Seemingly odd 'is' comparison.

[Tobiah]

>>print float(3.0) is float(3.0)
True

>>print float(3.0 * 1.0) is float(3.0)

False

>>>

Thanks,

Tobiah

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[Arnaud Delobelle]

Re: Seemingly odd 'is' comparison.

On Feb 13, 10:19 pm, Tobiah <t...@tobiah.or gwrote:

>print float(3.0) is float(3.0)

True

>print float(3.0 * 1.0) is float(3.0)

False

[You don't need to wrap your floats in float()]

>>def f():

... return 3.0 is 3.0, 3.0*1.0 is 3.0
...

>>f()

(True, False)

>>import dis
>>dis.dis(f)

2 0 LOAD_CONST 1 (3.0)
3 LOAD_CONST 1 (3.0)
6 COMPARE_OP 8 (is)
9 LOAD_CONST 3 (3.0)
12 LOAD_CONST 1 (3.0)
15 COMPARE_OP 8 (is)
18 BUILD_TUPLE 2
21 RETURN_VALUE

As you can see when "3.0 is 3.0" is evaluated the same float object is
put on the stack twice so the 'is' comparison is True (LOAD_CONST 1 /
LOAD_CONST 1 / COMPARE_OP 8).

Whereas when "3.0*1.0 is 3.0" is evaluated, *two* different float
objects are put on the stack and compared (LOAD_CONST 3 / LOAD_CONST
1 / COMPARE_OP 8). Therefore the result is False.

HTH

--
Arnaud

[Christian Heimes]

Re: Seemingly odd 'is' comparison.

Tobiah wrote:

>>>print float(3.0) is float(3.0)

True

>>>print float(3.0 * 1.0) is float(3.0)

False

>>>>

Thumb rule: Never compare strings, numbers or tuples with "is". Only
compare an object with a singleton like a type or None. "is" is not a
comparison operator.

Christian

[Erik Max Francis]

Re: Seemingly odd 'is' comparison.

Tobiah wrote:

>>>print float(3.0) is float(3.0)

True

>>>print float(3.0 * 1.0) is float(3.0)

False

>>>>

It's implementation dependent what values these expressions will take.

If you're trying to test equality, use `==`, not `is`.

--
Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
There is nothing more incomprehensibl e than a wrangle among
astronomers. -- H.L. Mencken

[Asun Friere]

Re: Seemingly odd 'is' comparison.

On Feb 19, 12:27 pm, Asun Friere <afri...@yahoo. co.ukwrote:

But given the nature of immutables

I meant to write "given the nature of mutables" of course ... :/