On 03/01/13 03:18, Alan Gauld wrote:
This is going somewhat off-topic but my curiosity is roused...
On 02/01/13 15:16, Oscar Benjamin wrote:
When the idea was discussed in the run up to Python 3, Guido raised
exactly this case and said
"""...
(BTW Pascal also had the division operator right, unlike C, and we're
... If we had done it that way, we wouldn't have had to introduce
the index() builtin and the corresponding infrastructure (__index__
and a whole slew of C APIs).
I don't get the reference to index here.
Why would adopting Pascal style division remove the need for index?
No, the comment about division was a parenthetical aside:
"(BTW Pascal also had the division operator right, unlike C, and we're
finally fixing this in Py3k by following Pascal's nearly-40-year-old
lead.)"
The bit about __index__ refers to using trunc():
"I still really wish I had followed Pascal's lead instead of C's here:
Pascal requires you to use trunc() to convert a real to an integer. ...
If we had done it that way, we wouldn't have had to introduce the
index() builtin and the corresponding infrastructure (__index__
and a whole slew of C APIs)."
I don't know what this "index() builtin" is, it doesn't appear to exist.
But __index__ is a special method that converts to int without rounding
or truncating, intended only for types that emulate ints but not other
numeric types:
py> (123).__index__()
123
py> (123.0).__index__()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'float' object has no attribute '__index__'
The purpose is to allow you to use custom integer-types in sequence
indexing, e.g. if n = MyInteger(42), you can use mylist[n] and Python
will call n.__index__() to convert to a proper int. In the past,
Python would only allow actual ints or longs for indexing.
Python cannot use the int() builtin or the regular __int__ method to do
the conversion because they will happily convert floats and strings, and
you don't want to allow mylist['42'] or mylist[42.0] to succeed. So
there needs to be a second special method for converting integer-types
to real ints.
As is often the case, this need was driven by the numpy community, if I
remember correctly, which has int8, int16, int32 and int64 types that
don't inherit from int.
--
Steven
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