Difference between 'is' and '=='

[mwql]

Hey guys, this maybe a stupid question, but I can't seem to find the
result anywhere online. When is the right time to use 'is' and when
should we use '=='?

Thanks alot~

[Rene Pijlman]

Re: Difference between 'is' and '=='

mwql:[color=blue]
>Hey guys, this maybe a stupid question, but I can't seem to find the
>result anywhere online. When is the right time to use 'is' and when
>should we use '=='?[/color]

http://docs.python.org/ref/comparisons.html

--
René Pijlman

[Max M]

Re: Difference between 'is' and '=='

mwql wrote:
[color=blue]
> Hey guys, this maybe a stupid question, but I can't seem to find the
> result anywhere online. When is the right time to use 'is' and when
> should we use '=='?[/color]

"is" is like id(obj1) == id(obj2)
[color=blue][color=green][color=darkred]
>>> 100+1 == 101[/color][/color][/color]
True
[color=blue][color=green][color=darkred]
>>> 100+1 is 101[/color][/color][/color]
False

They don't have the same id. (Think of id as memory adresses.)

--

hilsen/regards Max M, Denmark

http://www.mxm.dk/

IT's Mad Science

Phone: +45 66 11 84 94
Mobile: +45 29 93 42 96

[Fuzzyman]

Re: Difference between 'is' and '=='


mwql wrote:[color=blue]
> Hey guys, this maybe a stupid question, but I can't seem to find the
> result anywhere online. When is the right time to use 'is' and when
> should we use '=='?
>
> Thanks alot~[/color]

'==' is the equality operator. It is used to test if two objects are
'equal'.

'is' is the identity operator, it is used to test if two
names/references point to the same object.

a = {'a': 3}
b = {'a': 3}
a == b
True
a is b
False
c = a
a is c
True

The two dictionaries a and b are equal, but are separate objects.
(Under the hood, Python uses 'id' to determine identity).

When you bind another name 'c' to point to dictionary a, they *are* the
same object - so a *is* c.

One place the 'is' operator is commonly used is when testing for None.
You only ever have one instance of 'None', so

a is None

is quicker than

a == None

(It only needs to check identity not value.)

I hope that helps.

Fuzzyman
http://www.voidspace.org.uk/python/index.shtml

[Joel Hedlund]

Re: Difference between 'is' and '=='

> "is" is like id(obj1) == id(obj2)
<snip>[color=blue]
> (Think of id as memory adresses.)[/color]

Which means that "is" comparisons in general will be faster than ==
comparisons. According to PEP8 (python programming style guidelines) you should
use 'is' when comparing to singletons like None. I take this to also include
constants and such. That allows us to take short cuts through known terrain,
such as in the massive_computa tions function below:

--------------------------------------------------------------
import time

class LotsOfData(obje ct):
def __init__(self, *data):
self.data = data
def __eq__(self, o):
time.sleep(2) # time consuming computations...
return self.data == o.data

KNOWN_DATA = LotsOfData(1,2)
same_data = KNOWN_DATA
equal_data = LotsOfData(1,2)
other_data = LotsOfData(2,3)

def massive_computa tions(data = KNOWN_DATA):
if data is KNOWN_DATA:
return "very quick answer"
elif data == KNOWN_DATA:
return "quick answer"
else:
time.sleep(10) # time consuming computations...
return "slow answer"

print "Here we go!"
print massive_computa tions()
print massive_computa tions(same_data )
print massive_computa tions(equal_dat a)
print massive_computa tions(other_dat a)
print "Done."
--------------------------------------------------------------

Cheers,
Joel

[Roy Smith]

Re: Difference between 'is' and '=='

In article <e08mr1$7lu$1@n ews.lysator.liu .se>,
Joel Hedlund <joel.hedlund@g mail.com> wrote:
[color=blue]
> Which means that "is" comparisons in general will be faster than ==
> comparisons.[/color]

I thought that == automatically compared identify before trying to compare
the values. Or am I thinking of some special case, like strings?

[Peter Hansen]

Re: Difference between 'is' and '=='

Roy Smith wrote:[color=blue]
> In article <e08mr1$7lu$1@n ews.lysator.liu .se>,
> Joel Hedlund <joel.hedlund@g mail.com> wrote:[color=green]
>>Which means that "is" comparisons in general will be faster than ==
>>comparisons .[/color]
>
> I thought that == automatically compared identify before trying to compare
> the values. Or am I thinking of some special case, like strings?[/color]

You must be thinking of a special case:
[color=blue][color=green][color=darkred]
>>> class A:[/color][/color][/color]
.... def __cmp__(self, other): return 1
....[color=blue][color=green][color=darkred]
>>> a = A()
>>> a is a[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> a == a[/color][/color][/color]
False


-Peter

[Clemens Hepper]

Re: Difference between 'is' and '=='

Roy Smith wrote:[color=blue]
> In article <e08mr1$7lu$1@n ews.lysator.liu .se>,
> Joel Hedlund <joel.hedlund@g mail.com> wrote:
>[color=green]
>> Which means that "is" comparisons in general will be faster than ==
>> comparisons.[/color]
>
> I thought that == automatically compared identify before trying to compare
> the values. Or am I thinking of some special case, like strings?[/color]

Even for strings there is a performance difference:
[color=blue][color=green][color=darkred]
>>> timeit.Timer("' a'=='a'").timei t()[/color][/color][/color]
0.2685978412628 1738[color=blue][color=green][color=darkred]
>>> timeit.Timer("' a' is 'a'").timeit()[/color][/color][/color]
0.2173049449920 6543

mfg
- eth

[Dan Sommers]

Re: Difference between 'is' and '=='

On Mon, 27 Mar 2006 14:52:46 +0200,
Joel Hedlund <joel.hedlund@g mail.com> wrote:
[color=blue]
> ... According to PEP8 (python programming style guidelines) you should
> use 'is' when comparing to singletons like None. I take this to also
> include constants and such ...[/color]

This does *not* also mean constants and such:

Python 2.4.2 (#1, Feb 22 2006, 08:02:53)
[GCC 4.0.1 (Apple Computer, Inc. build 5247)] on darwin
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> a = 123456789
>>> a == 123456789[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> a is 123456789[/color][/color][/color]
False[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Regards,
Dan

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

[mwql]

Re: Difference between 'is' and '=='

It's really strange,

if
a = 1
b = 1
a is b ==> True

the same thing applies for strings, but not for dict, lists or tuples
I think the 'is' operator is useful for objects only, not for primitive
types,
I think I solved the mystery behind my bugged code =)

[Benji York]

Re: Difference between 'is' and '=='

mwql wrote:[color=blue]
> It's really strange,
>
> if
> a = 1
> b = 1
> a is b ==> True
>
> the same thing applies for strings[/color]

Not quite:
[color=blue][color=green][color=darkred]
>>> 'abc' is 'abc'[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> 'abc' is 'ab' + 'c'[/color][/color][/color]
False

--
Benji York

[Clemens Hepper]

Re: Difference between 'is' and '=='

Dan Sommers wrote:[color=blue]
> This does *not* also mean constants and such:
>
> Python 2.4.2 (#1, Feb 22 2006, 08:02:53)
> [GCC 4.0.1 (Apple Computer, Inc. build 5247)] on darwin
> Type "help", "copyright" , "credits" or "license" for more information.[color=green][color=darkred]
> >>> a = 123456789
> >>> a == 123456789[/color][/color]
> True[color=green][color=darkred]
> >>> a is 123456789[/color][/color]
> False[/color]

It's strange: python seem to cache constants from 0 to 99:

for x in xrange(1000):
if not eval("%d"%x) is eval("%d"%x):
print x

for me it printed 100-999.

- eth

[Diez B. Roggisch]

Re: Difference between 'is' and '=='

mwql wrote:
[color=blue]
> It's really strange,
>
> if
> a = 1
> b = 1
> a is b ==> True
>
> the same thing applies for strings, but not for dict, lists or tuples
> I think the 'is' operator is useful for objects only, not for primitive
> types,
> I think I solved the mystery behind my bugged code =)[/color]

The reason that "is" works for small numbers is that these are cached for
performance reasons. Try
[color=blue][color=green][color=darkred]
>>> a = 1000000
>>> b = 1000000
>>> a is b[/color][/color][/color]
False

So - your conclusion is basically right: use is on (complex) objects, not on
numbers and strings and other built-ins. The exception from the rule is
None - that should only exist once, so

foo is not None

is considered better style than foo == None.

Diez

[Felipe Almeida Lessa]

Re: Difference between 'is' and '=='

Em Seg, 2006-03-27 às 08:23 -0500, Dan Sommers escreveu:[color=blue]
> On Mon, 27 Mar 2006 14:52:46 +0200,
> Joel Hedlund <joel.hedlund@g mail.com> wrote:
>[color=green]
> > ... According to PEP8 (python programming style guidelines) you should
> > use 'is' when comparing to singletons like None. I take this to also
> > include constants and such ...[/color]
>
> This does *not* also mean constants and such:
>
> Python 2.4.2 (#1, Feb 22 2006, 08:02:53)
> [GCC 4.0.1 (Apple Computer, Inc. build 5247)] on darwin
> Type "help", "copyright" , "credits" or "license" for more information.[color=green][color=darkred]
> >>> a = 123456789
> >>> a == 123456789[/color][/color]
> True[color=green][color=darkred]
> >>> a is 123456789[/color][/color]
> False[color=green][color=darkred]
> >>>[/color][/color][/color]

Not those kind of constants, but this one:

Python 2.4.2 (#2, Nov 20 2005, 17:04:48)
[GCC 4.0.3 20051111 (prerelease) (Debian 4.0.2-4)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> CONST = 123456789
>>> a = CONST
>>> a == CONST[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> a is CONST[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

--
Felipe.

[filipwasilewski@gmail.com]

Re: Difference between 'is' and '=='

Clemens Hepper wrote:[color=blue]
> It's strange: python seem to cache constants from 0 to 99:[/color]

That's true. The Python api doc says that Python keeps an array of
integer objects for all integers between -1 and 100. See
http://docs.python.org/api/intObjects.html.
This also seems to be true for integers from -5 to -2 on (ActiveState)
Python 2.4.2.

--
fw