numpy-discussion

[Numpy-discussion] attributes of scalar types - e.g. numpy.int32.itemsize

[Sebastian H. <ha...@ms...>]

Hi,
array dtype descriptors have an attribute itemsize  that gives the total 
number of bytes required for an item of that dtype.

Scalar types, like  numy.int32, also have that attribute,
but it returns "something else" - don't know what:
>>> a.dtype.itemsize
4
>>> a.dtype.name
'float32'
>>> N.int32.itemsize
<attribute 'itemsize' of 'genericscalar' objects>

Furthermore there are *lot's*  of more attributes to a scalar dtype, e.g.

>>> N.int32.data
<attribute 'data' of 'genericscalar' objects>
>>> N.int32.argmax()
Traceback (most recent call last):
  File "<input>", line 1, in ?
TypeError: descriptor 'argmax' of 'genericscalar' object needs an argument

Are those useful ? 

Thanks,
Sebastian Haase

Re: [Numpy-discussion] attributes of scalar types - e.g. numpy.int32.itemsize

[Travis O. <oli...@ie...>]

Sebastian Haase wrote:
> Hi,
> array dtype descriptors have an attribute itemsize  that gives the total 
> number of bytes required for an item of that dtype.
>
> Scalar types, like  numy.int32, also have that attribute,
> but it returns "something else" - don't know what:
>   
>
> Furthermore there are *lot's*  of more attributes to a scalar dtype, e.g.
>   

The scalar types are actual Python types (classes) whereas the dtype 
objects are instances.

The attributes you are seeing of the typeobject are very useful when you 
have an instance of that type.

With numpy.int32.itemsize you are doing the equivalent of

numpy.dtype.itemsize


-Travis

Re: [Numpy-discussion] =?iso-8859-1?q?attributes_of_scalar_types_-_e?= =?iso-8859-1?q?=2Eg=2E=09numpy=2Eint32=2Eitemsize?=

[Sebastian H. <ha...@ms...>]

On Friday 18 August 2006 11:38, Travis Oliphant wrote:
> Sebastian Haase wrote:
> > Hi,
> > array dtype descriptors have an attribute itemsize  that gives the total
> > number of bytes required for an item of that dtype.
> >
> > Scalar types, like  numy.int32, also have that attribute,
> > but it returns "something else" - don't know what:
> >
> >
> > Furthermore there are *lot's*  of more attributes to a scalar dtype, e.g.
>
> The scalar types are actual Python types (classes) whereas the dtype
> objects are instances.
>
> The attributes you are seeing of the typeobject are very useful when you
> have an instance of that type.
>
> With numpy.int32.itemsize you are doing the equivalent of
> numpy.dtype.itemsize

but why then do I not get the result 4 ?

-Sebastian

Re: [Numpy-discussion] attributes of scalar types - e.g. numpy.int32.itemsize

[Sebastian H. <ha...@ms...>]

On Friday 18 August 2006 15:25, Travis Oliphant wrote:
> Sebastian Haase wrote:
> > On Friday 18 August 2006 11:38, Travis Oliphant wrote:
> >> Sebastian Haase wrote:
> >>> Hi,
> >>> array dtype descriptors have an attribute itemsize  that gives the
> >>> total number of bytes required for an item of that dtype.
> >>>
> >>> Scalar types, like  numy.int32, also have that attribute,
> >>> but it returns "something else" - don't know what:
> >>>
> >>>
> >>> Furthermore there are *lot's*  of more attributes to a scalar dtype,
> >>> e.g.
> >>
> >> The scalar types are actual Python types (classes) whereas the dtype
> >> objects are instances.
> >>
> >> The attributes you are seeing of the typeobject are very useful when you
> >> have an instance of that type.
> >>
> >> With numpy.int32.itemsize you are doing the equivalent of
> >> numpy.dtype.itemsize
> >
> > but why then do I not get the result 4 ?
>
> Because it's not a "class" attribute, it's an instance attribute.
>
> What does numpy.dtype.itemsize give you?
>
I'm really sorry for being so dumb - but HOW can I get then the number of 
bytes needed by a given scalar type ?

-S.

Re: [Numpy-discussion] attributes of scalar types - e.g. numpy.int32.itemsize

[Travis O. <oli...@ie...>]

Sebastian Haase wrote:
> On Friday 18 August 2006 11:38, Travis Oliphant wrote:
>   
>> Sebastian Haase wrote:
>>     
>>> Hi,
>>> array dtype descriptors have an attribute itemsize  that gives the total
>>> number of bytes required for an item of that dtype.
>>>
>>> Scalar types, like  numy.int32, also have that attribute,
>>> but it returns "something else" - don't know what:
>>>
>>>
>>> Furthermore there are *lot's*  of more attributes to a scalar dtype, e.g.
>>>       
>> The scalar types are actual Python types (classes) whereas the dtype
>> objects are instances.
>>
>> The attributes you are seeing of the typeobject are very useful when you
>> have an instance of that type.
>>
>> With numpy.int32.itemsize you are doing the equivalent of
>> numpy.dtype.itemsize
>>     
>
> but why then do I not get the result 4 ?
>   
Because it's not a "class" attribute, it's an instance attribute.

What does numpy.dtype.itemsize give you?

-Travis

Re: [Numpy-discussion] attributes of scalar types - e.g. numpy.int32.itemsize

[Travis O. <oli...@ie...>]

Sebastian Haase wrote:
> On Friday 18 August 2006 15:25, Travis Oliphant wrote:
>   
>> Sebastian Haase wrote:
>>     
>>> On Friday 18 August 2006 11:38, Travis Oliphant wrote:
>>>       
>>>> Sebastian Haase wrote:
>>>>         
>>>>> Hi,
>>>>> array dtype descriptors have an attribute itemsize  that gives the
>>>>> total number of bytes required for an item of that dtype.
>>>>>
>>>>> Scalar types, like  numy.int32, also have that attribute,
>>>>> but it returns "something else" - don't know what:
>>>>>
>>>>>
>>>>> Furthermore there are *lot's*  of more attributes to a scalar dtype,
>>>>> e.g.
>>>>>           
>>>> The scalar types are actual Python types (classes) whereas the dtype
>>>> objects are instances.
>>>>
>>>> The attributes you are seeing of the typeobject are very useful when you
>>>> have an instance of that type.
>>>>
>>>> With numpy.int32.itemsize you are doing the equivalent of
>>>> numpy.dtype.itemsize
>>>>         
>>> but why then do I not get the result 4 ?
>>>       
>> Because it's not a "class" attribute, it's an instance attribute.
>>
>> What does numpy.dtype.itemsize give you?
>>
>>     
> I'm really sorry for being so dumb - but HOW can I get then the number of 
> bytes needed by a given scalar type ?
>
>   
Ah, the real question.  Sorry for not catching it earlier.  I've been in 
"make sure this isn't a bug mode" for a long time.

If you have a scalar type you could create one and then check the itemsize:

int32(0).itemsize

Or you could look at the name and parse out how big it is.

There is also a stored dictionary-like object that returns the number of 
bytes for any data-type recognized:

numpy.nbytes[int32]


-Travis

Re: [Numpy-discussion] =?iso-8859-1?q?attributes_of_scalar_types_-_e?= =?iso-8859-1?q?=2Eg=2E=09numpy=2Eint32=2Eitemsize?=

[Sebastian H. <ha...@ms...>]

On Friday 18 August 2006 16:51, Travis Oliphant wrote:
> Sebastian Haase wrote:
> > On Friday 18 August 2006 15:25, Travis Oliphant wrote:
> >> Sebastian Haase wrote:
> >>> On Friday 18 August 2006 11:38, Travis Oliphant wrote:
> >>>> Sebastian Haase wrote:
> >>>>> Hi,
> >>>>> array dtype descriptors have an attribute itemsize  that gives the
> >>>>> total number of bytes required for an item of that dtype.
> >>>>>
> >>>>> Scalar types, like  numy.int32, also have that attribute,
> >>>>> but it returns "something else" - don't know what:
> >>>>>
> >>>>>
> >>>>> Furthermore there are *lot's*  of more attributes to a scalar dtype,
> >>>>> e.g.
> >>>>
> >>>> The scalar types are actual Python types (classes) whereas the dtype
> >>>> objects are instances.
> >>>>
> >>>> The attributes you are seeing of the typeobject are very useful when
> >>>> you have an instance of that type.
> >>>>
> >>>> With numpy.int32.itemsize you are doing the equivalent of
> >>>> numpy.dtype.itemsize
> >>>
> >>> but why then do I not get the result 4 ?
> >>
> >> Because it's not a "class" attribute, it's an instance attribute.
> >>
> >> What does numpy.dtype.itemsize give you?
> >
> > I'm really sorry for being so dumb - but HOW can I get then the number of
> > bytes needed by a given scalar type ?
>
> Ah, the real question.  Sorry for not catching it earlier.  I've been in
> "make sure this isn't a bug mode" for a long time.
>
> If you have a scalar type you could create one and then check the itemsize:
>
> int32(0).itemsize
>
> Or you could look at the name and parse out how big it is.
>
> There is also a stored dictionary-like object that returns the number of
> bytes for any data-type recognized:
>
> numpy.nbytes[int32]

Thanks, that seems to be a handy "dictionary-like object"

Just for the record - in the meantime I found this:
>>> N.dtype(N.int32).itemsize
4

Cheers,
Sebastian

Re: [Numpy-discussion] attributes of scalar types - e.g. numpy.int32.itemsize

[Charles R H. <cha...@gm...>]

Hi,

On 8/18/06, Sebastian Haase <ha...@ms...> wrote:

<snip>

Thanks, that seems to be a handy "dictionary-like object"
>
> Just for the record - in the meantime I found this:
> >>> N.dtype(N.int32).itemsize
> 4


And on x86_64 linux python ints are 8 bytes.

In [15]: asarray([1])[0].itemsize
Out[15]: 8

Interesting. Looks like one needs to be careful about the builtin python
types.

Chuck