 25.09.2012 16:11, schrieb alex23:
> On Sep 26, 12:08 am, Peter Otten <__pete...@web.de> wrote:
>> Jayden wrote:
>>> In the Python Tutorial, Section 9.4, it is said that
>>
>>> "Data attributes override method attributes with the same name."
>>
>> The tutorial is wrong here. That should be
>>
>> "Instance attributes override class attributes with the same name."
>>
>> As methods are usually defined in the class and data attributes are usually
>> set in the instance it will look like data override method attributes.
>
> But you can assign attributes on the class, which has the same impact,
> so the tutorial is correct.

You can assign attributes of the class or the instance, and you can
assign with functions or data (actually, both functions and data are
objects, Python doesn't make a distinction there). The important thing
is that lookup first looks in the instance (where data attributes are
usually set) before looking in the class (where method attributes are
usually set). Observing typical use and deriving a rule from this is
misleading though.


>> No, you're right. Please file a bug report athttp://bugs.python.org
>
> Didn't you just demonstrate the behaviour you're now saying is a bug?

I think he meant a bug in the tutorial, not in the implementation of Python.


Uli

Re: data attributes override method attributes?
 

On Wed, Sep 26, 2012 at 12:54 AM, Peter Otten <__peter__@web.de> wrote:
> To me
>
> "Data attributes override method attributes with the same name"
>
> implies that data attributes take precedence over method attributes, not
> that they replace them only when there is an assignment of data after the
> method definition.
>
> I would even consider replacing the whole paragraph
> with something like
>
> "Data attributes and method attributes share the same namespace. To avoid
> name conflicts consider using verbs for methods and nouns for data
> attributes"

Instance attributes override (shadow) class attributes. Since methods
tend to be on the class and data tends to be on the instance, the
original sentence does make some sense. The section is talking about
conventions, so it's not inherently wrong, but perhaps just needs a
comment about methods not usually being attached to the instance.

ChrisA

Re: data attributes override method attributes?
 

Am 25.09.2012 16:08 schrieb Peter Otten:
> Jayden wrote:
>
>> In the Python Tutorial, Section 9.4, it is said that
>>
>> "Data attributes override method attributes with the same name."
>
> The tutorial is wrong here. That should be
>
> "Instance attributes override class attributes with the same name."

I jump in here:

THere is one point to consider: if you work with descriptors, it makes a
difference if they are "data descriptors" (define __set__ and/or
__delete__) or "non-data descriptors" (define neither).

As http://docs.python.org/reference/dat...ng-descriptors
tells us, methods are non-data descriptors, so they can be overridden by
instances.

OTOH, properties are data descriptors which cannot be overridden by the
instance.

So, to stick to the original example:

class TestDesc(object):
def a(self): pass
@property
def b(self): print "trying to get value - return None"; return None
@b.setter
def b(self, v): print "value", v, "ignored."
@b.deleter
def b(self): print "delete called and ignored"


and now

>>> t=TestDesc()
>>> t.a
<bound method TestDesc.a of <__main__.TestDesc object at 0xb7387ccc>>
>>> t.b
trying to get value - return None
>>> t.a=12
>>> t.b=12
value 12 ignored.
>>> t.a
12
>>> t.b
trying to get value - return None
>>> del t.a
>>> del t.b
delete called and ignored
>>> t.a
<bound method TestDesc.a of <__main__.TestDesc object at 0xb7387ccc>>
>>> t.b
trying to get value - return None


Thomas

Re: data attributes override method attributes?
 

On 9/25/2012 11:03 AM, Chris Angelico wrote:
> On Wed, Sep 26, 2012 at 12:54 AM, Peter Otten <__peter__@web.de> wrote:
>> To me
>>
>> "Data attributes override method attributes with the same name"
>>
>> implies that data attributes take precedence over method attributes, not
>> that they replace them only when there is an assignment of data after the
>> method definition.
>>
>> I would even consider replacing the whole paragraph
>> with something like
>>
>> "Data attributes and method attributes share the same namespace. To avoid
>> name conflicts consider using verbs for methods and nouns for data
>> attributes"
>
> Instance attributes override (shadow) class attributes.

except for (some? all?) special methods

> Since methods
> tend to be on the class and data tends to be on the instance, the
> original sentence does make some sense.

but it *is* wrong


The section is talking about
> conventions, so it's not inherently wrong,

The suggestion to Capitalize method names and prefix data names with '_'
are wrong with respect to the style guide.

but perhaps just needs a
> comment about methods not usually being attached to the instance.
>
> ChrisA
>


--
Terry Jan Reedy