what is the keyword "is" for?

[daniel]

I'm so confused by the keyword "is" and "==" equal sign, it seems they
could be exchanged in some contexts, but not in others, what's the
difference between them in terms of comparation?

thanks...

daniel

[sjdevnull]

Sybren Stuvel wrote:

Dan Bishop enlightened us with:

>>a = b = 1e1000 / 1e1000
a is b

True

>>a == b

False

If "a is b" then they refer to the same object, hence a == b. It
cannot be otherwise, unless Python starts to defy logic. I copied your
code and got the expected result:

>a = b = 1e1000 / 1e1000
a is b

True

>a == b

True

Probably depends on the implementation. 1e1000/1e1000 yields a NaN
here, and I get True for "a is b" but False for "a==b". Presumably
comparing a NaN for equality (any comparison?) always yields False.

[Simon Forman]

Sybren Stuvel wrote:

Dan Bishop enlightened us with:

>>a = b = 1e1000 / 1e1000
a is b

True

>>a == b

False

If "a is b" then they refer to the same object, hence a == b. It
cannot be otherwise, unless Python starts to defy logic. I copied your
code and got the expected result:

>a = b = 1e1000 / 1e1000
a is b

True

>a == b

True

Sybren

Python 2.4.3 (#2, Apr 27 2006, 14:43:58)
[GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

|>a = b = 1e1000 / 1e1000
|>a is b
True
|>a == b
False

Huh. Weird. I copied it too and got the "wrong" result.

[Marc 'BlackJack' Rintsch]

In <sl************ **********@schu imige.stuvel.eu >, Sybren Stuvel wrote:

Dan Bishop enlightened us with:

>>>>a = b = 1e1000 / 1e1000
a is b

True

>>>>a == b

False

If "a is b" then they refer to the same object, hence a == b. It
cannot be otherwise, unless Python starts to defy logic. I copied your
code and got the expected result:

>>>a = b = 1e1000 / 1e1000
a is b

True

>>>a == b

True

I get the same as Dan:

In [13]: a = b = 1e1000 / 1e1000

In [14]: a is b
Out[14]: True

In [15]: a == b
Out[15]: False

In [16]: a
Out[16]: nan

On my platform the division results in "Not A Number". Two NaNs compared
are always `False`. You could argue that this is the very same NaN but to
get this effect the interpreter has to take care that every NaN produced
while a program is running is unique. Quite huge overhead for such a
corner case IMHO.

Ciao,
Marc 'BlackJack' Rintsch

[Dan Sommers]

On Wed, 16 Aug 2006 10:06:03 +0200,
Marc 'BlackJack' Rintsch <bj****@gmx.net wrote:

In [14]: a is b
Out[14]: True

In [15]: a == b
Out[15]: False

In [16]: a
Out[16]: nan

On my platform the division results in "Not A Number". Two NaNs
compared are always `False`. You could argue that this is the very
same NaN but to get this effect the interpreter has to take care that
every NaN produced while a program is running is unique. Quite huge
overhead for such a corner case IMHO.

The interpreter isn't doing anything special; nans have [the equivalent
of] an __eq__ method that always returns False.

Regards,
Dan

--
Dan Sommers
<http://www.tombstoneze ro.net/dan/>
"I wish people would die in alphabetical order." -- My wife, the genealogist

[Sion Arrowsmith]

Simon Forman <ro*********@ya hoo.comwrote:

>Python 2.4.3 (#2, Apr 27 2006, 14:43:58)
[GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

|>a = b = 1e1000 / 1e1000
|>a is b
True
|>a == b
False

I agree with you:

$ python
Python 2.4.1 (#2, May 5 2005, 11:32:06)
[GCC 3.3.5 (Debian 1:3.3.5-12)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

>>a = b = 1e1000 / 1e1000
a is b

True

>>a == b

False

Or maybe I don't:

$ python2.3
Python 2.3.5 (#2, Sep 4 2005, 22:01:42)
[GCC 3.3.5 (Debian 1:3.3.5-13)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

>>a = b = 1e1000 / 1e1000
a is b

True

>>a == b

True

See:

http://mail.python.org/pipermail/pyt...ry/022133.html

--
\S -- si***@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
___ | "Frankly I have no feelings towards penguins one way or the other"
\X/ | -- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

[Alex Martelli]

Sybren Stuvel <sy*******@YOUR thirdtower.com. imaginationwrot e:

Dan Bishop enlightened us with:

>>a = b = 1e1000 / 1e1000
a is b

True

>>a == b

False

If "a is b" then they refer to the same object, hence a == b. It
cannot be otherwise, unless Python starts to defy logic. I copied your

Python also needs to respect standards (such as IEEE 754 for
floating-point arithmetic, and the SQL standards) which do specify the
existence of special objects that "are not equal to anything" including
themselves -- Nan and NULL respectively for these two standards.

We're talking about extremely widespread international standards
developed by huge body of professionals which do include professional
logicians, so I doubt they "defy logic". Python tries to delegate FP to
the underlying hardware and SQL to an external relational DB engine, so
it should be as compliant as the pieces of infrastructure it's using --
which seems to me to be a good architectural decision (not just for
speed and ease of coding, but to ensure any anomaly is somebody else's
fault:-).
Alex